M42 swap wiring nightmare...

[Formulatwo]

Changed the name and purpose of this thread.  Instead of making a new post...


I have a 1987 325e.  I swapped a 1991 318i Convertable engine into it yesterday.  Everything is in and bolted up, but I have run into wiring issues.

All of the problems seem to be coming from the c101 connector. (The round one on the driver's side firewall).  I was under the impression that it would just be plug and play,  but apparently it isn't.

I hooked everything up and reconnected the battery,  when I turned the key,  a wire under the hood smoked and melted.  

It appears that only ONE wire melted.  a red/yellow wire that runs to the ABS system.  I disconnected this wire entirely so that it wouldn't be a problem....

Now whenever I turn the key nothing happens,  the car doesn't crank or anything.  Although the dash board does light up. I'm not sure if the fuel pump turns on either.

Anyway,  can anyone get me some information on the differences in the c101 circle connector from 1987 to 1991?

If I hardwire and match colors together will this solve my problems?

[rob_e30]

We did this with our race car.  You need to take the engine, transmission, and prop shaft.  You should be able to take the exhaust from the M42 also.  You'll also need the wire harness but you should not have to touch the ABS.  The motor mounts follow the engine, and you'll have to figure something out about the radiator and AC if you have those.

[DesktopDave]

I'd say you'll have to cut a bigger hole in the firewall for the larger Motronic plug.  You'll need the whole exhaust, drive shaft and definitely the diff (your 2.93 won't be good with an M42...the 4.10 or 3.93 are much better).  You might have to splice the auto plug if you're doing an auto/manual too.  You'll have to figure out a bracket for the coils, unless you do a coil on plug setup.  The pre-facelift sedans have some body differences with mounts & studs, etc.

You'll have to check them all out one by one.  Have you been to RealOEM.com to check them both out in detail?  I'll be swapping a '91 325i power train into an '87 325e when the weather clears.  I'll let you know what I find.  I think the rad expansion tank might be trouble too.

But those are the only differences...the e30 remained basically the same car from '84 to '93.  Someone else will chime in...

Yep...motor mounts too.  They're like $100/each.  Check everything out carefully!

[Formulatwo]

Okay,  so I'm assuming that the harness plug on the firewall will not have to be re-pinned?

Both cars are manual.
My exhaust won't be an issue,  as my catalytic converter has been replaced by flex hose...I also won't be running A/C.  What is the situation with the radiator?  I'm assuming custom mounts,  or is there also a wiring issue?

Desktop dave, you mentioned $100 motor mounts,  couldn't I just use the 318's mounts?  Also, thanks for the realoem website, I am checking that out now.

[E30PQ]

Okay,  so I'm assuming that the harness plug on the firewall will not have to be re-pinned?

Both cars are manual.
My exhaust won't be an issue,  as my catalytic converter has been replaced by flex hose...I also won't be running A/C.  What is the situation with the radiator?  I'm assuming custom mounts,  or is there also a wiring issue?

Desktop dave, you mentioned $100 motor mounts,  couldn't I just use the 318's mounts?  Also, thanks for the realoem website, I am checking that out now.

If your going E30 to E30 just use the harness from the donor car and the ECU from the donor car.  

Just the trans from the donor car, the drive shaft from the donor car and the exhaust from the donor car.  The M20 exhaust is different from the M42 version.

You should use the radiator from the M20 as it is larger and should provide added cooling.  You might need to change the fan thermostat switch from the M42 rad into the M20 rad.

I don't see why you couldn't use the M42 motor mounts unless they are worn out.

[DaveM-sport]

If your going E30 to E30 just use the harness from the donor car and the ECU from the donor car.  

Just the trans from the donor car, the drive shaft from the donor car and the exhaust from the donor car.  The M20 exhaust is different from the M42 version.

You should use the radiator from the M20 as it is larger and should provide added cooling.  You might need to change the fan thermostat switch from the M42 rad into the M20 rad.

I don't see why you couldn't use the M42 motor mounts unless they are worn out.

Not if the cars have different car looms. facelift to facelift are fine but if your putting an facelift engine into a pre-facelift car, you need to do a small little bit more work but not anything too difficult.


E34 525i engine mounts work too and are alittle cheaper.
740iL are exactly the same, not sure on price.
E28 M5 mounts work quite nicely.
No reason to change them unless they are in sh!t though.

[Formulatwo]

Made some progress today...




The motor mounts appear to be okay, but he transmission mounts fell apart.

[Formulatwo]

Bump,  any help for my wiring issues would be appreciated!

[DaveM-sport]

What happened the calypso red E30?

[Formulatwo]

Bought the totaled red e30 for $300,  parting out the remaining parts in the for sale section.

I really need to figure out this wiring issue.

[wannam42]

Not if the cars have different car looms. facelift to facelift are fine but if your putting an facelift engine into a pre-facelift car, you need to do a small little bit more work but not anything too difficult.

Hmmm... What sort of "small little bit more work" are you talking about? :confused:

Not to threadjack but I'm putting in a 09/95 M42 to replace an M20.

[deekay]

if your m42 'vert donor car is like mine it'll have an open small-case with a 4.27 gear. definitely carry that over, it'll be a hot setup in a coupe chassis.

[Vladi]

I hooked everything up and reconnected the battery,  when I turned the key,  a wire under the hood smoked and melted.  

It appears that only ONE wire melted.  a red/yellow wire that runs to the ABS system.  I disconnected this wire entirely so that it wouldn't be a problem....

Congratulations! You just fried your ECU! A ton of people think the wires are plug and play, but as you found out it's not. And they fried their ECU's.

If you make me a diagram of wire colours containing the car's side of the harness and the engine's side harness I'll tell you wich wire goes where.