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Old 05-08-2018, 08:09 PM   #1
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Need simple answer ECM ground '94 Supreme

1994 Supreme ragtop, ECM in the engine bay behind right headlights area. Does someone know specifically where this one gets it's ground from?

ECM is shutting down randomly. I see evidence someone was probing for answers before in its power supply. I ran power to it direct and still shuts down, suspect ground now or just plain fried ECM. Need to verify ground path before condemning ECM though.
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Old 05-08-2018, 08:34 PM   #2
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Re: Need simple answer ECM ground '94 Supreme

Terminals B32 and C1 are grounds....all black with white tracers...

Terminals B16 and C17....tan with white tracer
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Old 05-08-2018, 08:43 PM   #3
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Re: Need simple answer ECM ground '94 Supreme

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Originally Posted by Tech II View Post
Terminals B32 and C1 are grounds....all black with white tracers...

Terminals B16 and C17....tan with white tracer
much appreciated...but what I'm shooting for is where, physically, do they pick up their ground?

Used to be easy...cluster of ground wires on the thermostat housing stud or whatever...easy to find. This one I have no idea where it actually picks up the ground from.
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Old 05-08-2018, 09:24 PM   #4
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Re: Need simple answer ECM ground '94 Supreme

If anyone likes a good mystery and wants to see if anything here screams relevant...this car (my son's) has 215,000 on it. It's the mighty 3.1, and it still runs great despite the miles.

When the weather first had a couple of fluke high 70's days back in Feb or so, it was running hot in traffic/idling, and he said it was at that point when it began this random shut-down BS. It never overheated, but it got up there pretty high.

I have limited time and physical ability to throw at car stuff anymore, but I went to look and saw the primary cooling fan was not working. I told him to use the A/C until we could get a better handle on it. Forcing the condenser fan to run. That solved the problem at the time.

I later found the fan was fine, relay fine, etc, just no signal to turn on. I told him to replace the fan's temp switch but might as well have been telling a fish to go ride a bicycle. It never happened.

What DID happen is that one of his friends who is good with electrics but not wrenching so much did a pretty slick little install of a relay near the passenger's side power distribution box, taking power from the same stud where the ECM gets its power with the key on, and just made the primary fan run whenever the key is on.

That solved the hot running, but obviously not an ideal solution. Anyway the car ran well enough until recently, then the stalling became really frequent and he was having trouble driving it.

THEN, it wouldn't start the other day. I went to look, and it acted like the starter was dead but he showed me that sometimes if he rolled the key back all the way to ACC and then forward to start it would start and run, but die immediately. When it wouldn't start all the dash and radio and such would come to life, but no starter. When it WOULD start, you'd hear relays closing and the fuel pump priming, etc. It was obvious one side of the power was not going where it needed to most times, without playing with the key positions.

So just for laughs we flushed out the cylinder with WD-40 and scuffed the VATS chip's contacts...I was half prepared to try a VATS bypass, but after cycling the key a few times it would start more and more often, until it went back to normal again, started every time.

AT THE SAME TIME, the engine was progressively running longer and longer before shutting down, but was idling way high, like 2000 rpm...and the volts looked a little low on the gauge. Under 14v anyway. But still enough...

Well...a little overwhelmed and short on tools, money, and patience, I just decided to dc the battery and let the ECM reset and do an idle relearn for starters. After hooking it back up, the idle was perfect...rock steady and we let it run 20 minutes and it never died.


He took it out on the road with me chasing in my car...and it performed perfectly. We went to eat, left, it did great going home. Not being one to accept that things fix themselves, I expected more trouble.

So yeah, next day he says he was driving and it is shutting down randomly again.

I went to look some more today and spent some time pulling connectors apart, spraying contacts, cleaning, pulling all the fuses and relays and making sure contacts were good...took that passenger's power box off and turned it over and saw some corrosion where wires came up into it, cleaned/sprayed them best I could...took a little time trying to play with those wires while it ran to see if I could aggravate it into stalling and narrow it down, but never developed any pattern.

Apparently this is not the first time....I see where some clod whittled notches of insulation off the wires in that area, so they could probe them...I taped those up to rule out any cross-ups and that made no difference.

To the best of my knowledge I've tried shaking/bending every stretch of harness that has anything to do with power to the ECM side and from there out, but never found anything that said "the problem is near here"...just random stall. Might run 10 seconds, might run 10 minutes. Regardless of what I do.

Before I forget...when I was behind him driving, the lights looked weak and the turn signals flashed slow. That was when it was running perfect. Today it won't stay running but it shows me 14.2v every time I checked while running. I tried an ECM reset again but didn't change anything.

One more thing that might be telling...I don't remember how good OBD1 was at recognizing a misfire, but a while back it had a bad plug and would misfire continuously for minutes at a time until we changed the plugs and wires...but the CEL never indicated a problem. That seemed weird to me. But like I said I don't remember how capable it was in that respect.
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