M20B25 wiring harness - need help badly!

[mike87]

Hi everyone. I’m in dire need of some help with my wiring harness and was hoping someone who likes a puzzle can kindly help me figure out what’s going on and missing here.

In short, I bought an M20B25 (pulled from a 1987 325i convertible) from a seedy craigslist individual to rebuild and it came with a mangled/cut wiring harness because the seller had no idea what they were doing and decided to go the lazy route with a pair of scissors.

I pieced the wiring harness together, and it looks like it’s missing the short section that goes through the firewall bulkhead which connects the ECU-end inside the cabin to the rest of the harness in the engine bay. I circled the main section in red which looks to be missing, which houses the wires for the:

1. O2 sensor (I think 4x wires)
2. Ignition coil (I think 2x wires)
3. Ground on top of the strut mount (I think 1x big bundle of wires)

Does anyone know what colors these wires are and any advice as to how to piece these two halves of the wiring harness together? I was planning on soldering in wires to connect the color-matched wires from the two halves together, but can’t figure out where the ~7 wires go that connect to the 3 things listed above.

I also have the E30 electrical wiring diagram book but can’t find the right page that has this info.

Thanks in advance


Example photo from eBay showing the piece that I'm missing:



Actual piece of the harness that I'm missing (I already know that the stub taped in blue goes to the battery terminal block):



Left side is in the cabin (I know the big thick white wire with the two smaller wires inside is somehow related to 2 of the O2 sensor wires), right side is in the engine bay:

[jeffnhiscars]

Hey Mike. Wouldn't it be easier long term to get an intact harness ?

[cheffy30]

agreed with jeff... why struggle with it? sent from hell using Tapatalk

[mike87]

In an ideal situation, yes I would just buy a good harness, but a fully in-tact harness would run another $250 (per ebay) that I would rather not spend at this time.

I'm pretty slick with a soldering iron and 3D printer/lathe so I can make most of the connections myself since this mangled wiring harness that I have is about 90-95% complete. I just need some help figuring out the connections.

Plus I love a good project...

[jeffnhiscars]

many ebay sellers are on crack. Real world price is probably half that

[JehTehsus]

I would recommend against trying to fix that harness, especially right there. Whether solder or crimp joins those are going to be sitting right where the grommet goes through the firewall, and that is tight and IMO a perfect place to stress and break joins. I just got my buddies M20 running on megasquirt and we had issues where just pulling the slightly modified harness through the grommet/firewall damaged a couple connections.
It certainly could have been poor soldering on my part though, as another thing I will note is soldering the harness wire is a bit of a nightmare, it does not want to take solder and you really have to get it hot and sit on it for a while.

[sweet3]

o2 sensor wiring:
Pin 1: yellow
Pin 2: black
Pin 3: brown
Pin 4: green/blue

ignition coil:
Terminal 1: black (also from diagnostic port Pin 1)
Terminal 15: green (from c101 Pin 7)

ground terminal:
usually brown and brown/orange wires

[808ETA]

many ebay sellers are on crack. Real world price is probably half that

I agree! I just paid $150 for a harness with DME from one of the FB groups. Sweet3 actually modified it for my Motronic 1.3 upgrade and head swap. He did it quick and it looks great!

[TobyB]

try these:

http://www.wedophones.com/BMWManualsLead.htm

and if it looks too scary, buy a harness.

t

[bmwman91]

That is a total write-off. It COULD be fixed, but to do it right you would probably need more than $100 in proper splices, crimp tools, sheathing, etc.

$250 for an E30 DME harness is bogus nonsense. I have been buying ones in great condition on here and eBay for $75-120 for years (and these are M42 harnesses, which are considerably less common than M20 ones).

Shop around a bit and get one that is fully intact. Do not mess with that hacked-apart thing. Put up a WTB thread in the WTB-Classifieds sub and you'll likely have a deal in no time. That's what I have done with great success many times.