Confusion around the ABS over voltage relay & pin 20 C101

[Theo]

Car is a 1989 M3

The wiring diagram shows a separate over voltage relay but there is no relay or socket for this on the car body loom. Does this over voltage relay now reside in the ECU itself or was it just discarded altogether?

As I understand it the ABS ECU is powered by pin 15 on the ABS ECU connector and this comes from the blue alternator wire.

Power for the ABS pump and valves is from a separate heavy duty lead direct from the positive terminal on the bulkhead.

There are 3 red/yellow wires which are joined at a splice within the loom. One is connected to pin 1 of the ABS ECU, one feeds the coils on the valve and pump relay via pin 10 on the ABS pump plug and the third is connected to pin 20 on the C101 plug,

What is the purpose of the connection at the C101? There is no corresponding pin on the engine loom but presumably this existed in a previous version on the loom? Will my ABS function as things stand, the car is in pieces so I can't do a road test!

Thanks

[Brianmoooore]

Car is a 1989 M3

The wiring diagram shows a separate over voltage relay but there is no relay or socket for this on the car body loom. Does this over voltage relay now reside in the ECU itself or was it just discarded altogether?

Discarded altogether. Whether it was just found to be unnecessary or because of improvements in the durability of electronic components, I've no idea

As I understand it the ABS ECU is powered by pin 15 on the ABS ECU connector and this comes from the blue alternator wire.

This is just to tell the ABS ECU that the engine is running

Power for the ABS pump and valves is from a separate heavy duty lead direct from the positive terminal on the bulkhead.

Correct

There are 3 red/yellow wires which are joined at a splice within the loom. One is connected to pin 1 of the ABS ECU, one feeds the coils on the valve and pump relay via pin 10 on the ABS pump plug and the third is connected to pin 20 on the C101 plug,

Pin 20 of the C101 is the power feed to the ABS ECU, and replaces the relay part of the old overvoltage protection 'relay' used in the early version. The relay was fed with 12 volt + directly from the battery and was switched on by a green wire from the ignition switch. Instead of replacing the overvoltage relay with a standard one, someone had the bright idea that there was already a relay switched on by the ignition switch - the DME relay in the engine loom.
It looks like your car has been fitted with an engine and loom from an earlier model.

There is no corresponding pin on the engine loom but presumably this existed in a previous version on the loom? Will my ABS function as things stand.

Your ABS won't work as things are. Pin 20 of the engine loom needs to be fed with 12 volts+, ignition switched, so you need to add a wire from either pin 87 of the DME relay to pin 20

[Theo]

Thank you very much Brian, I was very much hoping you would reply.

I had my suspicions around the ABS not working but the engine loom is original to the car (which had an S38 conversion back in the 90's) so it must have been driven around without ABS since then. Incredible!

[Brianmoooore]

If it didn't have power to pin 20 of the C101 or have the overvoltage relay, then it can't have. On a RHD car the relay was clipped above the body coloured bit of steel that the catch for the glovebox is bolted to - not obvious at first glance. I've no idea where it should be on a LHD car, which I presume your M3 is.
What about the ABS warning lamp? That should be on with no supply on the red/yellow, unless the lamp has been sabotaged.

[Theo]

The loom is currently removed from the car so I can't check the lamp, the bulb is there which is a surprise!

I did have a couple of questions unrelated to the ABS, if I may. There are two permanent live wires in the door loom. One is for the heated door lock but what is the other one for? It's on a 3 pin circular plug. It's years since I've

Is the heated door lock on at all times? Surely it would drain the battery if this was the case?

Thanks very much

[Brianmoooore]

There's only one permanent live in the door loom that I know of, but it does use two different pins in the A pillar plug - one is power in and the other is power out. It was done like this to make the addition of the optional central locking loom easier.
There's also an ignition switch position 1 switched live in the door looms for the electric mirrors/heated glass.
The heated door lock only operates when the external door handle is pulled out. There's a simple leaf switch attached to the back of the handle.
The 3 pin plug is for two different options - the heated door locks, and the interior light delay off option, which also uses a crude leaf switch on the back of the door handle.
I've seen cars with one option or the other, but never with both. If it's possible to have both, it would need some kind of double leaf switch, which there doesn't appear to be provision for.

[Theo]

Very interesting, I think it does have the delay relay fitted will that still function to some extent?

[Brianmoooore]

If you connect the handle switch terminal on the module to the door pin switch terminal, the delay will work both when you get in and exit the car, instead of just when you get in. This is how most owner fitted delay modules are wired, since most don't buy and fit the handle switch when they buy a second hand delay unit.