1995 olds 88, tracking down electrical problem

[haggardrichard]

Hello, new member here.

I have a 1995 Oldsmobile 88. This all started about a year ago.
Twice during highway driving, it felt like a very harsh misfire under a slight load. Went away after a few minutes, no CEL. A couple other times the vehicle died when coming to a stop, and upon startup the CEL would be illuminated, but would be off after another key cycle.

Soon afterward my AC compressor was grinding badly, found the pulley separated. After replacing the compressor, adding oil, and recharging, I found that clutch wouldn't engage. The CEL was constantly on at this time. This vehicle is a pain to pull codes with this OBD1.5 BS, I had to use a Tech 1 with a 16-pin adapter. I used this equipment at school, and do not currently have access to it.

Codes were p1640 and p1670. Only 1 code was current, I believe it was 1640. It involves the driver module that controls the TCC solenoid, cruise control, and evap solenoids if I remember correctly. I noticed that cruise control was inop, I could not fill the gas tank without the pump stopping, and I could not get the TCC to engage while driving. So this all made sense considering the DTC. It made me wonder if the harsh misfire I felt before was the TCC engaging repeatedly, and stayed engaged those times that it died when coming to a stop.

At the time I was more worried about the AC working properly, and I found that the two pink wires that give power to the AC relay were shorted to ground. they had continuity to ground and I did not have voltage there. To make the AC work and verify the rest of that system was ok, i rewired it from the relay center underhood, so it had power directly from there. I fused it with a 10A fuse. AC works perfectly, cycles on and off, etc.

Now I'm tracking down the short to ground, I figure that is what is setting the DTC and shutting down the quad driver module. Hopefully I'm not wrong there. Unfortunately the schematic im working off of doesn't seem to be completely accurate. It's reference a fuse that doesn't exist, but the rest seems correct. Well, I now have the dash completely out tracking down the wires, found the splice referenced in the schematic, and find that another wire in this splice leads to a blown cruise control fuse....

So am I correct in thinking that there is a short to ground in the cruise control circuit, or at least any circuit related to this splice? Would a blown cruise control fuse set p1640? I would hope that I haven't done all this just for a blown fuse. Either way I guess I need to find what it is that blew the fuse.
I'm not too familiar with older cars and the lack of electrical information compared to today's cars is not helping.

Just found that with the PCM plugged in, there is continuity from that splice to ground, very high resistance around 400kohms. Again with PCM plugged in, there's is about 500ohms from the ac relay power wires to ground. With PCM unplugged there is no continuity for either. Is my PCM internally shorted?

[maxwedge]

First of all is the cruise and it's components powered by that quad driver? Disconnect all the included cruise components and see if the code returns, if so continue on check the remaining high load solenoids, tcc solenoids were prone to failure.

[haggardrichard]

I could really kick myself for never checking that cruise control fuse. Though the schematics dont reference this whatsoever, that splice i mentioned is powered through that fuse. One of the wires in that splice is one of the 12v wires to the PCM. That quad driver module doesnt power cruise control, in fact it's basically the opposite. Im pretty irritated that that fuse powers so much more than is referenced.

The wires in that splice go to the AC relay, PCM, EGR, 02 sensor 1, and i believe the MAF if i remember correctly. So all of this was caused by a blown fuse. I was unable to find anything obvious that wouldve blown that fuse. After replacing fuse, everything works correctly and no check engine light.