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Discussion Starter · #1 · (Edited)
I’m attaching a few pages from the 2003-2005 Honda Service Manual for reference for this thread.

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Now that everyone can see what I’m referring to, I’ll start by describing the VTEC system as I best understand it.

When the PCM wants VTEC, it sends 12v (EDIT - this is a square wave power signal, so it probably does not read 12v on a multimeter) onto the GRN/YEL wire to the VTEC solenoid, which is grounded to G101 through a black wire. The VTEC solenoid activates and sends oil pressure to the VTEC pressure switch.

The BLU/BLK wire carries 12v from the PCM to the VTEC pressure switch, which is in a normally closed position (continuity) allowing the voltage to be routed through the BRN/YEL wire to G101 ground. Since the wire is grounded, the PCM reads 0V on the BLU/BLK wire.

When the VTEC solenoid opens and sends sufficient oil pressure to the VTEC pressure switch, the switch opens (becomes discontinuous), and the BLU/BLK wire path to ground through the BRN/YEL becomes disconnected. This causes the PCM to read 12v on the BLU/BLK wire.

A scanner will read the PCM data VTEC SOL and VTEC PRESS SW.
When VTEC is off:
VTEC SOL = OFF, 0V on GRN/YEL wire, VTEC PRESS SW = ON, 0V on BLU/BLK wire.

When VTEC is on:
VTEC SOL = ON, 12v on GRN/YEL wire, VTEC PRESS SW = OFF, 12v on BLU/BLK wire.

P2646 is vtec pressure switch circuit LOW, which means that the computer was expecting 12v on the wire, but got 0V on wire. To say it in a different way, if we ever have a situation where VTEC SOL=ON and VTEC PRESS SW=ON, that means a P2646 code will appear. In English, when the PCM activates the solenoid, it expects the vtec pressure switch to respond by saying it sees pressure.

Potential causes of P2646 would then either be:
1.Computer shorted to ground at BLU/BLK pin, abnormally reading 0V
2.BLU/BLK wire shorted to ground, abnormally reading 0v
3.vtec pressure switch stuck in closed position, always in continuity, 0v
4.vtec pressure switch not getting sufficient oil pressure to open the switch, such as in a a) bad solenoid not opening, b) clogged vtec spool valve screens, or c) low engine oil pressure (from a multitude of causes)

Ok, now that we are on the same page, take a look at the P2646 algorithm listed. I have no problems with steps 1-5. That’s all basically checking oil level and confirming the P2646 is still present.

The problem I have starts with step 6. Disconnect vtec pressure switch connector. Then in step 8 it says, check to see if VTEC PRESS SW = ON. Well, if we disconnected the vtec pressure switch connector, we have made an open circuit. In an open circuit the BLU/BLK wire will measure 12v and we have artificially made the VTEC PRESS SW = OFF.

The only way VTEC PRESS SW = ON when we have disconnected the vtec pressure switch connector is if we have a short to ground on the BLU/BLK wire or a short to ground inside the PCM. But what goes the manual say? The manual says if VTEC PRESS SW = ON the next trouble shooting steps is to the check the vtec oil pressure. That can’t be right. We should be checking for a short to ground.

The manual’s other leg says if the VTEC PRESS SW does not say ON, then replace the vtec pressure switch. Well, that also can’t be right. We disconnected the vtec pressure switch connector in step 6. This causes the VTEC PRESS SW to stay in the OFF position all time time because we made and open circuit. It is currently normal since we caused it.

The other bizarre discrepancy is steps 12+13. it says to run the VTEC test, which basically revs the engine up to 3000ish rpms and check the vtec oil pressure. In the manual the cutoff limit is 7psi which can’t be right. 7psI is an idle speed reading, not a 3000rpm reading. FWIW, the similar test in a 2003 CRV manual puts the cutoff limit at 57psi.

It is my interpretation that the factory manuals’ wiring diagrams are correct, while the step by step algorithm is wrong.

