No pulse from ECU on #3 coil
 

Since being SORNed and sitting on the driveway through all of the covid lockdowns, my UK-spec GC8 classic is now refusing to start. I've traced this back to a lack of spark on #3 cylinder, which I've traced back further to a complete lack of pulse from the relevant pin at the ECU. Loom continuity, individual plugs and coils all checked and double-checked, and a spare known-good ECU now also tried. I'm seeing a pulse on the relevant pins for all 3 other coils.

Any thoughts on what could be causing the ECUs to do this?

What year, coil on plug or coil on plug conversion or coilpack with leads.

Sorry for the lack of detail. It's a '96, with original coil on plug.

Are you saying that there is no pulse from the "known-good ECU" as well as the original??
Have you run through the relevant diagnostics??
Have you swapped out the igniter for a known good one?? since your last thread
https://www.scoobynet.com/general-te...atibility.html

How did you trace it it back to the ecu when you've an ignitor in the circuit. Have you tried a different ignitor. Check your earth at the coil on number three also check the earths at the ecu. White block connector next to ecu.

:mad::lol1: You done it again Don.

I wrote my reply earlier today but somehow didn't post it :facepalm:

Ok, to try and answer as many of the questions as I can, here's a breakdown of what and how I've been testing.

Step 1. Attach flashing-light style coil tester to one coil at a time, with a known-working spark-plug, and turn over engine.
Result: Spark and flashing light from all coils except #3.

Step 1a. Repeat on coil #3, with one of the coils from the other cylinders.
Result: Still no spark or flashing light from coil on the harness for #3.

Step 2. Test continuity between 4-pin igniter harness to the big engine bay multi-plug (using resistance on multi-meter), and then from the multi-plug back to each coil connector.
Result: Good readings all round.

Step 3. Test +ve voltage to the relevant wire on each of the individual 2-pin coil connectors.
Result: Good readings all round.

Step 4. Test voltage readings on the 5-pin (ECU-side) igniter harness connector, one pin at a time (+ve multimeter lead to connector signal pin, -ve multimeter lead to connector earth pin, using probe inserted into the back of the connector), while cranking over the engine.
Result: Good readings (pulsing in the 0.1v to 5v range) on pins for 1, 2 and 4, constant reading of 0.006v on pin for 3.

Step 4a. Repeat step 4 with igniter completely disconnected, using probe connected straight into the open side of the harness connector.
Result: Exactly the same as step 4.

Step 5. Test continuity between 5-pin igniter harness connector and ECU harness connector for all igniter pins.
Result: Good readings all round (resistance identical for all 4 coil positions).

Step 6. Test voltage readings for the relevant signal pins directly on the back of the ECU harness one pin at a time ( using probe inserted into the back of the harness), while cranking over the engine.
Result: Good readings (pulsing in the 0.1v to 5v range) on pins for 1, 2 and 4, constant reading of 0.006v on pin for 3.


So if I now cross-check what I've tested, against the flow-chart on page 2-7b T8C0 in the diagnostic guide (big thanks again Don for the link), in theory the result I've had should mean a faulty ECM, except that I've already tried a second known-working one, and still get the same result.

I'll read through the diagnostic guide some more tonight, but right now this has me pretty stumped.

No you saved it waiting on me posting :cry::D

You really need a scope for that carry on. Check continuity of everything with ohms on the meter never mind turning the engine over and report back :thumb:

Well, I've checked and double-checked resistance/continuity everywhere, and all the readings are good. Still keep coming back to the same thing - from page T8C4 of the diag guide, "Check input signal for ignitor", I'm getting positive voltages for 1, 2 and 4, but never for 3.

Is there something that could stop the ECM from sending this? Bad crank sensor, cam sensor, other? Whatever it might be, why would it only affect #3?

Have you opened your ECU at all? Check for any signs of damage/corrosion,

I've checked both of them, no sign of anything untowards on either.

next step is throw a spare wire from the ecu pin to the igniter.

Right, so test steps and results from today.

Step 1. Hook up the spare (metal-case) igniter with a temporary harness, using wiring guide from Don's link in my other thread.
Result: Spark on all coils except #3.

Step 2. Run a bypass wire directly from the back of the harness connector on the ECU (using a pin-probe pushed through the back of the plug), straight to relevant pin on igniter.
Result: No spark on coil.

Step 2a. Run the same bypass wire for one of the other igniter pins on the back of the ECU.
Result: Coil sparks perfectly.



In theory then, everything is still pointing to the ECU not sending a signal for the igniter pin for coil #3. What I'm struggling to get my head around is how/why two separate ECUs would fail in exactly the same way, and not show any other signs at all of an issue.

The weird thing is the car should still start anyway,it would sound rough due to.pot 3 not firing but should still.start. .

Possibly your ignitor is knackered and its blew the drivers in both ecu's. As said it should still run on 3 can you hear fuel pump has it an immobiliser being a UK car.

Still no joy, but to answer latest questions:
Ignitor module is definitely not faulty - tested by using bypass wire from one of the signal points on the back of the ECU, direct to the pin for coil #3 on the igniter (coil #3 sparks just fine doing this).

The fact it won't even start and then just run rough also has me wondering if there isn't some other issue lurking, as well as the bad coil signal. I just don't know what that issue might be.

One of the other signal pins or the pin for No3 cause if not you've answered your own questions. Do not go directly to the coilpack otherwise you will have problems.

OK, so just to put this one to rest. Car is now running nicely, the problem was timing after all. More specifically, when changing the cambelt while the car was laid up, I'd used the triangle-shaped mark on the crank sprocket, instead of the straight mark (:brickwall:facepalm:). Sorted that out over the weekend, and car started up on first turn of the key this morning.

Thanks all (especially Don and Joe) for the awesome help. It's been a very educational, if slightly frustrating experience!

You need a kick up the arse :lol1: don't believe you ever once mentioned you'd been meddling with belts :facepalm:

basically 90deg out then..........