There is a local 96 Z that is for sale. The owner claims the car to have 2k original miles. I carfaxed the car and it came back clean. I was wondering if there was any way I could tell if the car was rolled back or the true miles have been tampered. Does the ecm hold anything of this sort? He is the original owner, so I guess its possible for him to have just unplugged the speedo and stopped the odometer from counting.

 

I guess I've never gone actively searching for an odometer in the PCM but it's not something I've ever seen on a scanner. The data for the dash odometer is kept on a chip in the dash assembly. I don't work with OBD2 a whole lot, the scanner I do use for them is fairly generic.

I don't even think these PCM's have the hardware in the PCM to do such a thing. The memory on board is volatile and is cleared when the PCM loses power, with the exception of the EEPROM's that are flash programmable.

I'm sure an OBD2 guru will stick his head in here and correct me if I'm wrong.

 

I do know that with a scanner, OBD-II will tell you the mileage when the last clearing of codes was done, so I suspect the mileage could be gotten out of it somehow.

 

I guess the question would be, does it remember that after losing power? Just like the trouble codes I believe that information is kept in volatile memory. Any time the 12 volt supply is cut from the PCM, it's cleared.

To store that information permanently requires a whole different type of hardware that would cost money. Is there a good reason for GM to spend that money? If not, it's probably not there.

 

Have you looked over the car real good? A 1996 Z28 with only 2K original miles should be perfectly clean inside and out. Underneath and body should be perfect and the interiour should still look brand new. In fact if it don't look like it rolled off the show room floor then there is no way it only has 2K. Thats my only advice.

 

Well, you know they gotta have some memory that is saved so that the "change oil" calculations can be made on cars that use it. If that was erased every time the power was lost, it would not be very useful. Just speculating.

 

I thought that was also kept in the dash module or a BCM? I really don't know for sure either

We're all just speculating

 

I would believe the PCM should record the milage in it; but others have stated that this is stored in the cluster...

The speedo takes a filtered signal from the PCM that gets it from the tranny, so the PCM "sees" the pulses from the tailshaft of the tranny, converts those pulses to the appropriate ones to drive the dash speedo correctly (Taking into acct tires size, gearing, etc); this leads me to believe that the milage calculation and storage would be done in the PCM.

There are memory types that are non-volitale (Such as the EEPROM), infact EEPROM can be re-programmed line-by line, and such the PCM most likely re-programs one line of code in the EEPROM memory whenever the milage clicks over (It only takes a few nanoseconds to flash and re-write a single block of memory depending on the chip in the PCM.

I know this is a complex description and won't make sence to a lot of you, but this is most likely how it works; per my knowledge of the 68HC processor series (Which I believe is used in our cars), and how EEPROM/Flash/Volitale/Non-vilitale memory works.

I've been working on some side-projects using a PIC microcontroller that does something similar actually, and it's a MUCH slower processor and I'm erasing and re-writing a 2 lines of code (Am limited to 2 lines by the erase function) in ~ 30ms if I remember my code correctly.

 

It is stored in the cluster, whether or not it is also stored in the PCM is the big question. You're absolutely correct on your assessment of how the electronics work. I don't know the specifics on the OBD2 PCM's to have any clue what is or is not possible though

Typically GM has only used the PCM for critical engine and transmission functions. Everything else is handed off to other systems to handle. The less work the PCM does for anything but engine and tranny, the better.

Whomever told you a scanner could read the odometer, maybe they were thinking of a Tech 2 which can read many of the systems. Is it possible that a Tech 2 can poll the gauge cluster's memory as well and that's what they were seeing?

 

I was just looking at my son's 1996 Camaro today and when I used my Autoxray scanner, it would tell me what the mileage was when data was last cleared from the PCM. It had to be many years ago, because of the mileage stated. The battery was replaced in between that time, so killing the power to the PCM did not erase this register.

 

I agree with Buttercup. The memory in the mileage cluster is, without question, definately nonvolatile. Common sense would tell ya, that short of electrical destruction/short, the clustor memory can not be erased. (Sorta like those movie pay-per-view boxes. ) You get a cluster used, it displays what mileage the vehicle, it can from had. Odometer/speedo LCD clusters aren't cheap by construction design.
I don't BELIEVE.... pcm stores any mileage data on it's own. What data it receives, it passes on to the cluster memory chip.

 

After a little further study, the mileage that I was seeing on the scanner is actually stored in the PCM. It looks to be part of the freeze frame data for when trouble codes and their related info are stored.

 

Well, using this thinking, wouldnt buying a new gauge cluster from Granetelli, technically make you car have 0 miles? Or am I not understanding what you said?