Roygarth

Two nights ago the 'ABS' and 'Differential' warning lights came on. I drove home slowly. In the morning and since then the lights has not come on? Have I got an intermittent problem or a 'ghost in the machine'? Presume car is safe to drive?

Thanks in advance.

Piers
1990 C4 Coupe
58K Miles

Adrian

Dear Piers,
Firstly your C4 is PERFECTLY safe. You can continue to drive normally. Just be aware that you do not have ABS. In the dry and warmer weather this is also not an issue.
If the lights came with the warning horn on this says you have a problem. If just the PDAS and ABS lights came on whilst driving it suggests six possible problems,
1/. Failed wheel speed sensor.
2/. Failed differential lock.
3/. Failed accelerometer.
4/. Failed ABS return pump.
5/. ABS solenoid valve mechanical failure.
3/. Dual solenoid valve mechanical failure.

If the failure happens beofre you move off the problem will be;
1/. Control unit failure.
2/. ABS solenoid valve electrical failure.
3/. Dual solenoid valve elecrical failure.

Do you remember which light came on first. Sometimes they co-operate and one wil fail before the other. The one that comes on first is the system to be looked at.

If you are not a DIY person then it is off to use the Bosch hammer. If you are a serious DIY person you can do quite a lot at home. However the hammer is the quickest solution. Please remember that no fault codes and having a failure tells us a huge amount as well.

Just a reminder. The C4 is perfectly safe to drive. Your post is also lacking because it does not say if the warn horn came on and the master caution came on. My post assumes it did.

Ciao,
Adrian
911C4

Roygarth

Many thanks Adrian

I had music on full blast but don't think buzzer sounded, I 'think' the central warn light came on. The lights came on at the same time.

I've just been for a drive and no warn lights came on.

Does this mean I'm in the clear (he asked hopefullly!)?

Piers

Andy Roe

Can't this also indicate that your brake fluid is a bit low?

Andy

Adrian

Dear Andy,
The brake pressure light has to come on without warn horn and without ABS light to indicate a brake fluid level problem.
Dear Piers,
Are you in the clear. No a chance. When my system started playing up it was around once every three weeks. Eventually it got more and more frequent.
I hope for you that it is just one of those things but I would not put any money on it. Warnings are there for a reason and it certainly detected something wrong. Pity you cannot confirm the warn horn though. Ah well, next time.
Ciao,
Adrian
911C4

Roygarth

Dear Adrian

Although I hope you're wrong! When it happens again I'll be ready and will let you know what happens.

Best regards
Piers

Toddimus

It seemes like this thread may be over, but I've got a similar question...

First of all, we're talking about a '90 C4. I'm new to the 964 series. I've had 3 earlier 911's (77,78,81), doing all of the work on them myself. I plan to do as much of the work myself on the C4 as possible. I just got finished with a valve adjustment. Much to my suprise, it was easier than on the earlier 911's!!

Enough of the preface to my question, here's the real question...

If the car has been sitting for over a couple of hours, and I start the engine, both the "brake" and "PDAS" (the picture of the AWD drivetrain) lights come on. They remain on for approximately 10-12 seconds, while the pump pressurizes the hydraulic system. If I touch the brake during this time it feels soft and sometimes the ABS solenoids are activated (while already at a complete stop). If I depress the brake, sometimes the warning horn comes on momentarily, and the time for the lights to go out increases. Once the lights go out, I happily drive away without any apparent problems.

What do you think?

Cheers,
Todd

P.S. (TOTALLY UNRELATED SYMPTOMS, I hope) My "airbag" light just came on. What does this entail? Is it just reminding me to get it checked out for it's two-year checkup?

Johnny G Pipe

'If it doesn't leak oil, then its not a porsche...'