What I would love to hear from you guys is if you think I am correct and the manual’s step by step algorithm is wrong and should just be ignored completely … or is there a misprint in the manual that would make everything make sense? Or am I interpreting the steps in the manual incorrectly?
 

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I’m attaching a few pages from the 2003-2005 Honda Service Manual for reference for this thread.

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Now that everyone can see what I’m referring to, I’ll start by describing the VTEC system as I best understand it.

When the PCM wants VTEC, it sends 12v onto the GRN/YEL wire to the VTEC solenoid, which is grounded to G101 through a black wire. The VTEC solenoid activates and sends oil pressure to the VTEC pressure switch.

The BLU/BLK wire carries 12v from the PCM to the VTEC pressure switch, which is in a normally closed position (continuity) allowing the voltage to be routed through the BRN/YEL wire to G101 ground. Since the wire is grounded, the PCM reads 0V on the BLU/BLK wire.

When the VTEC solenoid opens and sends sufficient oil pressure to the VTEC pressure switch, the switch opens (becomes discontinuous), and the BLU/BLK wire path to ground through the BRN/YEL becomes disconnected. This causes the PCM to read 12v on the BLU/BLK wire.

A scanner will read the PCM data VTEC SOL and VTEC PRESS SW.
When VTEC is off:
VTEC SOL = OFF, 0V on GRN/YEL wire, VTEC PRESS SW = ON, 0V on BLU/BLK wire.

When VTEC is on:
VTEC SOL = ON, 12v on GRN/YEL wire, VTEC PRESS SW = OFF, 12v on BLU/BLK wire.

P2646 is vtec pressure switch circuit LOW, which means that the computer was expecting 12v on the wire, but got 0V on wire. To say it in a different way, if we ever have a situation where VTEC SOL=ON and VTEC PRESS SW=ON, that means a P2646 code will appear. In English, when the PCM activates the solenoid, it expects the vtec pressure switch to respond by saying it sees pressure.

Potential causes of P2646 would then either be:
1.Computer shorted to ground at BLU/BLK pin, abnormally reading 0V
2.BLU/BLK wire shorted to ground, abnormally reading 0v
3.vtec pressure switch stuck in closed position, always in continuity, 0v
4.vtec pressure switch not getting sufficient oil pressure to open the switch, such as in a a) bad solenoid not opening, b) clogged vtec spool valve screens, or c) low engine oil pressure (from a multitude of causes)

Ok, now that we are on the same page, take a look at the P2646 algorithm listed. I have no problems with steps 1-5. That’s all basically checking oil level and confirming the P2646 is still present.

The problem I have starts with step 6. Disconnect vtec pressure switch connector. Then in step 8 it says, check to see if VTEC PRESS SW = ON. Well, if we disconnected the vtec pressure switch connector, we have made an open circuit. In an open circuit the BLU/BLK wire will measure 12v and we have artificially made the VTEC PRESS SW = OFF.

The only way VTEC PRESS SW = ON when we have disconnected the vtec pressure switch connector is if we have a short to ground on the BLU/BLK wire or a short to ground inside the PCM. But what goes the manual say? The manual says if VTEC PRESS SW = ON the next trouble shooting steps is to the check the vtec oil pressure. That can’t be right. We should be checking for a short to ground.

The manual’s other leg says if the VTEC PRESS SW does not say ON, then replace the vtec pressure switch. Well, that also can’t be right. We disconnected the vtec pressure switch connector in step 6. This causes the VTEC PRESS SW to stay in the OFF position all time time because we made and open circuit. It is currently normal since we caused it.

The other bizarre discrepancy is steps 12+13. it says to run the VTEC test, which basically revs the engine up to 3000ish rpms and check the vtec oil pressure. In the manual the cutoff limit is 7psi which can’t be right. 7psI is an idle speed reading, not a 3000rpm reading. FWIW, the similar test in a 2003 CRV manual puts the cutoff limit at 57psi.

It is my interpretation that the factory manuals’ wiring diagrams are correct, while the step by step algorithm is wrong.

