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Discussion Starter · #1 ·
Maybe some of the tech experts on the EOC forum can chime-in: Why the heck is the odometer data (or any persistent data, for that matter) stored in a presentation device?

I was inspired to ask this question after coming across the TSB thread (http://www.elementownersclub.com/forums/showthread.php?t=42065) for 2007 gauge cluster needles falling off. Apparently, part of the replacement procedure requires the technician to transfer the odometer and oil life data to the new gauge cluster.

I replaced the gauge cluster in a 2001 Civic once and was appalled that the odometer data was stored in there. I mean, I understand that once upon a time, when gauges were totally mechanical, that the odo would be in the speedometer.

But nowadays? Is there a good reason for this?
 

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you've got me... I feel the same way as you. There should be a computer storing that info elsewhere in the car, such as the ECU and not the gauge cluster.
 

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Discussion Starter · #5 ·
My internet research on the topic leads me to believe that most car companies put the odometer data in the gauge cluster.

I saw mention of a few vehicles that store the value in both the ECU and cluster (WTF?), but have yet to turn up a vehicle that keeps the odometer value solely in the ECU.

At this point, I have to believe it's an example of old habits dying hard. I can't imagine anyone would actually design things this way if they were starting from scratch.
 

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If you were truly designing a vehicle from scratch, with no regard to consumers' preconception, nor manufacturing technology, it's doubtful that SUVs, let alone the Element, would have been built. But I digress.

The question has two parts - why is an odometer needed, and why is it controlled by the gauge CPU instead of the ECM/PCM?

A. Legal
1. Manufacturers vehicle warranties and maintenance procedures are specified in terms of distance operated as well as time owned. Until the average consumer becomes fluent in conversing electronically with ECMs, it's a safe bet that a basic odometer display will be standard equipment on all civilian vehicles.

2. When it comes time to sell a vehicle, most states require that you indicate the mileage at the time of sale, presumably for the benefit of the purchaser. Since there are laws against falsifying (rolling back) mileage indicators, they must take this seriously.

B. Engineering
Historically, gauges/monitoring systems have been viewed as more reliable and durable than control systems. Both the instrument cluster and the ECM/PCM systems now utilize computer processors, but basically the gauge cluster continues to be a relatively simple monitor with limited functions, while the ECM/PCM continues to evolve into a more sophisticated and complex closed loop control system.

Sometimes it's less easier and less expensive to provide a second system, than to add a function to an existing system.

Why isn't the clock built into the ECM or the gauge cluster instead of the radio? The clock uses a standard chipset to perform a dedicated function, and only needs a display - and buttons to set it. A radio provides the display and easily labelled buttons. An odometer is a simple counter. Having the odo built into the instrument cluster instead of the ECM/PCM, and the counting done by the gauge CPU, make it simpler to provide a trip odo function. If the ODO logic/memory were in the ECM, signals would have to be sent from the switches back to the ECM/PCM.

If you bury the functionality of an odometer in the software of an emission control system, it becomes very difficult to tell if or when there has been tampering. If you isolate a counter, set its range to handle 95% of the probable range encountered during operation, and restrict its programability to positively incrementing, it becomes much more tamper-resistant.
 

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Discussion Starter · #7 ·
@psschmied, you bring up some interesting points. I don't really question the need for an odometer.

But all of your bullets under the "Engineering" heading do not seem to propose a compelling reason to implement an odometer completely in the gauge cluster. If anything, they imply the obscurity of it all might make it harder to hack.

I think it's a stretch to compare the radio clock to the odometer. The radio is known to be a user-replaceable and upgradeable component. The gauge cluster - not so much. If anything, you would be replacing it with a stock component (in my opinion). And unfortunately, because of the manufacturers' designs, you must go through a costly process to receive a replacement cluster that has the correct odometer value stored in it.

I guess I (ignorantly) consider the ECU to be much more than intake/emissions. Maybe I assume it to be a more generic center of control than it really is. In that case, I agree - a real-time motor control device might not be the place to store the odometer value.

Regardless, separating the storage of the value from the mechanical dials that fail, would be better, in my opinion.
 

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separating the storage of the value from the mechanical dials that fail, would be better, in my opinion.
Separating mechanical movements from the vehicle's monitoring display (and moving them to the obsolete technology bin) would be better yet.

The only reason that I can think of for the persistence of meter movements is the basic inaccuracy of the cr*ppy fuel quantity measurement system, and the cost of replacing floats with weight/stress senors (used in industrial digital scales) or an optical fluid level system. Either could be combined with an algorithm that corrects for the shape of the tank to give a more accurate measurement.
 

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Discussion Starter · #9 ·
I'm pretty sure Tesla's Model S delivers on that progression - their dash gauges are an LCD screen, I believe.

Good call on the fuel tank level idea!
 

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I'm pretty sure Tesla's Model S delivers on that progression - their dash gauges are an LCD screen, I believe. . . .
An ECM/PCM probably doesn't provide the distance traveled function on the all electric Tesla. It's probably all integrated with the dash display computer.
 

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Discussion Starter · #11 ·
An ECM/PCM probably doesn't provide the distance traveled function on the all electric Tesla. It's probably all integrated with the dash display computer.
That may be, but I bet the display has it's own part number, and can be replaced without losing any data. But I'm totally speculating :)
 
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