Jump to content

M20 Wiring, Basic Question(S)


Recommended Posts

Hey guys, I should be pretty darn close to getting my M20 mounted this weekend and than tackling the electrical part. Only real question so far is from the C101 round plug unit, should I just splice wires from the 02 harness into that unit or is it a bit deeper than that?



Link to comment
Share on other sites

ooo, c101.  you have these, of course:




If I was doing this, I would make every attempt to keep everything on the engine

side of c101 stock E30.  More and more harnesses are failing, so if you ever have

a problem (or an engine fire) you'll be days ahead if you have a stock engine harness.


The E30 needs remarkably few inputs THROUGH c101, so it really isn't too hard

to get it running.  Getting all the parts to work, though, takes a bit longer.  But it's

not bad.


While you're at it, it's worth looking at the circular connector under the intake.

(I forget the number).

If it's pristine, you can leave it, but most have corrosion in them, and as this connector

hooks the injectors and some sensors to the ECU, any trouble here is a real stinker

to diagnose.





Link to comment
Share on other sites

what toby said.


get a chassis side c101 connector and wire that to the 2002 harness side.  then you just disconnect the c101 to remove the engine harness. i did this in my s14 install.  not all c101s are wired the same!  pay attention to the wiring diagram for the year and model of the engine you have.


and definitely check the connector below the intake.  this is a very common failure point on M20's.

Link to comment
Share on other sites

hmm, im thinking it might be time to take a quick peak at my wiring than (its been a while since ive really looked at it). I know looking over some wiring posts people have made that there is definitely a need to find the exact year (mine being an 86) and go from there.


From what I can gather though, I need to basically cut that C101 plug and move it over to the firewall. After that I will lengthen the wires I just cut, back to the M20. Thats all fine and dandy, but the one thing I am confused on is do I simply splice in the appropriate 2002 wires somewhere between the M20 and the big C101 plug? Is it really that easy?

Link to comment
Share on other sites

okay, I gotcha now


Many thanks, Im sure I will be cursing myself for thinking this will be easy but thats how it goes!

Thanks again, off to work I go finishing my mounts and getting things ready!

Link to comment
Share on other sites

Since you have a 1986 donor, get a body- side plug (if you don't have it already) and make it right for the 1986 (first is) diagrams.


Then you'll have no problems.


The troubles arise when you try to put a 1990 engine into a pre- 9/87 E30 body- there are a few connections that short out.

(says the man who used the fire extinguisher)


But if you just make it right by the 1986 ETM, you're golden.


And really, I think you need power, start,  and only a couple other things, and off you go.  The hardest part

is deciphering WHAT goes through C101, and where it does.  To do this, pull the engine- side connector

boot back, and make a list, pin- by- pin.  It takes 15 minutes.  After you have wire colors,

you're golden, because the ETM lists wire colors. 


BMW in the 1980's was really a methodical German company.  When they did the E36, THAT was when the coke started

flowing like water....

(really, some of the stuff they did to the E36 electrical system is hard to understand even WHEN you're coked to the gills...)



Link to comment
Share on other sites

Its not that hard at all. I dont know why but the hardest part for me was getting right wires from the e30 engine harness to the correct terminals on the starter. I had to try a few different combinations to get the solenoid to engage and the motor to turn over.

Here is the e30 c101 to 2002 chassis harness. Easy


325 Harness                                   Function                                 2002 Harness

#1 Blue                                           Alternator                               Blue of Voltage Regulator

#4 Brown/Violet                              Coolant Temperature             Brown/White

#5 Black/Brown                              Oil Pressure                           Brown/Green

#6 Green                                         ECU Power                            Green

#7 Green                                         Coil Power                             Same Green as above

#9 Black                                          Tachometer                            2 Black Wires (18 Gauge)

#13 Violet/Red                                 Fuel Pump                            Direct to Fuel Pump

#18 Black/Yellow                             Starter                                   Black (10 Gauge)



It will be necessary to peel back all tape from the main wires running across the engine and along the left fender to identify them. Once they have been identified, the other wires can be put aside for now and taped up or cut away later. Identify the Voltage regulator and throw it away since you have a 325I alternator. The electrical part can be tricky, so take your time and identify each wire before cutting. Some special items will need to be procured for this part. Put a 25A fuse in Fuse slot #5 and place a 15A fuse in the line running to the fuel pump from the harness. Also, in the 1987 325I models, there is a plug near the control unit that contains three wires. The black one is for the tachometer. You will use this wire instead of #9 on the round plug. Don’t solder or permanently fix the wires until you have the engine running and in the car, that way you’ll be able to correct mistakes if you made any. Once the electrical part is finished, mount the harness plug to the firewall for easy removal of the engine later on. If you don’t have the factory plug, etc. then simply cut and solder all wires to look as clean as possible. Pull all wires up and out of the way of the engine, but leave them connected. Turn the key and see if she runs.

