This car... What a piece of work. It’s been offline since Friday, 10/11. Two days prior, I started to have issues with a runaway idle one minute, and the next it would idle normally. That Friday I tried to get in and go, and it would only idle real fast or not fire up at all. Give it a little throttle when it actually did start, and it’d die. I also had a defective battery for the second time in a year, which threw me for a loop when not only would it not fire up, it wouldn’t turn over either.
I’d actually put around 800-1000 miles on it the previous week. I got caught in a flood outside of New Orleans that started to sweep the car away, but it somehow caught traction and lived to see another day. I think the week before that I saw a mouse in the engine bay so I suspected one or the other could have caused my issue.
I had a Code 22, low TPS voltage and a low coolant light. Strangely, these two share a ground along with a few other management sensors like MAP and IAT. 1V reference was getting to the TPS instead of the nominal 5. Therefore, it was sending negligible voltage back to the ECM when you advanced the throttle. Good ground all around. After testing voltage at the ECM, which was the same as at the sensor, continuity across every remotely relevant wire, and fixing every potential short I could find just to be thorough, I noticed that the coolant level sensor was also getting 1V of reference where it should be getting 5. They get their 5V from different pins on the ECM, which I thought was strange.
I unplugged the CLS, and my TPS is all of a sudden getting 5V and the car is running okay. Also, the pigtail for the CLS is getting 5V reference unplugged. Plug it back in, it all goes to hell. At this point, I’m convinced the sensor had to somehow short the reference leg to the ground leg internally from sitting in that rusty radiator goo for 15 years, but I’ve never heard of such a thing. I need to test continuity across the CLS reference wire if I really want to rule everything out, but I’m 99% sure the sensor is doing something weird where it’s shorting out the reference voltage, and even though the TPS and CLS have different pins for their respective 5V to exit, maybe they derive that 5V from the same place internally. I think one is C7 and the other is C12 on the ECM pinout. Strangely enough, I know the MAP and the TPS share their reference voltage source, but the MAP sensor has been working fine through this whole ordeal.
This is all after having very little time, about less than a day per week on average, to fool with it, replacing corroded fusible links, getting a junkyard ECM when I saw my voltages were wrong straight out of the computer, ordering a hard copy GM service manual from eBay, and having no access to serial data. I also did about thirty or so different voltage, voltage drop, ground, and continuity tests and repeated them about five times each just to be certain. What a joke, but I’m getting there.
If anyone’s had a sensor act like a burnt pilot on a string of Christmas lights, please share.
