marcb993

Hi guys - It looks like my abs control unit has packed up, and am looking for a reasonably priced fully functioning salvage part as replacement.

It's throwing up the ABS warning light on the dash and the OBD says error code 34 - valve relay open circuit, apparently. Swapping the unit with a working unit from another car results in no faults.

It's a 1990 C4 and the part number is 964 618 126 00 (or 01)
I just missed one for $375 - the seller sold it a month ago.

My C4 has been converted to rear wheel drive, so I would not necessarily rule out the C2 version of the unit 964 618 119 00 (or 01), but it would not be my first choice as it would involve a conversion process instead of just being plug and play.

If anyone has or knows of a spare one available, please let me know.

Goughary

There is a thread on here somewhere about buying a cheep Mercedes 190 abs unit and swapping parts. That seems the cheepest way to go

Duck

They are super rare and even used can be a fortune. I have a spare unit that does not work. I wish I could find a company to repair it to have a spare.

964TT

I replaced mine with a BWM M3 CUP unit, so I have one spare laying around, C2 though.

I'm really happy with the cup version, it is so much more faster and aggressive.

marcb993

Thanks Goughary - I will search for that thread and see if I can find it.

Duck, FWIW I sent mine to this company http://ecu-rc.nl/ in the Netherlands (no affiliation). They charge EUR400 to repair it and refund you EUR 340 if the can't. In the end they were not able to help me, so the net cost to me was 60 euros, but turnaround and the refund was prompt - not sure if that is something you'd want to consider in the future - maybe you'll have more luck with your unit.

@964TT - I didn't fully understand your reply - What is much faster and aggressive ? The ABS brakes ?? Can you please elaborate ?


By the way - how do you include your location in the avatar section ? I've searched high and low for this in profile settings..

964TT

Yes the ABS is more aggressive. I replaced the whole unit, the pump and the brain. I really disliked the original ABS, especially when driving on the limit.
The M3 Cup unit it a pretty straight forward swap as it is also 3channel which the C2's have.
I couldn't be happier with the change.

robt964

We need ALL the details about this please

ras62

It is usually the small pistons that have seized inside the pump. The unit can be removed and dismantled and the pistons can be freed off with a little effort. It is just as simple to swap the unit for one on a similar age Mercedes such as the 190. The units are identical or you can swap over the internals if you want originality.
The problem is these pumps are often sitting in a damp atmosphere and many are never exercised. I actuate the ABS occasionally to make sure the seizure doesn't reoccur.

Duck

Thank you so much for the suggestion. I will definitely check them out.

964TT

The Bosch unit from BMW is similar to the 3 channel one from the C2. It's a pretty straight forward swap, except for the speedometer.
We had to add another sensor just for the speedometer to get everything working again. First I tried a GPS unit which made the speedometer very accurate but not so good when you pass through tunnels and some bridges. It would loose the signal and retract the spoiler...
So another sensor was added to get the regular speedometer functionality back.

The ABS unit is a significant upgrade though, worth the hassle. The company here in the Netherlands where I got it installs upgraded systems in the old M3's. So they usually have some original Cup units laying around and I had it refurbished before they installed it.

marcb993

Thanks ras62, so are you suggesting that the pistons seizing in the pump could be what is causing the control unit to signal a fault ? That could well be possible, but I think that in my particular situation that is not the case, because when I switched my control unit with that from another car, the ABS warning light on the dash was no longer triggered, which suggests to me that the fault lies with the ABS control unit rather than with the ABS pump, otherwise I would have thought that both control units would have produced a warning signal.

@964TT - which company in the Netherlands did you use ? I'm intrigued by this cup conversion.

cajonfan

There is no feedback for the ABS controller to sense (or be damaged by) a stuck pump. The controller just switches a relay to turn the pump on and off. Stated another way, if the pump is stuck, there will not be any pressure relief in the brake circuits and the wheels can lock, but the controller will never know.

Sounds like you have at least a connection issue with your C4 controller, take a good look at pin 32.

Re-pinning a "new" connector to use a C2 controller would not be too bad. You would be going the easy way (C4 to C2), something like 24 connections.
The interface and chassis side equipment are all fairly easy to test.

Good luck

ras62

As Cajonfan said, there is no feedback from the pump to indicate a fault from the pump, although relay faults would be seen. I was confusing the ABS unit comment for the hydraulic part not the electronic part. If you have swapped the latter with good results I would try connections or failing that a repair. It could be simply dry solder joints..or of course component failure on the solenoid output. You could read this following thread as it sounds very similar to your issue
https://rennlist.com/forums/964-foru...c4-1990-a.html

marcb993

@964TT - I just read the thread on your 964TT build - wow, what an odyssey ! It sounds amazing tho.

@cajonfan - Which is pin number 32 ? And should i be looking on the unit, in the unit, or on the connector plug ? Is there anything in particular I should be looking for ? I apologise for the basic questions, but car electricals are not my strongest point.

@ras62 - thanks very much for locating that thread. Much appreciated - I will check it out.

Vandit

Off topic, but how was your C4 converted to RWD? Did you replace the whole transaxle with a G50 from a 2WD car or modify the G64 gearbox to RWD only?