'90 B25 into an '87 iS: Differences in Cooling and Electrical Systems

[NMusolino]

Picked up a "new" M20 to fix up and swap in to my Dad's '87 iS, however it is from a late model ('90) so not all of it is a direct fit. I know the differences between the two have been covered before, however I want to make sure I have it straight so we don't order the wrong parts.

I know the ignition and cooling system differ between the early and late models, and I have the late model ignition system as well. I plan to go convert it to the late model ignition system, however, I'm not sure about the cooling system.

Can I put an early model water pump on the late model engine? The rest of the cooling system in the car works fine, and I'll be doing the timing belt on the new engine anyway so I'm going to replace the water pump. If I could use the pump from the early models, that'd mean I wouldn't have to convert the rest of the cooling system to the late style, so it'd be good to save a little money there.

My other question is whether there is a difference in CPS location. I know all late models had it on the front of the motor, while some early models had it in the transmission bell housing. The build date of my car is 12/86 for what it's worth.

Thanks in advance for any help.

[GCAutosports]

You can use the early model water pump in late engine as long as you keep the early cooling system

[Kershaw]

The block is physically the same. Just swap everything that's currently in his car onto this new block. All i models have the CPS in the same location. it's the e models that have it in the transmission.

Cheaping out and not replacing the water pump is kind of meh. It's $50, you might as well replace it while you have everything apart. Your time is worth money too.

[Som]

Just as confirmation, I've got a 12/86 build 325i and the CPS is on the front.

I did the conversion to late model cooling, putting a late model pump on the original block. Fit fine. I imagine the reverse is fine, too.

Only other difference I can think of is that your timing belt cover will have a threaded bolt hole in it for mounting the late model hard pipe. Since you won't have the hard pipe, you'll have an empty bolt hole on the front. So don't freak out. :) Though, you could just swap the upper cover from the original car.

Som

[modernist]

Early model cooling system will fit any M20 perfectly.

The ignition system on all Motronic 1.1 and 1.3 M20s is the same ... correct me if I'm wrong.

[Som]

I was thinking the same thing as modernist - I wasn't aware of my ignition system being any different than on the late model cars. I'm pretty sure they're the same.

Som

[TobyB]

It's not the ignition, it's the engine harness- and an 87 'should' have a later harness.

But use the harness that goes with the car and you will have no problems...

t

[Som]

Just checked today and my car has a 1.3 ECU, which means it was swapped out by one of the POs. Also means I can't confirm/refute whether the engine harness needed to be changed to accommodate the changed ECU.

That said, everything in the engine bay looked pretty old and brittle and unmolested and stock looking when I took it all apart last year. *shrug*

Som

[modernist]

1.1/1.3 harnesses are compatible. Early harnesses have a 3 pin plug in the glovebox going to tach/econ gauges; late harnesses route this through C101. Some very late harnesses power the ABS through the main engine harness. Late harnesses have the plastic fuel injector rail and a round plug under the intake manifold. Functionally identical though.

[TobyB]

Some very late harnesses power the ABS through the main engine harness.

This was what I was thinking- 9-90 did that, and it's not compatible with early (pre 8-87?)
bodies, where that power (pin20?) lands on a grounded chassis pin.

BTDT. Smoke gets out.

t

[modernist]

This was what I was thinking- 9-90 did that, and it's not compatible with early (pre 8-87?)
bodies, where that power (pin20?) lands on a grounded chassis pin.

C101 pin 20, red/yellow. My '86 eta is running a harness from a '91 cabrio; I just snipped the wire and removed it from the harness. ABS in models before (9/90? 12/88?) doesn't have anything to do with the engine harness.

Edit: more details below

Early
[ign. switch] =====> [OVP relay] =====> [ABS computer, hydraulic unit]

Late
[DME] =====> [Main relay] =====> [C101 pin 20] =====> [ABS computer, hydraulic unit]

Basically, late harness in early model is a nonissue. Just remove the red/yellow wire between C101 and the main relay. Early harness in late car will need some wiring work if you want your ABS to work.

[achtung-e30]

good info