What I would love to hear from you guys is if you think I am correct and the manual’s step by step algorithm is wrong and should just be ignored completely … or is there a misprint in the manual that would make everything make sense? Or am I interpreting the steps in the manual incorrectly?
I think (always dangerous) that when the test refers to oil pressure switch ON what they really mean is that VTEC operation has been commanded ON by activating the VTEC solenoid which opens the VTEC oil pressure switch if all is working. Thus at idle the ECM terminal B11 shows 0 volts because the 12 volt bias voltage is pulled to ground, as you stated. Conversely, when the VTEC solenoid is commanded ON the ECM terminal B11 shows 12 volts through the internal ECM pull-up resistor. Oil pressure at the VTEC oil pressure switch needs to be 57 psi minimum. In contrast, when air-checking the VTEC rocker arm operation the manual requires 42 psi of air pressure to test. The whole thing is needlessly convoluted.
 

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Discussion Starter · #3 ·
I think (always dangerous) that when the test refers to oil pressure switch ON what they really mean is that VTEC operation has been commanded ON by activating the VTEC solenoid which opens the VTEC oil pressure switch if all is working. Thus at idle the ECM terminal B11 shows 0 volts because the 12 volt bias voltage is pulled to ground, as you stated. Conversely, when the VTEC solenoid is commanded ON the ECM terminal B11 shows 12 volts through the internal ECM pull-up resistor. Oil pressure at the VTEC oil pressure switch needs to be 57 psi minimum. In contrast, when air-checking the VTEC rocker arm operation the manual requires 42 psi of air pressure to test. The whole thing is needlessly convoluted.
Hi.Thanks for the response rusty. I always appreciate your posts.

Thanks for confirming my thoughts on how ECM terminal B11 works. I also read the diagram as a pull up circuit.

I agree, I don’t like trying to figure why someone decided to call the data parameters ”ON” or “OFF”. They just are what they are. They would have made things more clear if they just went with closed/open or 0v/12v for the VTEC PRESS SW data.

Thanks, I found the rocker arm 42psi air pressure test under the engine cylinder head section.

I’m still under the impression that the manual‘s step by step instructions is wrong for P2646. I think folks are better off just using the wiring diagram or getting the 2003 CRV P1259 information and following that instead.
 

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Discussion Starter · #4 ·
I’m including what I have for the 2003 CRV P1259. If you look at this and the vtec wire diagram from the Element, every step makes sense. I do realize that P1259 combines P2646, 2647, 2648, and 2649 into one code.

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I've been a mechanic for 45 yrs, never ever follow electrical trouble trees. I study the circuit and figure out how it works. With VTEC the system is should be considered 2 systems, one mechanical, other electrical. Thats what makes it difficult to diagnose by tossing parts at it. No "pattern" failures here, it's case by case. To me the most suspect part would be the pressure switch, mainly due to the low amount of power going through the contacts or connectors, even a small amount of resistance here will trip the code. If mine ever acts up, first would be looking at that ground signal going into the PCM, and not disturb the connection at the sensor/switch. Notice that is step 10 in the chart. Way to far down the tree for me. Which reminds me, that it's a good idea to coat the switch/wire connector with dielectric lube, as preventative maintenance.
 

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It is misleading. I can not speak for the HDS but when monitoring live data with an Autel, the VTEC solenoid valve and the VTEC pressure switch are always opposite when working properly. At idle, the solenoid is commanded OFF and the switch will show ON because the oil pressure is not great enough to open the circuit. When you get to the right RPM, the ECM commands the solenoid ON, oil pressure is allowed through, the pressure switch goes OFF and the ECM likes that there's oil pressure when there should be. Maybe, and just maybe, the HDS uses ON/OFF backwards. Cause yeah, step 8, if ON is indicated replace the switch? No... if the engine is not running the switch circuit is closed... ON.

