2003 Suburban 2500 Speedometer/Odometer Issue (Not Stepper Motor Issue

jdwheelz · January 31, 2013

First I'd like to say this is my first post, and thanks for all the great information here and sorry for the novel.

I just purchased a 2003 Sub 2500 2wd 8.1L 4.10 rear ~132k miles with stock rims and tires (245/75/16) from a used car dealer, so I have no history info other than carfax. On my first test drive I noticed a very pronounced shift into each gear. It would hold a few hundred RPM more than I thought it should and when it shifted it was VERY firm. It was consistent and never any slipping, just firm. It made me think someone had an aftermarket tune with aggressive shift points/pressures. The dealer offered to have the local Chevy dealer check out the vehicle and they said there was no evidence of a tune but they did flash with the newest program. After the new program the truck shifted like a stock vehicle so I picked it up.

While driving on the highway I had the cruise set at 78 mph, my normal cruising speed. I noticed that I was catching far more vehicles than normal, I downloaded a speedometer app on my phone and this verified that the speedo was off by 10%, if speedo says 70 I'm going 77. I have verified I have the 4.10 by checking RPM vs Speed and all checks out.

Where does the speedometer calibration logic reside? PCM or BCM? When they loaded the new program did they only load one of those modules and not the other?

With everything stock and presumably stock code I'm confused as to why my speed and mileage are off.

If any of our wonderful techs on the site are in my area I wouldn't mind driving 30 miles or so to see you instead of my local dealership. I am in Murfreesboro TN.

Again thanks for any help you can give.

Jason

txab · January 31, 2013

PCM. Perhaps they put in wrong info on tire size, etc...

gmtech4 · January 31, 2013

your tire size has to be put in to the reprograming for speedo ,...

have shift adapts reset in the TCM for giggles and have the truck learn your driving habits ,

just take truck in and have it scanned as harsh shifts CAN be from codes in transmission defaulting to max line pressure ,,,having said that the SES light should be on ,,,

welcome and take it in ,,,,im a big advocate for not throwing parts at trucks ,,,be smart with your money

nice powertrain you have ,they are a beast

jdwheelz · February 1, 2013

Thanks for the info. I was going to head to the dealer this weekend but now I have a water pump to replace. Oh well, maybe next weekend.

jdwheelz · February 13, 2013

Just wanted to follow-up with everyone. I stopped by the dealer this morning, the tech hooked up the Tech II and said that all updates had been programmed. He looked through the menus and only found one place he could adjust the tire size, it was in the BCM (Brake Control Module) and it was set correctly. He said this would probably only adjust the ABS operation but we could give it a try. So we went on a test drive together with my GPS as he monitored the Tech II. We set the tire size to a few different sizes and.........nothing, it didn't make a difference in the speed. He then looked at the wheel sensor speed and they were all reading correctly but when he looked at the vehicle speed in the PCM it was 10% low, as well as the instrument cluster speed. This led me to believe that the speed sensor is getting/sending incorrect information but it seems like if it was, the truck wouldn't shift properly or erratically. Perhaps this is why the shifting was so harsh before I purchased it, although I don't know why the original flash would have cleared that up.

He said that if I were to put different size tires on the truck and come in to have them fix the speedo they have to call GM. They give GM the tire size and IF they have a program for that tire size it costs the dealer $150 for the programming. They then load this new program to the PCM and this would fix the speed. In my case, stock tires, they just input the VIN and cannot modify any settings to adjust the speedometer calibration. My visit today cost $100 for one hour of labor and didn't fix my issue. I guess I'll just have to live with this or get a custom tune or programmer I guess.

Just for fun, I swapped the speed sensors, ISS and OSS, to see if that would make a difference but it didn't.