Wiring Disaster and bad pin 31 ECU ground

Hello DeLorean fans,

I recently inherited a DeLorean and I'm trying to get it running but there appears to be an electrical problem with it. Unfortunately, it looks like there has been quite a few electrical modifications behind both seats near the ECU and by the fuses. I'll probably be asking some other questions about the electrical system once I get the car into a running state.

The car turns over but it sounds like it's having a hard time getting enough juice to turn over. So I looked in the Technical Information Manual and saw that it has a troubleshooting section, I found C1 - IGNITION SYSTEM DIAGNOSTIC CHART (pg 122/137). On #2, it had me probe pin #15 at the coil against ground and said it should be 6-8 with the key on. However, I see more like 10.5V and the troubleshooter suggested that the system has a bad ground. To dig in, it says:

Quote:

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Check resistance of primary coil winding (0.95 - 1.4 Ohms)
Check for battery voltage at contact pin #15 of ignition ECU module, using voltmeter with ignition in Run' position" I started to dig out the ECU and I am left with lots of questions.

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I got into the compartment behind the driver's seat and found the three boxes that are all apparently called the ECU :hmm:. Anyway, I see that they are all attached to a metal plate, I pulled the bolts from the metal plate so I could see how everything was mounted and probe the correct pin. When I probe it, pin 31 appears to be just grounded to this metal plate. What worries me is that the place where this plate mounts is wood/fiberglass/rubber (insulated) so that plate grounding wire is probably not the same "ground" as the engine will see.

Attachment 44051

How is this supposed to work? Seems to me like there should be a fat ground wire running from the engine back to this plate so they have the same 0V reference voltage. What is this ground system supposed to look like when stock?

Thanks,
-Greg

Welcome,

The thing about manuals is they don't account for previous owner mods and hack jobs. Before diving into the repair, you need to figure out what all those extra wires are for, if they are even hooked up, and where they go. Chances are it's some kind of stereo or alarm but you need to know for sure because those mods could be part of the problem. Once all that is sorted out (removed) then start diagnosing.

From the factory: ECU's are grounded to a junction under the console, thence to a junction in the relay compartment, thence the rear bulkhead bolt (main grounding point for the entire car body):

Attachment 44054
Attachment 44055

PO mods can of course change factory configuration.

Bill Robertson
#5939

Quote:

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Originally Posted by content22207_2

From the factory: ECU's are grounded to a junction under the console, thence to a junction in the relay compartment, thence the rear bulkhead bolt (main grounding point for the entire car body):

Attachment 44054
Attachment 44055

PO mods can of course change factory configuration.

Bill Robertson
#5939

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AWESOME, Bill! I just found a long thread of yours called Ground-Location-List and it has a lot of really good detail. I printed out a few pages already. If you have any photos of that rear bulkhead bolt so I know exactly where to find it? I'll likely do the 4ga grounding reinforcement you posted found in this thread: voltage-gauge-quot-health-quot-assessment.

If you pull the carpeted bulkhead off you can't miss the bolt. It's right next to the ignition coil (that's the other side of the bolt).

Speaking of which: ignition circuit is totally unprotected (no fuse, fuse link, or circuit breaker), despite the coil being mounted to a metal plate that grounds pretty much the entire car. I added a fuse link into the resistor grid.

Bill Robertson
#5939

Please note: my ground bus does not depend upon the original bulkhead bolt to frame wire (attached under the coolant expansion tank), nor the frame to battery wire (community wisdom is that terminal can come loose). Bulkhead bolt is tied into the bus, but the bus has its own wire directly to the battery. Being located inside the car, this direct connection is weather protected.

Bill Robertson
#5939

ECU is short for "electronic control unit".

The metal plate behind the drivers seat is normally grounded with one or two black wires with ring terminals (can't remember how many is stock). The lambda ECU (metal case) has electrical ground connection inside the ECU to it's case. So normally there is no current flowing with the metal case ground circuit. It's more of a electrical noise shield.

The meal plate behind the passenger seat is not grounded with stock wiring.

That ring terminal is ganged into the ignition ECU ground (pull back the rubber boot and Pin 1 has two ground wires -- one to the tray and one to the junction under the console).

Bill Robertson
#5939

The wiring jumble and that little clear topped box on the right side appear to be related to the power antenna located in the rear, driver's side quarter vent. And generally they would be for the power up and down feature of the antenna. One of those thicker black wires is also the radio antenna wire. The little clear topped box is not factory, and as Michael mentioned, it could be tied into some sort of alarm or ignition shut-off switch. The speaker and antenna wiring from the factory was not normally the prettiest thing you've ever seen, but it usually wasn't quite that messy. Those wiring colours also aren't factory and indicate some PO hack-job.

Here's a schematic of just that Lambda ECU (the black one lying flat on top of the metal bracket there) and the two connectors on the back of it. All of these schematics are included in the forum resources section along with the master wiring diagram. I would also recommend you taking some time to sort out wiring that isn't right. I know you would like to get the car running and then sort it out, but you may never get the car running depending on how messed up the wiring is. And it could be as simple as an extra fuse that got unnecessarily added but is now blown that stops you.

Attachment 44056

Clear case relay is factory OEM for the power antenna (factory antenna didn't have an integrated relay as aftermarket units do).

To be brutally honest OP's ECU compartment looks stock.

Bill Robertson
#5939