GM LB7 Tire Size Calibration Challenges
I'd like to share a story about something I ran into yesterday while programming a new ECM on a GM C4500 LB7 (VIN: 1GBE4E1173F508647). It's a bit of a long story but I need to start from the beginning and I'll try to keep it as short as possible.
So this shop called me Friday to come check this truck because after they replaced the batteries the speedometer was reading double the actual speed. Once you got to 40mph the speedometer would read 80 and the speed limiter would kick in. This truck had no other problems prior to this. I have ran into several of these in the past doing this and have found that the ecm loses its ring gear and revs per mile programming when the battery voltage gets low. So I went out to check it and found the tire revolutions per mile programming was at 0 according to the Tech2.
I looked up my tire size and entered the numbers. It said programmed successfully and I verified by selecting "read". I took it out and it appeared to work ok however, the speedometer was off by about 10mph. I pulled over and started adjusting the revs per mile number to fine tune it to match the abs wheel speed pid as well as my handheld GPS. I tried numerous times but nothing seemed to change it. Then out of the blue I attempted to start the truck and it would not crank. Also the Tech2 beeped and said no communication. I tried to read codes but it would not communicate. Also the gauges that get signals from the ECM were all at their minimum position.
I diagnosed it and found the ECM was at fault. All powers, grounds, and communication circuits were there. I had no shorted 5 volt ref or CAN | Class 2 circuits. They got a new ecm and I went out yesterday to program it. Right away I noticed when I turned the key on that my gauges were working so I programmed it with TIS. The flash took and it started right up with no issues. I grabbed my tech 2 and programmed the gear ratio and tire revs per mile successfully. I took it out to drive it and just like Friday, the speedometer was off by 10mph.
So I went back in with the Tech2 and again started trying different revs per mile numbers. Again nothing seemed to change it so I took it back to the shop to do a little research into what could be going on. I found PIT3294G: Reprogramming Speedo Gear Ratio & Tire Size which I tried it and it said successful. Hit the key to start it and nothing. No gauges, no ECM communication, and won't start once again. I double checked all inputs and found no problems. I tried a hard battery reset with no change. I tried to flash the ECM again but it would not communicate to accept the flash.
I'm now where I left off on Friday with a faulty ECM. After doing some researching, I read something that mentioned the key having to be left off for 60 seconds in order for the ECM to accept the revs per mile calibration. I did not see this anywhere when looking up in GM Service Info so I did not do it and is likely why the changes I made did not work. That being said, it appears that whatever data block in the ecm holds this revs per mile programming can "overflow" so to speak and will corrupt the ecm. This is just my theory since both ECM's are now locked up after multiple attempts of programming the revs per mile without waiting 60 seconds has bricked the ecm's.
We have another ECM coming tomorrow since this new one was doa right out of the box but I just wanted input. I feel like this is a "hole" in the software that likely no one has ran into before but I have no way of proving it. I talked to John Rogers, Sean Miller, and Michael Burmester yesterday needing a shoulder to cry on lol. I was hoping they had seen this before and wanted to share my odd story. I'll update here tomorrow with how this all goes. If anyone knows a GM engineer have them call me.