The main problem with following an engineers troubleshooting steps is that they assume everything else is working perfectly. When I get a P2646 I check the oil level first then drive it while watching live data. If there is a lag between solenoid ON and switch OFF during acceleration it's usually just clogged screens. If there is a lag between solenoid OFF and switch ON during deceleration it's usually a sticking/bad switch. If the switch has just gone completely bad and stuck closed (ON) it will usually trip the P2646 code. Stuck open (OFF) will usually trip P2647. Many other things can trip P2646 though. Fun stuff.
 

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Discussion Starter · #7 ·
I've been a mechanic for 45 yrs, never ever follow electrical trouble trees. I study the circuit and figure out how it works.
@07lmnt thanks, I totally get that. Many times the trouble tree steps are inefficient ways to find an answer. Since I’m a DIY guy, I’ve got the leisure time to study the circuit, and then also mentally go through the trouble tree steps. Every trouble tree I have needed to study makes sense with the circuit diagram, albeit done somewhat inefficiently. Except this one. The P2646 trouble tree doesn’t make any sense. When I look at the related P2647, P2648, and P2649 and the corresponding CRV’s P1259 the trouble trees for all those make sense when compared to the circuit.



It is misleading. I can not speak for the HDS but when monitoring live data with an Autel, the VTEC solenoid valve and the VTEC pressure switch are always opposite when working properly. At idle, the solenoid is commanded OFF and the switch will show ON because the oil pressure is not great enough to open the circuit. When you get to the right RPM, the ECM commands the solenoid ON, oil pressure is allowed through, the pressure switch goes OFF and the ECM likes that there's oil pressure when there should be. Maybe, and just maybe, the HDS uses ON/OFF backwards. Cause yeah, step 8, if ON is indicated replace the switch? No... if the engine is not running the switch circuit is closed... ON.

The main problem with following an engineers troubleshooting steps is that they assume everything else is working perfectly. When I get a P2646 I check the oil level first then drive it while watching live data. If there is a lag between solenoid ON and switch OFF during acceleration it's usually just clogged screens. If there is a lag between solenoid OFF and switch ON during deceleration it's usually a sticking/bad switch. If the switch has just gone completely bad and stuck closed (ON) it will usually trip the P2646 code. Stuck open (OFF) will usually trip P2647. Many other things can trip P2646 though. Fun stuff.
@Locksmith thanks for responding. I use a Launch x431, and the data parameters for VTEC SOL and VTEC PRESS SW are the same as your Autel, as in, they are always opposite when working properly. I believe @philharmonic who has a Foxwell has also reported the same. It is possible one of them original made the code, and the others copied it. However, as you said, even if you flipped the ON/OFF in the trouble tree, it still does not work out right.
 

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I should have been more specific. I meant just the VTEC pressure switch test in step 8.

But now that I'm thinking about it a bit more, I think I got it. Ok so, in a KOEO state the solenoid is OFF and the switch is ON. In step 8 they are telling you to unplug the switch and check its state. They are asking you if it shows ON. If it doesn't show ON, that means the circuit is good and just replace the switch. If you unplug it and it still shows ON you have further diagnosis to do. I just wish they would move the "inspect the VTEC system" in step 13 up before replacing parts because checking the screens would be part of that.
 

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Discussion Starter · #9 · (Edited)
I should have been more specific. I meant just the VTEC pressure switch test in step 8.

But now that I'm thinking about it a bit more, I think I got it. Ok so, in a KOEO state the solenoid is OFF and the switch is ON. In step 8 they are telling you to unplug the switch and check its state. They are asking you if it shows ON. If it doesn't show ON, that means the circuit is good and just replace the switch. If you unplug it and it still shows ON you have further diagnosis to do.
Yes I think we’re partially on the same page, in step 6 they say unplug the switch and in step 8 read the data parameters.

If it doesn’t show ON, that means the circuit is good. But just because the circuit is good, it does NOT mean the switch is bad. They have not accounted for low oil pressure, the vtec solenoid being bad or gunk on the screens that can also be present. Once you have confirmed vtec oil pressure is good then you know the switch is bad.

If it shows ON they next recommend measuring oil pressure, but that can’t be right, because the only things that can cause it to say ON is a short to ground, either in the wire or in the PCM.
 

