How do I check the grounds on the TBI harness?

[dpilot83]

I have a 93 K3500 w/ a 7.4 and an NV4500 with the 8747 PCM. I've had a lot of strange problems with it and I was hoping to check some wiring things, starting with the grounds. Where all are they located? I'm only familiar with the ones that are on the thermostat housing.

How do I check them? I took one of the ones off of the thermostat housing even though they both looked good. I accidentally broke the ring terminal so I cut it off, stripped it and installed a new ring terminal. As I was doing that i discovered that the last 1.5 inches of the wire (or so) had melted insulation which made it difficult to strip the wires. It also appeared that it had very slight amount of corrosion in that area. I never would have seen it unless I had accidentally destroyed the ring terminal. I'm convinced even a close inspection would have yielded no indication that there was anything wrong with that wire. Ever since I fixed that, the brake light quit flashing a RWAL brake code like it has been consistently doing for months.

So how do I check other wires in the system (especially grounds) without cutting every wire open? And like I mentioned above, where are all of the grounds supposed to be? Thanks.

[dpilot83]

I can do a continuity check to measure the resistance with a voltmeter easily enough, especially if I knew where the other end of the ground wire was. I was hoping to not have to open up the entire engine harness to trace the wire to the other end. The other problem with that is that it may not show any corrosion problems. One thing I'm interested in trying is a voltage drop test while it's running. I would assume this would involve removing the ECM and seeing if I can get a probe into the ground pins on the ECM and then have the other lead of the multi-meter on the grounds on the thermostat. I don't know if that is hard on the ECM to run the pickup while the ECM is pulled out enough to get a voltmeter lead in there? Perhaps it needs to be mounted so that it can be grounded through the mounting bracket?

I have never had an ECM out so I don't know what is acceptable and I also don't know if it will be possible to probe into the pins on the harness without causing more damage than the solution to the problem i worth. Any thoughts? Thanks.

[buddrow]

The ecm case itself is not grounded but through the ground wires for the sensors and solenoids, injectors, etc. One would assume that since you have corrected the flashing RWAL/Brake lights then you should be good to go unless you have other issues you are trying to resolve. If you just want to chase wires and do voltage drop tests have fun with that but its not necessary. The corrosion on your t-stat housing grounds is likely from age, weather, maybe a poor ground at some point causing the wires to overheat enough to crystalize the copper wire and deform the insulation. Not much telling really and its moot at this point. Have a nice day knowing you fixed two issues at once and carry on.

Buddrow

[lionelhutz]

Don't ground the PCM case.

The connectors can be difficult to back probe but it could be possible if you had a fine probe.

There shouldn't be many grounds. Maybe 2 or 3 at the thermostat and another 2 or 3 on a back intake bolt.

It may be easier to just give them each a pull/bend test. If they pull off or feel crunchy when you bend them then cut them back and re-terminate.

[dpilot83]

The ecm case itself is not grounded but through the ground wires for the sensors and solenoids, injectors, etc. One would assume that since you have corrected the flashing RWAL/Brake lights then you should be good to go unless you have other issues you are trying to resolve. If you just want to chase wires and do voltage drop tests have fun with that but its not necessary. The corrosion on your t-stat housing grounds is likely from age, weather, maybe a poor ground at some point causing the wires to overheat enough to crystalize the copper wire and deform the insulation. Not much telling really and its moot at this point. Have a nice day knowing you fixed two issues at once and carry on.

Buddrow

Thanks. The other issue I'm working on is the hard start while the engine is hot. I was under the impression that bad grounds can cause all kinds of weird problems and I wanted to get the cheap basics out of the way before proclaiming an improper diagnosis on some other part. I've heard bad grounds can cause the CTS to be way off which can tell the engine to provide too much or not enough fuel while starting. Anyway, I'll keep plugging away at it. Thanks again.

[EagleMark]

I was under the impression that bad grounds can cause all kinds of weird problems....

Your under the right impression!

[dpilot83]

Don't ground the PCM case.

The connectors can be difficult to back probe but it could be possible if you had a fine probe.

There shouldn't be many grounds. Maybe 2 or 3 at the thermostat and another 2 or 3 on a back intake bolt.

It may be easier to just give them each a pull/bend test. If they pull off or feel crunchy when you bend them then cut them back and re-terminate.

Thanks. I've cleaned the two on the thermostat housing and put new terminals on them. I'll look for the ones on the back of the intake as well. I noticed I'm going to have to replace the fitting for the heater core on the back of the intake manifold as well (again) so I'll be digging around back there anyway. Thanks again.