Link to comment
Share on other sites

Well after I got my garage sorted out I got looking at my engine bay and my god what a mess. I am thinking this will be a bit of a headache as there is like random wires even before I pulled my engine laying about not tied into anything at one end. Fortunately I think alot of it can be sorted out by tracing it all back but it will just take alot of time. I do have a ways to go before I can even think about firing but the more and more research I do and the help you guys have provided me with makes me feel a bit more confident.


Right now its a matter of testing my mounts today (made a drivers side one out of 1/8th for now just so I can get an idea of how my engine will sit and where I can put my rad). So far it looks like I will have to buzz a couple notches into my front bumper supports as my Scirroco rad sits a little too high and a little to far in the engine bay for my liking. Once that is sorted out I will buzz some edges off my Getrag 260, cut the transmission tunnel *GULP* and figure out my transmission mounts from there. From that point I can figure out my shift linkage situation and see how that goes together, than I can measure for the size of driveshaft I need (local bmw dude has like 50 driveshafts from e30's and the like but all are slightly different sizes). Once that is sorted I can move on to locating a suitable fuel pump and line solution. Probably going to run my power lines to the trunk with my e30 cable and looking into fabbing a rear strut bar/battery box. After that I will yank the engine and tranny out again, put the new timing belt on, slap a flywheel on, install my new clutch, check everything again for loose connections, button up all my fittings and lines, and go about final assembly and wiring... Hopefully I can make the most of my time off before I land a new job and get this thing to being drivable before mid July...


Sorry for rambling on lol


Back to the topic at hand. I was looking last night and I need to go to the wreckers and grab a chassis side c101 connector as I have the engine one with the cap, and further along my harness is the Chassis side without a cap. I assume the piece that would connect on top of that to link the wiring to the chassis side (which is what I will run my needed wires to) is what I need.


Mlytle, I started reading your thread and it definitely helped me wrap my head around how to tackle this wiring. Also the car is amazing, I can just imagine how she drives!


TobyB, I looked at the other connections throughout and they are in pretty nice condition, no issues to report.


Deutschman, that is a much clearer list than the one I was looking at lol. I will just have to double check my years wiring lol

Edited by 13
Link to comment
Share on other sites

  • 7 months later...

Hey guys sorry to bring this up again but tomorrow I will be trying to get this wiring stuff out of the way. As you can see theres a ton of crap laying around in the engine bay and previous owners half assed fixes. I will be yanking out the engine and its wiring harness as I will be adding a bit of bracing where the footwells and frame meet, and hopefully getting this wiring stuff sorted for now.






My biggest question is just to make sure if I can source all the wires in this one little cluster of wiring wrapped in all this black wrap. Im also wondering that when I find these wires I need, do I just cut them and wire them to my chassis side C101 connector? If so I think I will be alright, I will be double checking my wiring for my year of 02 and m20 engine but I feel pretty confident it wont be too bad once I get in there with some light and room.. Fingers crossed, I have to cut my fenders for my flares aswell so its going to be a fairly stressful evening haha.


Once I get the wiring sorted, trim the crank nut and p/s pulleu, add bracing in the front end, cut fenders and bolt flares on, hopefully I can start working on getting bit of plumbing done, run my fuel lines/pump/filter, get my rad mount and transmount fabbed. If that gets sorted I should be able to have this thing almost ready for the second the streets are clear of snow in spring!

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.
With your permission we and our partners may use precise geolocation data and identification through device scanning. You may click to consent to our and our partners’ processing as described above. Alternatively you may access more detailed information and change your preferences before consenting or to refuse consenting. Please note that some processing of your personal data may not require your consent, but you have a right to object to such processing. Your preferences will apply to this website only. You can change your preferences at any time by returning to this site or visit our privacy policy.