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But just because the circuit is good, it does NOT mean the switch is bad.
Correct. I would imagine they are writing these troubleshooting guides assuming the car has had its maintenance schedule followed properly and they have to be able to be followed by any noob at a dealer service dept. In my experience, P2646 is usually just the switch or low oil level. In the real world, we know much differently. You get someone who only knows about the key and the gas cap and you know you in for a treat.

Once you have confirmed vtec oil pressure is good then you know the switch is bad.
I'm going to see if I can verify with my Autel but I believe the VTEC test in step 12 will force the solenoid on and you should see mains oil pressure at the switch. The only way I've done it so far is hooked up my oil pressure gauge and drove it. I've never been able to get VTEC to kick in while it's in park. That test will show if the solenoid is bad and you know you have to replace it no matter what else is wrong. But I agree it is a convoluted order of operations.

As far as wiring goes I've done tons of pigtails when the switch plug falls apart and gets wet inside. I can't remember if that tripped 2646 or 7 though. I've had a dirty ground on the intake cause trouble. I have not had to re-wire anything VTEC. Well, other than my own experiments. I've also had a 55 gallon drum of oil plague us with VTEC issues till we used it up. I think Lucas Oil ripped me off. I don't use them any more. But yeah, VTEC is a picky bitch and so many things can go wrong.

edit: I verified it. The Autel also has a VTEC test and you can command the solenoid ON and watch the pressure gauge shoot up. It's honestly easier to me to just drive the car but at least I know it works.
 

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Discussion Starter · #11 ·
I think you know where Im going with this thread. Someone comes with a P2646 every month on this forum. Im trying to de-mystify this code. I think it’s possible to put together a testing algorithm that makes sense.

I would imagine they are writing these troubleshooting guides assuming the car has had its maintenance schedule followed properly and they have to be able to be followed by any noob at a dealer service dept.
I’ll respectfully disagree with you there. The reason I disagree is in the CRV P1259 trouble tree I linked in post #4. You will see in step 26, the CRV manual goes on to say if measured vtec oil pressure is below 57psi, then inspect the vtec solenoid (I.e. clean the screens) and also check engine oil pressure. The person who put together the CRV manual did understand that things like sludge, bad oil pump, worn engine can decrease oil pressure causing codes.

I'm going to see if I can verify with my Autel but I believe the VTEC test in step 12 will force the solenoid on and you should see mains oil pressure at the switch. The only way I've done it so far is hooked up my oil pressure gauge and drove it. I've never been able to get VTEC to kick in while it's in park. That test will show if the solenoid is bad and you know you have to replace it no matter what else is wrong. But I agree it is a convoluted order of operations.

edit: I verified it. The Autel also has a VTEC test and you can command the solenoid ON and watch the pressure gauge shoot up. It's honestly easier to me to just drive the car but at least I know it works.
Thanks for verifying that. I have only run the VTEC TEST and watched the data parameters switch ON/OFF for the vtec solenoid/vtec pressure switch. I also note that in the CRV manual, they don’t ask to run the VTEC TEST, but rather to apply direct battery power and ground to the vtec solenoid, revving the engine up to 3000, and measuring the vtec oil pressure. So I do agree with you, I dont believe the PCM requests vtec activation when there is no load on the engine.

I don’t have the adapters to plug a gauge onto the vtec oil pressure switch port. I looked up the Honda parts, and it comes to $450+ for all the adapters. How did you piece it all together? I think I can piece all the parts together, but it would still cost me about $80 in adapters+pressure gauge. That M10x1.25 thread on the vtec pressure switch is what is making sourcing the tools difficult.

As far as wiring goes I've done tons of pigtails when the switch plug falls apart and gets wet inside. I can't remember if that tripped 2646 or 7 though. I've had a dirty ground on the intake cause trouble. I have not had to re-wire anything VTEC. Well, other than my own experiments. I've also had a 55 gallon drum of oil plague us with VTEC issues till we used it up. I think Lucas Oil ripped me off. I don't use them any more. But yeah, VTEC is a picky bitch and so many things can go wrong.
If the vtec pressure switch is shorting out because of water, it’s shorting to ground. Therefore you’re getting a P2646 because the BLU/BLK wire will read 0V when it’s not supposed to. If you’re having problems with the ground at the intake manifold (aka G101), that means you’re got an open circuit in the wire … simulating an open switch and the BLU/BLK wire will be reading 12v, thus a bad ground will give you a P2647.
 

