M42 Electronics Gurus....HELP!

[ClayW]

M42 in my 1600 stopped working. I didn't do anything different or do any maintenance; it just stopped.
It's in my 1967 1600, but I wired everything as if it was in the original 91 318is.

Symptoms are as follows:
CEL comes on with key and stays on.
Stomp test doesn't work.
Fuel pump doesn't prime.
Engine cranks, but doesn't fire.

Any guesses?

[wazzu70]

Check your fuel relay and master relay first.

[ClayW]

Check your fuel relay and master relay first.

Checked them both. They seem to be working.

[thebrelon]

crank sensor

[DesktopDave]

I'm worried about the continuous CEL. I'm wondering if you have a damaged O2 or huge vac leak? I'd figure the solid light is an emissions flag? As for the car not starting, I'd chase down the usual culprits. It could be two separate problems, like a dead O2 heater and a flaky crank sensor.

Can you get a stomp test completed? That should let you know what the DME is worried about.

I'll get a solid CEL when the car is running down hills, but never any other time.

[wazzu70]

Fuel pump should prime in the key on position, the ECU does not need any other input to make that happen.

Get this resolved first as the car cannot run without fuel Measure voltage at the pump itself and make sure you are getting voltage.

[ClayW]

Fuel pump should prime in the key on position, the ECU does not need any other input to make that happen.

Get this resolved first as the car cannot run without fuel Measure voltage at the pump itself and make sure you are getting voltage.

The fuel pump is not priming (no voltage). I checked continuity all the way to the relay and tested the fuel pump relay; it's just not getting a signal to switch on. I tried manually powering the fuel pump. The car still cranks, but doesn't fire.

I swapped out a crank sensor. No change.

I swapped DMEs with my spare (175), but to no avail. The stomp test doesn't work.

The only thing I can think of is that the harness might be compromised in some way......argh.

[DesktopDave]

Short the pump relay - does that make the pump run?

If the jumper works, I'd test both the crank sensors. Pairs 1&2 should be 680 ohms, 1&3 and 2&3 should be over 100Kohms.

If that works, test the engine harness from the crank senders. Could it be some water in a connector or something like that?

We don't have a complete troubleshooting guide, I guess that's something I should be working on, LOL.

[thebrelon]

how old is your fuel filter?

[wazzu70]

Make sure there is power to the fuel relay at all the proper terminals. Check the red wire with white stripe, that comes from the main relay.

If you get power at all the terminals, check the ground wires from relay to ECU.

This problem has nothing to do with the engine as the fuel pump works with or without an engine in the chassis!

[ClayW]

how old is your fuel filter?

Fuel filter has less than 2500 miles on it.

[thebrelon]

Make sure there is power to the fuel relay at all the proper terminals. Check the red wire with white stripe, that comes from the main relay.

If you get power at all the terminals, check the ground wires from relay to ECU.

This problem has nothing to do with the engine as the fuel pump works with or without an engine in the chassis!

is it? I thought that the ECu needs to "see" the engine revving through the crank sensor to send the fuel pump power command?

[wazzu70]

When you turn the key on the pump starts priming before the engine even cranks right?

Another thing you can check is the power supplys to the ECU. Bith constant voltage and key on voltage. If the ECU does not get power the relays will not ground/switch on.

B+ battery voltage is pin 26 on the ECU connector.
Ignition on voltage is pin 56 on the ECU connector.
Power from main relay is pin 54 on the ECU connector.

You may also want to verify the ECU grounds have good connection and the fuel pump ground has a good connection.

[thebrelon]

yes right, replied too fast... weird failure...

I would also check the ECU power supplies (permanent, switched on, grounds) before looking into the loom itself.

could you clarify/confirm that when you shunt the relay the pump works but the engine doesn't start?

[ClayW]

Palm in face.

When the car died, one of the first things I checked was the relays. I tested the fuel pump relay electronically with a positive result. I felt the main relay clicking ON in place when I turned the key on, so I took that to mean that it was good.

So, today, I removed the main relay, which is a 5-prong, and tested it electronically. Again, the 87 terminal clicked ON just fine, but I got a flickering result on the 87a terminal. I bought a new main relay from the dealership, threw it in, and the car fired right up. I still can't believe it was that simple.