Brian N

All,

I have just searched the archives to find any comments in relation to my current problem with no luck.

My ABS lights come on with ignition and after a short drive (or rev) clear appropriately. After driving if I sit the car and let it idle, the ABS & Brake lights come back on and stay that way. If I cut the iginition and restart the car it clears again.

Physically testing the braking on a loose surface the ABS appears to cut in (or let go) although it does appear as efficient as usual.

My local Porsche mechanic put the Hammer on it today and told me that he was getting NO readings from the ABS, no errors or no confirmations, and suggested I book it in for them to look at.

Any ideas ??

Regards
Brian
'91 C4 Cab Black on Black

Adrian

Dear Brian,
First let us get the light sequence in order.
With ignition on but not started you should get the following lights (related to this system) on
PDAS (traction control)
ABS
Brake Press Warn
When you start the C4 the ABS light should go out but the PDAS and Brake warn should stay on.
After 20-60 seconds both the PDAS and Brake warn lights should go out. Obviously we want them to stay out.
Now you are saying from your post that only the ABS and Brake warn lights come on. This is not correct operation in itself. Are you sure you are giving the correct description of the lights.
The normal fail light sequences are (others are possible under certain rare conditions)
PDAS and Brake warn with horn
PDAS and Brake warn without horn
PDAS and ABS warn with horn

Now I am assuming that the Brake warn you are telling us about is correct. This means that you DO NOT have an ABS problem nor do you have a PDAS or traction control problem. If you have the warning horn blasting away you have a brake pressure problem. If the warn horn is not activated (but is serviceable) you have a brake fluid level problem. These problems will NOT show up as fault codes on the PDAS/ABS control unit.

I strongly recommend you understand the sequence of lights being activated and whether the warn horn is sounding. Getting this correct will save you a lot of time and money and frustration.
Brake warn here is the key. If this light is on you have a brake system problem. Fluid level (very very common), fluid level sensor problem (very common especially if wet) air in system (also common after a DIY brake job), leaking pressure switch on hydraulic pump (located under black panel forward of the battery and getting more common on C4s), leaking pump seals(same location) or a failing pump (quite rare but does happen).
You can see by this that the lights and horn sequence is critical in helping you. The ONLY way to get an ABS light and not a PDAS warn as well on a C4 is for the PDAS warn to be disconnected.
Ciao,
Adrian
911C4

PS: If PDAS fails so does the ABS and vice versa. If there is a brake pressure a problem only the PDAS is normally de-activated.

Brian N

Dear Adrian,

Its midnight in Aust, perfect time to go out and test the car.

My start sequence agrees with your normal start mode. Also the warn horn is servicable.

The two lights that come back on some time later as I sit with my foot on the brake, are the PDAS and Brake Pressue Warning without horn.

I will follow down your supplied suspect list.

Thanks
Brian

Adrian

Dear Brian,
You have a brake fluid level problem. Also get your foot off the brake and see how long it takes to happen. When you press the brake pedal you are actually bleeding off pressure as well and this can create a red herring.
Anyway, brake fluid is your problem and this is easy to check. If brake fluid is okay (please do not forget that the level must be checked at the time the brake pressure pump first switches off). This means you need to either let the pump run off the battery and when you hear it switch off check the fluid level or start the engine and run around and wait for the pump to stop.
A C4 is sensitive with brake fluid levels. If the fluid level is okay that is above min and below max then you may need to pull off the brake sensor connectors (they are just spades) clean them up and clean up any water sitting in the cap. I am sure it is just brake fluid level.
Ciao,
Adrian
911C4

PS: There are four systems connected to the brake fluid tank for a C4 and three for all other 964s. I am starting at the tank because it is the easiest for people to imagine.
Normal brakes (either high pressure or vacuum boosted)
Traction Control (C4 only)
ABS
Clutch.
Four systems and identifying which one has failed is critically important.
Traction Control or PDAS uses PDAS warn light and ABS warn will follow within 30 seconds.
ABS uses ABS warn light and PDAS warn will follow within 30 seconds.
Brake pressure uses PDAS warn combined with Brake warn and warn horn and they will all come on together.
Brake fluid level uses PDAS warn combined with Brake warn light only.
For the clutch system there is no warning unless a major leak has been caused and then it will show up as a brake fluid problem.