Bob Krantz

The abs light has been coming on at startup in my 89 C4. I have searched the threads on this but have not seen the symptoms I have. First, the awd interlock and brake pressure warning lights usually come on in the morning (and sometimes after all day at the office) for about 30 seconds. My guess is the pressure is bleeding down and has to rebuild. But ocassionally the abs light also comes on at startup and stays on. If I restart the car it always goes out (so far). In the past, this happened about once every 6 months. Now it has happened 4 or 5 times in the past week. Anything I can check without the hammer?
Bob K

shaker71

Does it go out if you drive the car without restarting?

Phil Raby

Could be a voltage issue - check my thread on xenon headlamps

Phil

Adrian

Dear Bob,
The ABS light coming on suggests your battery voltage when you start is dropping to 9.9VDC. If the warn horn is not coming on this suggests your battery is most likely the problem. I you go to the rennlist.com tech section you will find a full explanation of this system.
You are quite correct, your pressure is bleeding off. Porsche require the system to hold pressure above 102 bar for three hours.
Ciao,
Adrian
911C4

Bob Krantz

Thanks to all. So far, the light will not go out until I restart. The battery is a recent Optima, so I may need to check the charging system. I did upgrade to H4's about a year ago with slightly higher wattage bulbs. No other modifications to the electrical system.
Bob K