C24FUN

All was well when I put the car away last October but when I brought her out this month I started getting ABS alarms. This is a 1990 C2 (no PDAS) that has had properly functioning ABS since I bought it 6 years ago. The symptoms are:

ABS alarm sounded when I first backed the car out of the garage (beeping and indicatorlight).
ABS alarm will sometimes activate as soon as the engine is started (before the car moves).
ABS alarm will sometimes not activate until I have been driving for anywhere from a few seconds to a few minutes, or occasionally not at all.
If ABS alarm has not yet sounded, a hard application of the brakes will cause the wheels to lock up and the alarm to sound.

I have not made any changes to the vehicle since parking it in October.

I have checked for brake fluid leaks - none found.
I have removed and reseated the ABS computer connector - no change.

The variety of conditions under which the alarm will activate has left me at a loss to come up with any theory other than the ABS actuator itself may be faulty. Unfortunately even though I have built the Porsche Diagnostic Interface it doesn't help because the C2 does not provide any ABS data to it. Knowing some detail about the sequence and function of the ABS`internal diagnostics would be helpful.

If anyone has any suggestions or experience with this type of problem I would greatly appreciate your input.

Thanks in advance,

Jim G.

38D

Any water under the drivers seat?

C24FUN

Colin,

No water anywhere (was stored in my nice dry heated garage). Not sure where you were going with that thought?

Jim G.

C24FUN

So, does anyone know the details of the internal test sequence that the C2 ABS goes through on startup? It might help my troubleshooting.

Jim

mojorizing

Hi Jim,
If everything else checks out, ie. not a weak battery, failing alternator, bad plug wires, because these have caused the ABS light to come on for other people, then I'll paraphrase what I read in Adrain book pg. 407.

ABS light will illuminate under the following conditions after engine start:

1) ABS goes out after 2.5-5 seconds and comes back on with audible. ABS control has failed or ABS solenoid valve inside ABS hyd. unit has shorted. Solenoid valves coils should read approx. 0.7-1.7 ohms.

2)ABS light goes out after 2.5-5 seconds, stays out till car reaches approx. 4mph plus a few seconds, then comes on again. This indicates a bad wheel sensor or ABS return pump in the ABS Hyd. unit. Sensors should read 600-1600 ohms.

hope this helps. Kevin

C24FUN

Kevin

Thanks for the suggestions. I'm kicking myself for not grabbing my copy of "The Book" before posting.

Jim

38D

Sometimes a little bit of water will get under the drivers seat and short the contacts. Once it dries out, it will work fine again.

Sten

If all else fails I have a used C2 ABS unit 964.618.119.00 which I bought by mistake!

C24FUN

Thanks Barry, I'll keep it in mind, although my experience with the cost of shipping from the UK to Canada makes me hope I won't need it!

C24FUN

Here's what turned out to be the problem:

For some reason Porsche uses a completly separate power lead for the ABS system which is terminated piggy-back fashion onto the main connector at the positive battery terminal (this is true for the '90 C2 but I'm not sure whether it is the same on all 964s). There are at least two potential problems with this setup that can lead to erratic ABS operation.

1. The ABS power teminal is held on by a small nut which can loosen over time leading to high resistance or open circuit.

2. Corrosion can form under the terminal (stray battery acid etc.) leading to a high resistance connection.

In either case the ABS circuit eventually can't get the juice it needs to operate and indicates a fault condition.

Hope this keeps someone else from going crazy trying to find this problem!

Jim G.

porsche356sc

Does anyone no if its possible to change the Solenoid valves in the abs unit

"1) ABS goes out after 2.5-5 seconds and comes back on with audible. ABS control has failed or ABS solenoid valve inside ABS hyd. unit has shorted. Solenoid valves coils should read approx. 0.7-1.7 ohms."

d3drturner

C24FUN, very useful. Saved me probably quite a bit of time. My anti-lock light on my 1991 C2 started coming on a few days ago. Cleaned the terminal and re-tightened and the light is out.

thanks!

C24FUN

Hey, that's great to hear! Glad it helped.

BTW, another curious problem happened with my ABS last summer. I was on a long drive and my ABS warning light started coming on intermittently. I thought it might be a loose sensor. About 10 minutes later my air conditioning cut out. Weird, I thought. A few minutes later my radio cut out. Wow, strange.

About a minute later the engine started to miss and very quickly died. I was stuck on the side of the road with a completely dead battery! Fortunately I was travelling with a resourceful PCA-CWR tour group and by swapping batteries we managed to get home.

I eventually determined that the carbon commutator brushes on the alternator had, after 20 years, worn down to the point that no power was being generated to charge the battery. Not a single alarm or warning light had lit up on the dash (yes all the bulbs are good) so this is one the factory didn't figure on! So if your car is getting up there in age and the alternator is all original it might be time to change out the voltage regulator module that comes with new brushes (about $20 at your auto parts store).

Cheers!

Jim

Richard Curtis

Just to add to this thread: I had a problem with ABS light coming on intermittently (and only flickering then... it never stayed on longer than a split second). Seemed to be associated with going over bumps. I removed the ABS relay, looked at it oddly, then spread the tangs just a bit so that the relay would stay seated. Reinstalled. No more flickering ABS warning light. At least, so far. Fingers crossed.

Richard Curtis

Here we are 7 months later... and the flickering ABS light ^ happened again, this time alarmingly so. This time, I removed all four wheel speed sensors (simple to do) and cleaned the magnets. The fronts were relatively clean but both rear sensors were dirty with magnetized debris. I reinstalled figuring this would fix the occasional flickering ABS light. No such luck.

Then I began thinking again about that loose relay connection. This time I placed a simple but thick (about 5/16 inch) rubber washer atop the relay and reattached the fuse box cover. It obviously fit very snugly and very obviously pushing the relay down into the connection. I then drove the car 600 miles — some of it over a very bumpy interstate -- and the light did not reoccur.