1988 GMC C1500 Electrical

CONGANAS

MEMBER
1988 GMC C1500
5.0L
V8
4WD
AUTOMATIC

Prior to my work, my truck was running fine, no issues besides a miss. So I performed what was supposed to be a simple tune up. Swap out the spark plugs, changed or the cap and rotor and was about to change the wires, but I decided to start up the truck. The wires had felt pretty good, not dry. I was going to replace they anyway.

The truck didn't sound like it wanted too crank over. It would make a couple of turns then crank as if the battery was dying. I found the ground terminal was loose and there was a bit of corrosion on the pos terminal so I got some baking soda and water and cleaned everything. Cranked the engine same result but I noticed the pos side cable was slightly smoking while I turned the engine over.

So I dropped the starter thinking I may have loosened a connection and find there is a sensor behind the starter and behind the dip stick tube with a wire hanging by its last stand of wire. I have no idea what this is for and if it would cause the problems.

Images (Click to make bigger)

1 Reply

CARADIODOC

MECHANIC
33,873 POSTS

That looks like a knock sensor. Besides the fact someone used a butt connector that doesn't seal out moisture and is going to cause a problem, it has nothing to do with the starter circuit.

You already found the major clue screaming at you, but you aren't aware of it. Slow cranking is caused by a defective or run-down battery, or a high-resistance connection in one of the battery cables. That resistance is WAY too small to measure with an ohm meter, but we can see the RESULTS of it by measuring the voltage dropped across it during cranking. High resistance almost always occurs in a mechanical connection meaning where a cable terminal bolts to something. If you have a helper to crank the engine for you, you take voltage readings at various points in the circuit and look for one that's too high. In starter circuits you're allowed no more than 0.2 volts dropped across any one mechanical connection, AND no more than 0.4 volts throughout the entire positive or negative half of the circuit. I know that's confusing, and I can explain it better, however, you have a connection that has WAY more than 0.2 volts being dropped across it. When that happens, smoke is a common sight. Take that connection apart and shine it up, then make sure it's tight.