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I have the Lang TU-16A pressure testing kit. It has the 10MM-1.25 adapter but that's if you're only going to hook up the gauge. I got that 1/8-27NPT Tee fitting and male couplers from plumbing parts and chased the one outlet with a 10mm-1/25 tap to install the pressure switch. Much thicker layer of thread tape on the pressure switch since the one outlet on the Tee is threaded both ways.

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And the whole thing about the authors of the manual was purely speculation on my part. I like how the CR-V manual approaches it because they don't go straight to replacing parts that might not be bad. But it just has one general code for the VTEC right, it covers everything?

So what are you thinking, a way for someone who might not have access to the expensive tools to still be able to get to the bottom of 2646 and 7?
 

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Discussion Starter · #13 ·
I have the Lang TU-16A pressure testing kit. It has the 10MM-1.25 adapter but that's if you're only going to hook up the gauge. I got that 1/8-27NPT Tee fitting and male couplers from plumbing parts and chased the one outlet with a 10mm-1/25 tap to install the pressure switch. Much thicker layer of thread tape on the pressure switch since the one outlet on the Tee is threaded both ways.



And the whole thing about the authors of the manual was purely speculation on my part. I like how the CR-V manual approaches it because they don't go straight to replacing parts that might not be bad. But it just has one general code for the VTEC right, it covers everything?

So what are you thinking, a way for someone who might not have access to the expensive tools to still be able to get to the bottom of 2646 and 7?
Thank you for the Lang TU-16A kit link! I assume you just plugged in the gauge without hooking up the vtec pressure switch when you were testing your VTEC TEST? Or did you hook together your whole t adapter set up? I‘m also speculating here … but I would be apprehensive about trying to measure vtec oil pressure during a road test instead of using the VTEC TEST. Is it possible that the PCM puts the car into “limp mode” when you hit 3000rpms giving you little time to make an accurate reading while trying to watch the scan tool data parameters and traffic at the same time?

Yes, the CRV P1259 code looks to be an older code found in older Honda vtec PCMs. Ive seen mention of it in 1990 hondas a lot. Ive also met 2004 CRV owners, and it appears the CRV transitioned to P2646+ in 2004 models. P1259 covers P2646, P2647, P2648, and P2649 all in one. I assume that Honda made the change to try to simply diagnosis. And I do think it simplified the P2647, P2648, and P2649 codes but not P2646.

I am not sure we‘re going to be able to fully diagnose P2646 without a higher end scan tool, multimeter and the ability to measure both vtec oil pressure AND engine oil pressure. However, it may be possible to at least explain better why you do a specific step/test to offer more clarity. When folks understand why, then if they choose to ”change parts” on a guess, at least they know what they could be missing. I may try to put something together in the next few weeks and post it on the forum so you guys can pick me apart and tell me everywhere it is wrong.
 

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You're welcome. I had to have everything plugged in for the test because the Autel would not let me continue with any VTEC codes present. Driving it is not dangerous at all because you're not looking for any specific oil pressure, just that it spikes when VTEC kicks in. It should be near zero (up to 7psi is ok) with no VTEC and pump pressure when it kicks in. At that RPM it can be up to 80 or 90psi so it jumps up quick. It's quite obvious. You also do not have to be going fast. Just hold the transmission in 1 an go. I have a side road by my shop that's perfect to go back and forth all I want. There's plenty of room above the spool valve for the adapters and the hose is long enough to reach into the passenger window. I run the hose right by the hood hinge and hang the gauge on the passenger visor. I have found that when I clear VTEC codes and drive it, it takes a couple times into VTEC for the ECU to trip the code again.

