316 to 325 conversion wiring help c101 plug

[flat_out_kernow]

Hi everyone, I'm new to the forum and relatively new to cars, I've been a bike man until I broke my back, so with my time off recovering I've picked myself up a lovely little 316 and decided to swap the 325 lump in. All went to plan, engine in, rear axle with disks on, exhaust mounted up lovely and then the wiring.....
I've read up about making sure pin 20 is correct. Both the car and new engine are red/yellow wires. As soon as I connected the battery (ignition off) I saw a puff of smoke from the engine loom. Cut the loom open and the brown wire from my oil pressure sensor has burnt out. It connects into a load more brown wires?
What I have on the c101 plug is a complete mismatch of wires/colours
Car side
1 - light blue
2 - --------
3 - --------
4 - brown purple
5 - black brown
6 - yellow green
7 - green
8 - yellow white
9 - black
10 - -------
11 - blue white
12 - --------
13 - red purple
14 - black yellow
15 - black green
16 - -------
17 - -------
18 - black yellow
19 - ----------
20 - red yellow

Engine side

1 - light blue
2 - yellow green
3 - --------
4 - brown purple
5 - green brown
6 - green white
7 - green
8 - white black
9 - black
10 - blue
11 - green white
12 - grey
13 - green purple
14 - black white
15 - black green
16 - black white
17 - -----
18 - black yellow
19 - -------
20 - red yellow

I'm not 100% on the colours as it was dark and I had enough for the day lol. But it clearly doesn't match. What can I do? Electrics arnt my strong point and I can't find anything on the net.

Any help much appreciated

Thanks

[Brianmoooore]

The problem is a bad bit of design by BMW!
The main engine to body earth strap has a parallel path - the pin of the oil level sensor which has a brown/orange wire on it is connected to the metal of the sump via an internal connection in the sensor, and the other end of the brown/orange wire is connected, inside the engine loom, to a much thicker brown/orange wire, that then connects to the earthing point on top of the RH suspension turret.
If a poor connection occurs in the main earth strap (or you forgot to fit it?), when you try to start the car the current to the starter motor tries to pass through this wire, with the results you have seen.
If BMW had specified that the pin on the sensor should be isolated from the body, then the possibility of the problem wouldn't occur.

[flat_out_kernow]

I bought a heavy duty longer earth strap and amounted it bon the original 316 earth strap then onto the block. It burnt out the wire with the ignition switched off :-\ . are the other wires correctly matched and just sdifferent colours? Cheers for the quick response

[Brianmoooore]

Your C101 connections are all correct. You have the odd colour pair the wrong way around - for example, 6, yellow/green should read green/yellow, and 14 is blue/yellow, not black/yellow, but the wiring is correct.
As I said in my OP, the only thing that can burn out the brown/orange to the oil level sensor is a combination of BMWs design flaw and a failed main earth strap. The earth strap is connected to the alloy engine mount, rather than the block, so the joint between the arm and the block is also part of the earth path, as is the connection plate on the strut top.
I once bought an immaculate silver 320i touring unseen from a salvage yard - Cat C, engine fire, for next to nothing, back when they still had value. All the 'fire' turned out to be was the burnt level sensor wire like yours.

[flat_out_kernow]

Spot on mate, I shall go down to the yard and have a look later on today and see if I can get something sorted. Cheers for your reply mate, do you reckon any other damage could have been done by this short? Pressure sensor ? Other wires? Hopefully I'll get somewhere with it later on, don't think it helps having an old school aftermarket alarm bodger into the dash :-\ going to rip that out too.

[Brianmoooore]

The short won't have damaged any sensors, but there's a distinct possibility that the red hot wire will have melted into the insulation of other wires, and may be shorting to them, or may do in the future.
Ideally the loom should be replaced (same loom on Motronic 1.3 320s), or at least removed, unwrapped, the damaged wire removed, others along its route checked for damage and made good, and then the whole thing rewrapped with the correct tape.
No point in replacing the wire on your car, since four pot body looms don't have the level sensor wiring.
Anyone else reading this who has a M20 engine in an ex four pot car, should remove the plug from the sensor.