Maybe I don't know how this works, but when someone says...I have an 88 truck, I don't know if it is a 4 X 4, Automatic or what.
As posted, using the jumper between terminal 1 & 2 is ECM flash codes. Hell, I got an ACDelco key on my ring for this job right now. Yes, flashing code 12 three times is normal. If nothing comes after, your are done here. Code flash from low to high. ABS is a separate system added on by TRW usually and is a 3 channel system. It has the ability to stop, release or hold both front by themselves and both rear at the same time.
If it is an automatic, it could be a TH700R4. The sensor if driven by a color coded plastic gear to an electric two wire sensor, the driven and drive gear needs looked at. They will wear over time and "Apple Core" with the center worn down and not touching to hit and miss. Yes, the sensor can go bad, but in that era when they use a buffer or DRAC, either will take an A/C voltage, change it to digital "On-Off" signal, amplify it, then share it with then speedometer, but can have a separate wire to the ABS module.
I don't have the paper GM service manuals for these, and can't figure out how to find the vehicles information to give a good answer. What am I missing?
Even newer GMT800 use a high resolution VSS sensor, that is magnetic. It's target is a 40 tooth metal wheel on the output shaft and is heated, then pressed on. They can slip or move out of place backward. With any "PMG" sensor, air gap is critical, so loss of signal will happen. This occurs on Front-Drive transaxles as well. But, the ABS will use this high resolution signal from the PCM, and apply, release, or hold front brakes, but both rear brakes at the same time. This is three channel ABS and works fine.
I guess from my point of view, changing parts is guessing. We did not afford that privilege at the dealer, so only after basic visual, system check and a scan tools ability to communicate with the RWAL module, then a signal generator from VSS harness forward and check scan tool for readings. An A/C voltage test on the VSS sensor while turning the driveshaft, the is needed, a close inspection of both the drive and driven gears.
Yes, I still have those tools even though it's been over a decade since I worked at the dealer. But once again, I can't answer if I don't know what someone has. Is there a way to find out?