When I approach 2646 I imagine all the things that would cause the ECU to think there's not enough oil pressure and the opposite for 2647. I believe there may be cheaper scan tools that monitor live data. Being able to watch the states of the solenoid and switch in real time is very valuable in my humble opinion. Like I said earlier, they can lag and that can tell you a lot. None of the Honda diagnostics take that into consideration.

I really don't think we need to worry too much about the base engine oil pressure. If the car ran perfectly fine and just went into limp mode all of the sudden, it's not going to be the oil pump. Of all the Elements I've been through and all the ones I currently run, I have never had to repair or replace an oil pump. My engine has 538k miles on the original oil pump and none of my fleet have less than 250k mi. Some are knocking on my back door.

I would be more than happy to collaborate.
 

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Discussion Starter · #15 ·
You're welcome. I had to have everything plugged in for the test because the Autel would not let me continue with any VTEC codes present. Driving it is not dangerous at all because you're not looking for any specific oil pressure, just that it spikes when VTEC kicks in. It should be near zero (up to 7psi is ok) with no VTEC and pump pressure when it kicks in. At that RPM it can be up to 80 or 90psi so it jumps up quick.
Totally makes sense, if it’s spiking up to 80-90 psi normally that should be plenty to easily see.


Being able to watch the states of the solenoid and switch in real time is very valuable in my humble opinion. Like I said earlier, they can lag and that can tell you a lot. None of the Honda diagnostics take that into consideration.

I really don't think we need to worry too much about the base engine oil pressure. If the car ran perfectly fine and just went into limp mode all of the sudden, it's not going to be the oil pump. Of all the Elements I've been through and all the ones I currently run, I have never had to repair or replace an oil pump.
Yeah, the first I’ve heard of the switching “lag” is when you talked about it in post #6 although you may have alluded to it on other threads before. it’s significance should really be taken seriously if it is encountered, because it makes sense when the whole circuit is taken into consideration.

I do have one question for you. I believe you were the one that came up with using an aftermarket adjustable pressure switch as a substitute for the Honda vtec pressure switch. I think you may have said that you had some early failures of OE Honda vtec pressure switches and were looking for another option. But I think you said that you had tuned down the switch to the mid 20‘s psi … I had assumed you were you doing that to compensate for low engine oil pressure. Not the case? Perhaps you were just trying to calibrate the aftermarket psi switch point to the vtec activation psi?


I would be more than happy to collaborate.
Thanks.
 

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Discussion Starter · #17 · (Edited)
So I went ahead and “simplified” the VTEC diagram from post #1 to two charts, one for when VTEC is not on, and one for when VTEC is activated:

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I then highlighted the areas where a fault could cause a P2646 and then filled in what faults I think could be present:


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Any feedback would be appreciated.

*** EDIT - the GRN/YLW wire carrying the VTEC SOL signal is a square wave power, so it probably doesn’t read 12v when powered on.
 

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Discussion Starter · #19 ·
If I understand this correctly, I think you need to change the switch to "continuity, closed switch". If oil pressure is present and the switch does not open it will cause P2646. A switch that fails open will cause P2647.
yes, you’re right. I’m going to delete the chart with the yellow highlighted areas. It is a confusing slide.
 

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Discussion Starter · #20 · (Edited)
This endeavor is longer than I expected. Here is what I have for now:


First, confirm the code.
Make sure your oil level is right. Clear codes. Run the VTEC TEST on the scan tool**** or test drive the vehicle. Monitor VTEC SOL and VTEC PRESS SW. The CEL light should some on when the VTEC SOL = ON and VTEC PRESS SW = ON. You should retrieve P2646 to confirm. If you don’t have the P2646, you may have an intermittent fault*. Clear codes.

Second, test the wiring.

