I've got an 86 SE 2.8 auto that is missing two ground connections. I had the engine out about a year ago and I am just getting back to working on it. Manufacture date is 11/85 and acording to my Motors manual it has the 85 wiring harness. I don't have a ground on D12 and D13 (black w/red tracer and black respectively) of the ecm. I checked an 85 Fiero that I have and there is a 12 ga. wire at the firewall side trans mounting bolt along with the black wire and tan wire and black wire and lt. green wire. It seems I am missing the 12 ga. wire on the 86. Could that wire be run to a different location. I've looked everywhere that is visible and don't see it.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
The problem is that I can't find where the grounds come out of the loom in the engine bay. Thought maybe if some one knew where the factory may have connected them, that may give me a better idea of where to look. ?????
Just make a wire or find/buy heavy ground.... Car doesn't care much, just ground(s) carry the load between engine and frame and have a good connection.
Ideally, You only need 1-2 engine and frame grounds than takes Alt Peak Amps + ~25%. Alt is rated 100 peak amps + 25 = grounds should handle 125 amps. Big audio etc can eat power... Many will use big + wire and ignore the ground...
Most cars never need alt wires and engine grounds to handle peak alt power but if you make/buy a new one is easy to get ones that does.
Many GM cars have "underrated" grounds in that area because of reason above but ground everything is sounds good at first but really bad in the long run. Many people use too many grounds and these grounds can cause "Ground Loop" problems. (Google that...) One loose ground can cause noise etc.
Perhaps I'm misunderstanding your question, however here's how I understand it:
The wires running to ECM D12 and D13 go to the MAP, MAT, TPS, and CTS sensors... so if you're measuring the continuity of the connector pins D12 and D13, you're going to get the resistance of the sensors, not ground. You would have to measure the actual ECM pins D12 and D13 to see if they were grounded, not the connector pins on the harness. But that isn't advisable either because the ECM is plastered with warnings not to measure the resistance. Granted, these pins should be simple grounds but who knows what they go through inside the ECM before they are grounded.
So what are you supposed to do? Well, the only places where the ECM gets its ground, is through ECM pins A12, D6, and D1. If you check whether those pins on the harness are continuous to ground (again, do not test the ECM pins), then you have to have faith that they will ground the ECM properly when plugged in. If you don't get a direct path to ground on those three harness pins, then check to be sure the black and tan pair (G503) and the black and green pair (G504) of wires attached to the trans mounting bolt are free of corrosion and are tight. That is where ECM pins A12, D6, and D1 are connected to. Often though, they will break somewhere under the exhaust crossover pipe and only "appear" to be still connected.
If that's not your problem, and instead you are trying to find the location for the larger 12ga wire, then it's probably ground G501 that you're looking for, which is supposed to be connected to one of the studs near the oil dip stick. There should be a copper braid and a large diameter black wire attached to the stud.
The wires running to ECM D12 and D13 go to the MAP, MAT, TPS, and CTS sensors... so if you're measuring the continuity of the connector pins D12 and D13, you're going to get the resistance of the sensors, not ground. You would have to measure the actual ECM pins D12 and D13 to see if they were grounded, not the connector pins on the harness. But that isn't advisable either because the ECM is plastered with warnings not to measure the resistance. Granted, these pins should be simple grounds but who knows what they go through inside the ECM before they are grounded.
Ok. I should have known that. Here's the symptom. Car seems to be running rich (exhaust has rich fuel smell). However, O2 toggles ok and is not indicating constant rich condition. This could be because I'm just getting this thing put back together and haven't had a chance to drive it. Symptom #2, when I connect my Snap-on Brick to it, it goes directly to field service mode. I believe this is because the Brick is providing a missing grnd. ???? again.
