BlackShark

87 928 S4 Auto. For some time I'd had a constant ABS light. Comes on when you start the car, before moving. Not passing the initial self check. Even so I went through and checked the wheel sensors with a ohm meter, pulled each one to check and clean the sensor and toothed ring. Jumped the relay, checked the fuse, nothing.... Light stays on. Was reading the WSM last night on ABS and it didn't give me much to go on. Started to suspect the ABS brain, but how to check? Looked at the wiring diagram, maybe I could check to see if the signal was getting to the brain? Noticed there was a junction marked ABS ground. Maybe I'll go check grounds as well. Come to find out while poking around the ABS pump under the hood I noticed two wires that went to a juction, wiggled the wire and they were loose at the junction. Pulled that apart, cleaned up, put some dielectric grease on and re-assumebled. NO MORE ABS LIGHT! Turned out to the be simpliest thing. All hail the grounds...

AO

Technically it's not a ground, it's a "+", but I'm still happy for you that you found the issue.

Marine Blue

Good to know, I've cleaned up a bunch of the ground points but I haven't done this one yet. I'll have to get to this before putting the battery back in this spring!

BlackShark

Ok, I stand corrected. I found a loose + junction point for the ABS system. Hence the cover over the terminal. Sometimes it's better to be lucky than good, and why you should always disconnect the battery when working on any of the wiring. I'm still that I finally fixed this one!

Mrmerlin

make sure tha battery is disconnected before you remove or tighten the screw otherwise you will burn a hole into the edge of the fender, and possibly fry some other wires, this wire is usually removed when the timing belt is replaced, as its part of the engine harness, it is supplied by battery voltage

Bill Ball

I'll bet I know exactly how Stan learned this. Same way I did. ZAP!!!

ew928

How to Arc-Weld using 12V. By Bill Ball.

I'm so glad certain hot wires on the car are fused . . .
And did you know a screwdriver is a very good conductor.

IcemanG17

Very good find.....my 1st thoughts were RELAY.....mine slowly died...casuing an intermittent warning...but was an easy fix on the CE panel....there also is 2 relays on the ABS pump that can and do die as well.....those are harder to check, since often they are model year specific....so unless you have a working same model year to swap with you just gotta buy the relay to test it...which is a bummer...

The_Remora

My ABS light came on recently, thanks for the tip, this is the 1st thing I am going to check.

Bill Ball

Yep, that wire connector on my 89 has a big Philips screw instead of a nut. Big Philips screwdriver shafts conduct electricity very well.

Mrmerlin

Bill I didnt do any fender welding but , I have had to remove quite a few of these to do timing belts, the correct routing for the wire that goes to the engine is .... routed under and behind the ABS support bracket and along the frame horn, if its not routed this way the wire will rub on other parts like the power steering lines and either get oiled up or chafe so its a good idea to add some heat shrink to the part of the cable that runs under the bracket

John Speake

I had the same issue this week as Blackshark with the ABS on my late 86 Euro S2. (this has same ABS as the early S4s).

The light came on initially while driving. When the car was switched off, and then started, the light was on before the car was moving. So I reasoned that the issue might not have been a sensor problem, but perhaps a power supply problem.

I check ed the ABS fuse, then bench tested the ABS relay. both tested good.

I then found this thread, and cleaned up the 12v terminals he mentions, one of which looked somewhat corroded. To do this, I had to disconnect the battery ground strap....

When I put everything back together after cleaning the terminals, the light was off, and stayed off during a 10 mile test drive.

I am not fully convinced the dirty terminal was the cause of the light coming on.

So the question is, does the ABS ECU have a memory that will trigger the ABS warning lamp every time the engine is subsequently restarted ?

Alan

I don't believe it does - I think it does a self test for every start-up so will auto-reset once any problem is fixed.

Alan

the flyin' scotsman

Good question John, here's my experience:

intermittent ABS warning light, sometimes on after a few KMs sometimes on start up. replace ABS relay on CE, solved.

same problem returns 2 seasons later, clean all wheel sensors and rings, prob remains...........replaced pump and brain relays (the ones under the LH front wheel liner), prob remains..........put the old OEM CE relay in..........prob solved...............I got another new CE relay and alls well since.

And Brians absolutely correct; they're MY specific.

Alan

Unfortunately there are many that snake around the car that are not fused.... I'd give Porsche an F- for this, examples are:

The switched radio supply in the console (certain to be messed with)
The cooling fan feeders to the Central Electric Panel
The ABS brain feeder to the Central Electric Panel
The LH feeder to the Central Electric Panel
The EZK feeder to the central electric panel
The main feeder to the Central Electric panel (from the Jump Post)
The ABS Hydraulic Unit Feeder (the one in question here)

Of course the battery -> starter -> alternator -> jump post are also unfused - but that is just about inevitable on any car.

Many of these listed are large enough that you can do impressive welding with them until you totally destroy the battery - some are small enough that if shorted out they will just melt and burn up your wiring looms ...somewhere...!

Much of the 928 electrical system is well thought out - this particular part is a complete abomination ...

Alan