I'd like to add:

'..and if the abs/brake warn doesnt light up all the sodding time, then its not a 964 C4'

Welcome to the 'small pieces of black tape' club, Piers. These lights seem to be SENSITIVE. Maybe it's a temporary glitch, you never know.
Just to add to the confusion, my problem (but with warn horn and ABS overactivity) was fixed (rather inelegantly by OPC) by repairing a wheelsensor earth, but the problem is recently back, and I'm pretty sure is down to a dying battery (couple of hesitant starts lately): I'll let you know. I'm trying to source a cheap local Bosch silver.
Now, where's that black tape?

Adrian

Dear Todd,
I have copied some of your text and put the answers in the text.
Ciao,
Adrian.
911C4

If the car has been sitting for over a couple of hours, and I start the engine, both the "brake" and "PDAS" (the picture of the AWD drivetrain) lights come on. They remain on for approximately 10-12 seconds, while the pump pressurizes the hydraulic system.

AS: This is normal with age. What this says is that your system is slowly bleeding pressure and after 2 hours it is below 102 bar. I suggest you check the age of your brake fluid first. Must be changed every 2 years. You may have an internal brake booster component leak back to the brake fluid reservoir. The system is supposed to stay above 102 bar for at least three hours. I would not worry about it too much but I would recomend you check for evidence of any brake fluid leaks in the brake booster system.

If I touch the brake during this time it feels soft and sometimes the ABS solenoids are activated (while already at a complete stop).

AS: This is not possible. You will never hear the ABS solenoids operate. You should not touch the brakes whilst the system is charging. If you do the brake pedal will feel soft and the noises you are hearing is the interlock relay operating to isolate the PDAS or traction control system.

If I depress the brake, sometimes the warning horn comes on momentarily, and the time for the lights to go out increases.

AS: This is normal. 3 heavy presses of the pedal in the stopped position will do this.

Once the lights go out, I happily drive away without any apparent problems.

AS: Everything is normal. You have no real problems except your static pressure holding ability is reduced and this again is quite common with age and low temperatures as well.

Toddimus

Thanks Adrian, that's exactly what I wanted to hear. I suspected that this is the case, but I wanted to bounce it off of some knowledgeable folks.

I haven't bought the repair manuals yet, so I don't know the procedure for changing/bleeding the brake system. Are there any special procedures for doing so? Do I just bleed them until the old fluid is expelled (maybe using different colored fluid?). Or are there special procedures for the ABS and AWD systems?

Also, what do you think about the airbag light? Is it just an indicator telling me to get the system checked?

Cheers,
Todd
<img border="0" alt="[cheers]" title="" src="graemlins/beerchug.gif" />

schlicht911

Hi,

I have the same problem, with these lights :-)
last week i was at the nürburgring driving from hamburg and
back to hamburg.. on my way back the lights goes on after
5-10 minutes on the road.. then they go off... and after 15 min on again and stay on.. so i dont like this situation and will change the brakefluid next week.. but not by myself :-))
i will tell you what happen after that...

greetings from hamburg germany

christian

Adrian

Dear Todd,
There are a number of separate systems which use brake fluid. You have to decide which systems you wish to remove the fluid from. I recommend you do it all which means you will need a Bosch Hammer.
The systems are.
Clutch circuit
Front brake circuit.
Rear brake circuit (you bleed the rears first).
Brake booster circuit (safety precautions needed with lots of care).
Traction control circuit (Bosch hammer needed).
ABS. The ABS system is the tough one. There is no way to bleed this system. You can use a correctly configured Bosch Hammer to help with this. There is approximately 30 cc of brake fluid inside the ABS hydraulic unit.
You must use a pressure bleeder system for a C4 and you must follow all the instructions contained in workshop manual.
Ciao,
Adrian
911C4

Toddimus

Thanks for the reply, Adrian.

I don't mean to be a nag, but you still haven't answered the question about the airbag light. What does it mean? I realize that it could mean some sort of malfunction, but does it also go on to remind me to get it checked?

I figured that the hydraulic systems were pretty complex. By the way, what exactly is the "Bosch Hammer"? I have been trying to figure it out in the context of all this stuff. It would seem to be some electronic diagnostic tool, but then it also seems that it is some sort of hydraulic pressure tool. What does it do?

Cheers,
Todd