Hi, my buddy is having a problem with his odometer reading on his 02Z and we are trying to figure out what needs replaced to fix this error. We think that the problem is with whatever the module that stores the vehicle mileage.

Thanks a lot

 

The mileage is stored in the Cluster itself. Its updated once per mile based upon commands from the PCM/BCM(not sure which), via the serial data link.

 

The odometer reading is stored in the IPC (gauge cluster) and to change it the cluster must be sent to a service center. Even the dealer cannot update/change the odometer reading.

 

So then if we switch out his cluster to try to fix it. Will the mileage be wrong? If we put a used cluster in there would it read his correct mileage or whatever the used cluster has? If so who would we go to get his correct mileage into the used cluster?

 

If you switch the cluster it will display the mileage in the replacement cluster. You have to "correct" the mileage in the replacement cluster to display the real mileage on the car. There is a forum member Pacethis I think that can correct the mileage. A speedometer repair shop can do this also. Search for "mileage correction". The mileage is stored in the cluster in a 25010 1k serial eeprom chip, normally this chip has to be remove and reprogrammed for your correct mileage.

What problem are you having with the mileage? Is the DIC displaying other things correctly? If you change clusters and don't get the correct mileage in the cluster it will show up on things like CarFax at some point and will reduce the value of the car a LOT.

 

Ok, I understand. Sometimes the mileage just shows up as an error message occasionally from what he said. The only thing I have ever seen it do is the DIC displays random dots and junk characters. About the carfax thing you are talking about do you just mean if once the used cluster was in place all of a sudden if the mileage jumped from 10000 miles to 75000 miles that it would decrease the value if it were serviced before the mileage was corrected and they noted the mileage as 75000 and not 10000? Is that what you mean by losing value?

 

The laws state the mileage must match the actual on the car therefore changing the cluster out will cause you a legal issue with your department of motor vehicles. What speedyz is saying is that you need to make sure the new cluster is properly matched in mileage.
The odometer displaying the wrong characters or garbage data sounds like you need a new cluster or the owner/previous owner may have tried to get in there and alter it.

 

What really will lower the value is when CarFax shows the car used to have say 15000 miles and then all of a sudden it had 5000 miles. Anytime the mileage goes backwards CarFax (and others) puts a BIG red flag on the report which really makes people wonder what happened.

What it sounds like with your problem is not the odometer but the DIC itself. The DIC is still an internal part of the cluster, just a different part that stores the mileage. I'm assuming from what you said the DIC won't display things like the oil pressure/temp, water temp, or anything else correctly, is that correct? If everything else in the cluster but the DIC seems to be working correctly (specially the speedometer) then the mileage stored in the chip is probably still the correct mileage, and the DIC is just having problems displaying the information. So when you buy a replacement cluster you will need to send both clusters to a repair shop and have them pull the mileage out of the bad cluster and put that mileage in the replacement cluster. What the shop will do is remove the serial eeprom from the bad cluster and read the data, then program that data into the serial eeprom in the replacement cluster. When you put the replacement in the car it will show the real mileage for the car and everyone will be happy, even the CarFax people.

One other thing to note. While I have not put bad data in the serial eeprom on a C5 cluster to see how they respond, I do know that many GM clusters will display "ERROR" if the data gets corrupt. To prevent data corruption the same mileage data is stored in 3 different locations in the serial eeprom.