Yes, my lack of knowledge with this vehicle does make for primitive troubleshooting, hence the reason I posted (looking for help...hint, hint). All the problems are originating with the VSS, but it's the bias voltage from the VSS buffer (at the VSS signal input) that seems to dissapear at times, so I'm investigating grounds and power. I did change the sensor, and would like to change the VSS buffer, but throwing money at it in the hopes I've stumbled on to the correct solution goes against my grain. If I can ascertain that a particualr component is bad, then I don't mind spending the money. But I may visit the junkyard and see if I can get one. I think they are usually a reliable unit. I did learn something new today. I found out I can access ABS codes through the OBD1 connector by sorting pins A and H and coounting flashes from the ABS light. And of course I got Code 37 which is for an erratic VSS signal (which comes form the VSS buffer). The confusion here is that the ABS light has been on since we acquired this vehicle; the only ABS code says the VSS signal is erratic, yet we drove the truck for a couple of months before any of the speedometer and shifting issues arose. And of course, if the problem does originate in the VSS buffer, then why does making corners cause the problem? So...more primitive troubleshooting to come! LOL. But maybe not for a few days. Calling for rain and I don't like working outside in the rain very much.
It drove fine until I went around a corner. Yep, letting off the gas doesn't affect the speedometer any more unless I am turning corners at the same time. And with enough of this, eventually the computer sets Code 27 and the transmission stops shifting properly. Something is loose, and I must have had my hands on it as the problem has been reduced to just going around corners. Did find a rusted off right front air-bag sensor dangling, and fixed that. No change, but I didn't expect one either. Think I'll check out the ABS as the VSS feeds into it. Weird system. Just for the rear I think, but it uses the VSS for the rear and looks like it has additional sensors for each front wheel. Maybe I'll take out the ABS fuse and see what happens. I may be getting nowhere fast, but I'm learning a little about how the truck works.
And the code came back and the truck stayed in 1st gear. So, I reset the computer again, wiggled all grounds on the engine, pulled a few engine plugs and re-inserted them, ensured the heater fan, radio and cruise control were all OFF, and the truck ran perfectly. Better than last time as even the speedometer worked correctly. Will drive it some more and if it stays OK, then I'll start turning on the accessories one at a time and see if the problem comes back. If it does, then I will check power and grounds for the offending system.
Update: I disconnected the battery and stepped on the brake pedal to discharge the PCM and clear the codes. It now shifts normally, but the speedometer still drops when I ease off of the throttle, and then comes back up when I increase the throttle. My speedometer is acting like a throttle position indicator! No doubt the VSS system is still the culprit for that, and the shifting problem might come back if I don't fix it.
I managed to jumper pins A and B on the OBD connector and got two codes. The first was 12, telling me the PCM passed it's diagnostic check, and the second is code 27, which appears to have something to do with the transmission. I"m still researching what code 27 means, but am not getting anywhere as of yet.
Update: I removed the driveshaft and found the snap-ring intact and no slop in the output shaft. Next, I measured the VSS sensor output with the VSS Buffer connected. The signal was very erratic and dropped quickly when I let up on the gas, even though I didn't reduce my speed. Last, I disconnected the VSS Buffer and monitored the VSS Sensor. The AC voltage was lower, but it seemed to follow the truck speed quite well. It was getting late, so I need to test it again tomorrow to be sure. It was sort of difficult as with the VSS disconnected, the truck stayed in 1st gear and I couldn't get much speed without revving the engine too much.
I have a 1995 GMC Sierra 1500 4WD with an automatic transmission. Until a recent excursion where 4WD Hi was used, everything has worked fine. After the 4WDing, when switched back to 2WD, a problem has arisen. Everything seems normal until the transmission tries to shift. At that time, it seems to get hung up in neutral and the speedometer drops, sometimes to zero. Not sure which is the cause, and which is the result. Could be the speedometer drops to zero and the transmission goes to neutral, or other way around. I changed the VSS and checked wiring to the VSS buffer. Problem is the same. I think it's an electrical problem, but I'm not sure. Next step is to monitor the VSS Buffer 12VDC while test driving. However, as this all started after returning to 2WD after some 4WD action, could the transfer case have something to do with it? Can the speedometer tone ring in the transfer case slip, or does it stop rotating if the TC slips into neutral? I did try shifting the 4WD lever a few times in case a switch was sticking, but no change. The 1-2 shift sometimes is fine, sometimes does the "neutral and dropping speedometer thing". The 2-3 shift always has the problem. Or perhaps it's not the transmission shifting, but the TC dropping into neutral. I need to test drive some more, but the problem also seems to co-incide with letting off the gas a bit. During the 4WD excursion, water got a bit high, so I pulled all the connectors I could find to check for water. All dry. There's a bit of fluid on the front driveshaft yoke, apparently from the TC, but it's merely damp looking, not dripping, so that's likely normal for the age. But i did wonder if the TC is low on fluid. The description I read on it does mention an internal pump. So I'll check the fluiid level, although I'm not hopeful on that one. Maybe I'm not used to shifting a GM in and out of 4WD, but it doesn't seem like it's working right to me. First, I can't get it to shift into 4wd lo, unless I stop and put the Transmission in Neutral. In fact, I can't even get the TC to shift into Neutral unless the transmission is in Neutral. Once in 4WD LO, I'm not sure if problem is there or not as the engine winds up with the vehicle speed staying low (more or less expected), but it does seem to stay in first gear a long time, and then shift to 2nd with a significant lurch. Next, I can't shift the TC back, not even to Neutral unless I shut the engine off as well as leaving the transmission in Neutral. Any attempt to shift the TC case through neutral on the way back to the 2WD position while the engine is running creates a grinding of gears and the speedometer starts to rise even though the vehicle isn't moving. A highway test in 4WD Hi shows the same erratic shifting, loss of power to the rear wheels, and a dropping speedometer as experienced in 2WD. Tomorrow I'll check the TC oil level and look for linkage problems as well as more voltage checks. Anyone got any insight at all as to this problem, or even some info on how the TC works in a GM and wether mine is acting normal or not? Thank-you.
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