M42 ignition coil wiring questions

[ofak]

I got a shell that I'm putting together but I can't figure out how to match the wiring harness from the ECU to the coil, I don't see any marks to identify the harness order, does anybody know how to match them up?

[iansane]

Looks like browns are ground, green is power and the blacks are signal from the DME. If I'm reading it right; Pin25 goes to cyl#1, pin52 goes to cyl#2, pin24 goes to cyl#3 and pin 51 goes to cyl#4.

[ofak]

Awesome, thank you! I had one plugged in the wrong coil. I also replaced the crank sensor today but still no start, I'll give it another go tomorrow. Thank you again.

[iansane]

Make sure you have all your grounds. I know it's reiterated a lot but I had one hooked to the battery hold down bolt that didn't get put back after replacing the battery one time and it caused significant headaches.

[ofak]

absolutely, Ive had several 80s cars and am aware of the german electrical gremlins that come with them, this is just my first M42. Same with the vacuum leaks, I just got the correct sized vacuum hose last night.

As far as grounds, I've only seen 3, the 4 bolts that hold the DME, the one next to the RH motor mount and the one for the battery, am I missing another?

[iansane]

Is your battery trunk mounted? Like I said, I had a ground strap that bolted to the forward battery tray/strut tower area coming out of the main harness. Small 6ga-ish wire that was a real faded brown (looked like it could have been red but wasn't) with the black sheathing over it. Easy to miss, and easy to hook to power instead of ground. I think it was the ground for the dme and/or main relay.

[ofak]

Battery is in the engine bay.

I did find 2 small wires, based on length I figured which was positive and which was ground. Here are some pics of when I got it 2 weeks ago.

[bmwman91]

Did you use a Genuine BMW crank sensor, or the less expensive Bosch branded one? Both are made by Bosch, but the less expensive one has given a lot of people trouble for some reason.

[iansane]

I assume you're still getting a nostart? It does crank though, right? You've verified that the main relay is clicking as well as the fuel pump relay? The fuel lines aren't swapped feed/return are they?

Good luck.

[ofak]

Did you use a Genuine BMW crank sensor, or the less expensive Bosch branded one? Both are made by Bosch, but the less expensive one has given a lot of people trouble for some reason.

whoops, looks like I got the problematic one. The old one was so old that the plastic cracked off when I removed it.

I assume you're still getting a nostart? It does crank though, right? You've verified that the main relay is clicking as well as the fuel pump relay? The fuel lines aren't swapped feed/return are they?

Good luck.

Correct, crank, no start. I only checked the fuel pump relay, I'll check the other one today. It's the white one next to the fuel pump relay, correct?

Today I also plan on checking the fuel pump pressure, how many PSI/Bar should be coming from the pump to the fuel rail?

I know the fuel line from the pump is in the correct port in the fuel rail (the one closest to the firewall) but now i'm doubting if i have the return line plugged into the canister. No fuel is coming out of the canister though.


Thank you for your help.

[jsnppp]

FYI, fuel pressure issues can be very real. On my 318is I was loosing pressure in the pump in the tank (the only one on those cars) due to a faulty junction hose. It would start and run but would take 10 cranks.

So, what I'm saying is check the fuel pressure (I did it between the tank and fuel filter) and then pull the pump to make sure it looks ok.

I don't remember what pressure is needed...

Jason

[iansane]

I know the fuel line from the pump is in the correct port in the fuel rail (the one closest to the firewall) but now i'm doubting if i have the return line plugged into the canister. No fuel is coming out of the canister though.

I couldn't speak to which one is which, I always forget.

The charcoal canister? The return line doesn't go to the charcoal can...

[ofak]

I couldn't speak to which one is which, I always forget.

The charcoal canister? The return line doesn't go to the charcoal can...

Yeah, I mean that when I hooked them up, I was certain I had the return line and the charcoal can on the correct places but now I don't remember how I got to that conclusion. Sorry, me no English good sometimes.







Fuel pressure gauge is plugged in between the hard line on the firewall and the fuel rail with the fuel pump relay jumped, not sure if that's the correct way to do it or if that presure is good for the pump.

The main relay does click, tested with the relay from the other harness, same result.




I was kind of in a rush to get this running because I wanted to drive it to an event that happened yesterday but now I'm going to take more stuff apart, sarting with the intake manifold, I'll have a look at the injectors, injector wiring and all the hoses that are under the manifold.

[bmwman91]

Fuel pressure kooks good. You should see 45 psi with the engine off.

For how long did the car/engine sit for, and was the fuel rail empty? Letting the injectors sit dry for a long time can cause them to get stuck. If you crank the engine and then pull the intake boot from the TB and open the TB by hand, do you smell fuel?

[ofak]

That's the donor, we got it in April but it's unknown how long it sat for before it got here, I know the previous owner passed away in 2021.


The 2 door overheated in 2016, previous owner pulled out the engine and transmission to do a swap but life happened.


I emptied out all the fuel when I got it


I do smell fuel when/after cranking