[* NOTE: Please see posts #23 by 07lmt and post #28 after reading this ”test the wiring” portion. My initial written troubleshooting steps below do not take into account the potential for an intermittent short to ground at the VTEC pressure switch wire connector. I believe 07lmt’s method is post#23, of initially monitoring at the PCM before touching the VTEC pressure switch connector is a solid idea. It is currently my thought that you can monitor VTEC SOL and VTEC PRESS SW over a few drives while reseating the VTEC pressure switch in between drives to see if the fault is intermittent. Again, VTEC SOL OFF with VTEC PRESS SW ON is normal, VTEC SOL ON with VTEC PRESS SW OFF is normal, while P2646 is VTEC SOL ON with VTEC PRESS SW ON. If your fault is intermittent, changed by reseating the VTEC pressure switch connector, then you should really consider an intermittent fault (short) at right at the connector. If your VTEC PRES SW stays ON all the time, then continue on below:]

With Key on, engine off, disconnect the VTEC pressure switch connector. This creates an open switch and your scan tool will read VTEC SOL = OFF and VTEC PRESS SW = OFF if the wiring and ECM are working correctly. It should also cause P2647 to appear.
If your VTEC PRESS SW = ON, then you either have a short to ground on the BLU/BLK wire or a bad PCM. To test for a BLU/BLK short to ground, you disconnect the PCM B connector. Now the BLU/BLK wire should be disconnected on both sides, and you can test for the presence of ground on the wire. If there is ground, then you need to find where the short to ground is located by tracing the wire. I personally think that the most common cause of a wiring short to ground has to be at the connector for the VTEC pressure switch*. If there is no ground, then your PCM is either bad or needs a software update.

Third, test the VTEC oil pressure.

[ Edit: in future post #34 from locksmith, it’s worth noting you should inspect the VTEC spool assembly for any leaks before this part. Leaks can occur at the VTEC pressure switch or between the VTEC spool assembly and the engine. Any leaks should be fixed, and retested before going on to measure pressures. ]

You will need to connect your oil pressure gauge using a T adapter between your VTEC spool assembly and VTEC pressure switch. These special adapters are available from Honda****:
IMG_0720.jpeg


Then use the scan tool to activate the VTEC TEST. Monitor the vtec oil pressure, and your scan tool VTEC SOL and VTEC PRESS SW data. What should happen in a normal vehicle is that when running VTEC test, when the rpm > 3000, the VTEC SOL switches from OFF to ON, the oil pressure raises > 57psi, and the VTEC PRESS SW switches from ON to OFF. But since we are working on a P2646, that means the VTEC PRESS SW will either stay ON, or is switching too slowly to OFF.
If your VTEC oil pressure hits 57psi** and your VTEC PRESS SW=ON, your VTEC pressure switch is bad.
If your VTEC oil pressure does not reach 57psi**, or raises slowly after VTEC SOL turns on, then you remove your VTEC spool assembly and check for debris/sludge on the VTEC spool assembly gasket screen and repeat.

Fourth test, if your VTEC oil pressure is low, and the gasket is clean, check your engine oil pressure at the engine oil pressure switch.

This is a fairly standard oil pressure 1/8”-28 BSPT thread. If you rev your engine up to 3000rpm, your psi should normally be > 44 psi. If your engine oil pressure is normal, you have a bad VTEC solenoid.
If your Engine oil pressure is low, then you’re going to have to go through a search of the potential causes of low engine oil pressure***





*Intermittent faults to be in future discussion
** The 57 psi number comes from the Honda factory service manual. 35 psi is a number that comes from member Fleetwood here at EOC and his measurement of his VTEC pressure switch. 40psi is the number mentioned by “Kentucky home garage” on YouTube. It is purely my guess that Honda considers >57psi as normal, and that 35-40 is the minimum needed.
***causes of low engine oil pressure to be in future discussion
**** Yes, I’m assuming that you are willing to spend the money for a $200-$1000 scan tool with manufacturer data and bidirectional control, $500 for special vtec oil pressure measurement adapters, and $100 for a nice oil pressure gauge. I think can be done with less expensive tools. There is also a good chance that you can skip some steps or skip buying tools by doing a partial diagnosis/partial “parts cannon” method. Again, future discussion. Please see future post # 33




Any comments for the above?
 
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