Swapping a 91 m20 into my 87 325is

[JGood]

I picked up a complete motor/tranny from a 91 325i to put in my 87 325is for $150 (Friend hooked me up big time. Mine had 260k miles, this setup has 140k).

I'm wondering what would be best/possible... swapping the 87 harness onto the 91 engine, or using the 91 engine with the 91 harness that's on it and modifying it to work with the car.

I'm guessing if I use the 91 harness, I'd need to splice a few wires from it to go to the c104 in the car. I'd also need the later O2 sensor with the big connector.

With the 87 harness, I wouldn't need to do anything, right? It will just go right onto the 91 engine with no issues?

Some things effecting my decision:

-the 87 harness lit on fire at one point and is slightly burned... it runs and drives fine, but the coolant sensor wires are a tad bit crispy

-the 91 harness has the inductive pickup on the plug wire. Those plug wires are trashed, so I'd need to buy new ones ($$$).

-the 91 is missing the CPS. Would the 87 CPS even work with the 91 harness?

-Anything else???

[jlevie]

If the 87 harness is in good condition, just use it. If you swap to the later harness you'll have to make some modifications to the later harness.

The CPS is the same for all production of the M20B25, so you can use the one from your 87 engine. You'll also need switch out the water pump on the 91 to the early version like you have on the 87 engine.

[JGood]

If the 87 harness is in good condition, just use it. If you swap to the later harness you'll have to make some modifications to the later harness.

The CPS is the same for all production of the M20B25, so you can use the one from your 87 engine. You'll also need switch out the water pump on the 91 to the early version like you have on the 87 engine.

I'm mainly curious what exact mods would need done to the harness. The c104 is only 3 wires, is there anything else? I just got done doing an m50 swap and made my own adapter, so I'm not afraid of a little wiring. I just don't want to have to redo every connector...

Thanks for the info on the CPS. Yeah, I'm going to use the early coolant system and have an early water pump on order.

[JGood]

Well, after lots of reading, I decided to just use the 91 harness. I ran the green wire (pin 2) of the c104 to terminal 15 of the coil pack (switched hot). It should start like that, and not having a tach or MPG readout.

I started cranking, and it started smoking. Fried the main relay wires.

I didn't do a pinout chart because every person on this board who swapped late to early motors didn't have to change anything at the c101. I'll do that now obviously, but does anyone have any other ideas?

Ground and power hookups look correct.

Again, it's a 91 m20b25 in an 87 325is. Everything on the engine is from the 91 except the coolant spider hose and water pump (I converted to the early style)

[JGood]

It was pin 20. On the 87, it's a spare ground from the body that doesn't go to anything on the engine side. On 88 or 89+ cars, it's switched power from the main relay to power the ABS unit on the body side. So I connected the main relay positive voltage to ground. Yeah, that doesn't work.

I tore the harness apart, yanked that burnt wire, checked the others for damage, put it back together, and it fired right up.

Runs great!