[GMCnet] Running a new ground to the ECM

[Richard]

I'm slaying the sitting-coach-lot-of-work-done-gremlins one at a time, and I'm trying to work my way through some electrical issues. I'm pretty sure I have a/some bad ground(s) causing 12 volts on a number of ECM sensor ground pins that should be reading 0 volts, most gauges not working, etc.

I read a post Ken Burton wrote some time ago about running a new ground to the ECM while trying to help a GMCer limp home from a rally. How exactly is that done? Do I just select the wires which are supposed to be ground just before the ECM connectors and splice on a wire to each one running to the engine block? Is there another way to do it, maybe a single wire? I don't want to start hacking into the harness if it's not really necessary.

thanks

Richard



--
'77 Birchaven TZE...777
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[Ken Burton]

Richard,

I was unfamiliar with the ECM we were working with. After talking to a GM mechanic friend on the phone he suggested that many of the intermittent problems he had with ECMs of that era were grounding issues. I had no wiring diagrams and no first hand knowledge of that ECM. I could not figure out where the wires went once they went though the hole in the floor under the driver's seat where the ECM was located. So my solution was to run a new 16 gauge ground wire from the outside case of the ECM through hole in the floor direct to an engine bolt directly below the hole. It probably was a bolt on the intake manifold. I do not know if that fixed the problem or not because we fixed several other things like wire crimps on the the distributor at the same time. We also replaced a bunch of tune up parts at the same time.

I do not know it this helped you or not, but the engine is the master ground for the 12 volt system. So however you decide to hook up to the ECM, run the other end of the ground wire direct to the engine somewhere.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Chris Geils]

If it is like most GM ECMs you should have a couple pins dedicated to grounds. The goal is to have the ECM, and power supply, and all sensors on the same ground plane.
If you have a diagram for the ECM you'll see which pins are grounds. If not then any black wire in the OE harness will be a ground. You need those pins grounded, not just the case.
You should also have a good heavy ground strap running from the block or head to frame (GM typically uses a bare braided flat copper strap for this) and from the engine battery to frame or engine (not sure where the OE GMC engine battery ground is located yet). You need the engine battery, the engine block, and ECM to be grounded together (same plane). Otherwise voltage differentials will seek ways of finding that plane by traveling through other ECM devices and circuits. You can imagine what happens when current goes through a circuit that was meant for a sensitive 5v signal like a sensor, or the o2 sensor which operates on 1 V.
--
Chris Geils - twin cities
1978 26' Kingsley w/ very few mods; Headers, Progressive Dynamics 9040 battery charger, aux trans cooler, one repaint in stock colors, 43k mi

[Ken Burton]

The engine battery ground cable goes to the engine, NOT the frame. The master ground on a GMC (engine side) is the engine, NOT the frame. Yes there is a strap between the engine and the frame for cross connection the the house electrical system. It is located on the back of the transmission and is connected around the rear transmission mount. None of this has anything to do with grounding the ECM. The ECM ground needs to go direct to the engine.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Chris Geils]

The engine is the "master" ground in any automotive ECM. I'll repeat myself because the discussion of frame grounds got us a little sidetracked;
"You need the engine battery, the engine block, and ECM to be grounded together" and for the ECM this means the ground pins in the connector not just the ECM case. The case is not a reliable ground for items on the board.

--
Chris Geils - twin cities
1978 26' Kingsley w/ very few mods; Headers, Progressive Dynamics 9040 battery charger, aux trans cooler, one repaint in stock colors, 43k mi

[Ken Burton]

mrgmc3 wrote on Wed, 25 December 2013 10:27

> The engine is the "master" ground in any automotive ECM. I'll repeat myself because the discussion of frame grounds got us a little sidetracked;
> "You need the engine battery, the engine block, and ECM to be grounded together" and for the ECM this means the ground pins in the connector not just the ECM case. The case is not a reliable ground for items on the board.

OK, I misread or misunderstood. We are in agreement.

Ken

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

[Mickey Space Ship Shuttle]

what is ECM? m. ca. 77 pb.

[Richard]

mickeysss wrote on Wed, 25 December 2013 19:20

> what is ECM? m. ca. 77 pb.

It's the engine computer that controls the aftermarket fuel injection I have, and is also referred to by different acronyms...ECU, PCM, etc.

Richard
--
'77 Birchaven TZE...777

[Richard]

Gentlemen,

Okay, I understand about running the ground to the engine, but I'm still a bit hazy on the connections at the ECM. I've read about repinning at the connectors using a little punch type tool to remove the pins, but I'm not sure if that's what I should be doing for what may be a temporary repair/test. I thought that stripping some insulation along the ground wire*s) some bit back from the ECM connectors and running ground(s) to the engine from there would work. Does that sound like a reasonable way to do it, or is there a better way?

Richard
--
'77 Birchaven TZE...777

[Todd Sullivan]

Engine control module

Todd Sullivan

Sully
77 royale
Seattle

[Chris Geils]

Ken,
No problem. I understand GM ECMs but am a little hazy on a lot of the mechanization of the GMC (such as ground routing) having only owned it for 4 months!

Richard V. wrote on Wed, 25 December 2013 20:35
> Gentlemen,
>
> Okay, I understand about running the ground to the engine, but I'm still a bit hazy on the connections at the ECM. I've read about repinning at the connectors using a little punch type tool to remove the pins, but I'm not sure if that's what I should be doing for what may be a temporary repair/test. I thought that stripping some insulation along the ground wire(s) some bit back from the ECM connectors and running ground(s) to the engine from there would work. Does that sound like a reasonable way to do it, or is there a better way?
>
> Richard


Richard,
I'll assume that you are using a 90s vintage GM v8 ECM and that you have some portion of the wiring harness and ECM connectors (and that you don't have a wiring diagram for the donor). If this is the case any wire in the ECM harness that is all black (no stripe) will be a ground. These should go to a clean ring connector on a cyl head, block or Trans case. You don't need to repin them at the ECM, just extend the wires to reach your new ground location. I would not recommend pulling one of the ECM connectors apart unless you have the correct terminal tools and extra terminals. They are available from commercial sources like Power and Signal or Mouser.

--
Chris Geils - twin cities
1978 26' Kingsley w/ very few mods; Headers, Progressive Dynamics 9040 battery charger, aux trans cooler, one repaint in stock colors, 43k mi