I have a 1995 993 C2 Tiptronic. Every now and then the ABS and ABD warning lights light up and do not go out. It seemed like this was happening after I used to wash my car but now I have ruled that out since it happened again last week when she was completely dry. Sometimes after restarting the car the lights do not light up. I know that this is not a malfunction because even with the warning lights the ABS works fine. Has anyone experienced this problem? Any help will be appreciated.

Thanks in advance

Sandman

 

Sandman, I hate to tell you but it sounds like the wiring harness. You can go to my site for more info - the link is in my signature. Here's the worst wiring harness story you will hear since you have a tip tranny: One car with a bad harness started and engaged reverse and rolled through the garage door and stopped when it hit another family car in the driveway. I am not making that up.

Oh yea, it could also be the battery loosing charge. But that's easy to check with a multi-meter.

E. J.

 

Sandman,
My car does the same thing. I wish I could help you, but I’m clueless as well. I’ll tell you everything I know about the problem, in the hope that it will help you isolate it.

I’ve posted this question to the board before, and most of the answers seem to point to alternator/belts/battery charging kind of things. For example, one person had the alternator belt break, and the lights came on. Replacing the belt (which you must do immediately) caused the lights to go out. I don’t know why the ABD and ABS lights come on simultaneously, and the alternator warning light does not come on, though. It seems that an alternator/battery kind of problem should cause that light to come on, and not the ABD and ABS lights.

I’ve also posted this question to the PCA technical site. Joel Reiser answered. He said to concentrate on brake fluid pressure or low fluid level, etc. He also said that most people neglect flushing the brake fluid, which could be part of the problem. This is not the problem in my case, as I’ve had my fluid flushed recently.

I have not isolated any conditions that cause the lights to come one. Once they came on as soon as I cranked the engine over, and once they came on after 45 minutes of freeway cruising. Other times they have come on have been in city driving. They have not come on during track or autocross driving. Water in the wires is not a factor for me.

One guess I’ve had as to the cause is a faulty wheel speed sensor. It could be sending zero or infinity to the computer, while the other three wheels are sending the same speed. I haven’t verified this, though.

I’ve had my harness replaced and I still have the problem. Stopping the engine and restarting always causes my lights to go out. Like you, my ABS works fine.

The only thing I have left to do is to have the codes in the computer read by the “hammer.” I’m approaching my 60,000 mile service, and will probably have the codes read then. I’ve done the manual code retrieval shown <a href="http://batauto.com/Porsche.html" target="_blank">here,</a> but the only code that comes up is “no faults stored.”

Good Luck,
Tom
’95 993
<a href="http://batauto.com/Porsche.html" target="_blank">here.</a>

 

Sandman (whoever you are), don't know if this helps. I had an old Audi Coupe Quattro that had an ABS warning light come on. Turned out I had a bad wheel sensor.

You say "I know that this is not a malfunction because even with the warning lights the ABS works fine".... How do you know???? Do you nail the brakes hard on a slick surface and feel/hear the ABS working? Only way I would think you would know that it's working. I would imagine that if the warning system is lit up, the ABS/ABD is shut down. It was on the Audi, anyway - and Porsche literature indicates the same.

Others seem to indicate that it could be a wiring harness problem. Could Be!!! There is an input from the throttle plate sensor to measure throttle position, so it might be that this is wired thru the wiring harness (then again, literature sez this is from the DME, not a direct signal). Works OK on initial startup, but once you're moving there's an incompatible signal between the throttle plate and the speed signal (ABD shuts off above 70KPH) and on come the lights.

Make an appointment with your dealer and let him troubleshoot the sucker.

 

[quote]Originally posted by Ray Calvo:
<strong>Sandman (whoever you are), don't know if this helps. I had an old Audi Coupe Quattro that had an ABS warning light come on. Turned out I had a bad wheel sensor.

You say "I know that this is not a malfunction because even with the warning lights the ABS works fine".... How do you know???? Do you nail the brakes hard on a slick surface and feel/hear the ABS working? Only way I would think you would know that it's working. I would imagine that if the warning system is lit up, the ABS/ABD is shut down. It was on the Audi, anyway - and Porsche literature indicates the same.</strong><hr></blockquote>

Ray,

Yep, that’s exactly what I did. Stomp on the brakes and see if you feel/hear ABS, even with the lights on. Like Sandman, my ABS appears to be working fine. This makes me think the lights are not indicating a problem with ABS/ABD (including wheel speed sensors) but are indicating some other problem.


[quote]<strong>Others seem to indicate that it could be a wiring harness problem. Could Be!!! There is an input from the throttle plate sensor to measure throttle position, so it might be that this is wired thru the wiring harness (then again, literature sez this is from the DME, not a direct signal). Works OK on initial startup, but once you're moving there's an incompatible signal between the throttle plate and the speed signal (ABD shuts off above 70KPH) and on come the lights.</strong><hr></blockquote>

Since I had the lights come on immediately after starting the engine one time, I don’t think it’s related to the 70KPH ABD limit.


My current hypothesis is what others have said all along: it’s a charging problem. The last time I drove the car was last week (I didn’t take it on my 7/4 vacation). The lights came on three times that day, which is the most they’ve ever come on. As an experiment, I hooked up a battery charger last night. If the car had been charging normally, I would expect the charger to indicate a fully charged battery in a few minutes . If there is a charging problem, the charger would have to charge the battery for a while. Well, the results are in: it took a couple of hours or so of trickle charging before the charger indicated a full battery. Yes, it could have been worse, but it should have been better. Also, the lights did not come on during my morning commute today. When I take my car in for its 60,000 service, I’m going to have the alternator checked out thoroughly (in addition to having the computer codes read).

 

From tom_993 [quote]My current hypothesis is what others have said all along: it’s a charging problem.<hr></blockquote>

Hope you're right, although from personal experience I wouldn't bet on it. I've had the battery drained and recharged a few times on my '95, and never had the abs/abd lights come on.

No info on low voltage causing a trigger of the abs unit, altho is triggered by "brake light switch", "wrong gear", or "Throttle info". is ABD is killed also apparently based on time demanded on to avoid brake overheat. Also does an ABS test (solenoid valves, pump, hydraulics) when speed exceeds 6 kph, and a speed sensor test when speed exceeds 12 kph, and another "dynamic" speed sensor test when speed exceeds 40 kph. Literature also sez if lights are lit, abs/abd is defeated.

 

Hi Ray,
Thanks for all your help and literature references. Every data point helps refine the search. I agree with you that it doesn’t make sense that the ABS and ABD lights would indicate anything besides problems with the ABS or ABD. It also doesn’t make sense that my ABS and ABD lights would light when they do (e.g. once immediately after starting the engine and one while cruising down the freeway at a constant speed for nearly an hour), or that they light simultaneously. I could see one or the other coming on if the problem were with one of the systems, but both? Puzzling.

The best information I have is from others who have also had the two lights come on. There are two examples in this <a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=3&t=002286" target="_blank">thread</a>. They both point to battery/charging issues. Yes, I hope I’m right too, but like you, I haven’t made any bets!

The lights have not come on since I charged the battery the other day. Perhaps I’ll continue this experiment by charging the battery every few days or so and see if it keeps the lights off.

Thanks again,
Tom
’95 993

 

Thanks alot guys for the update.

My car has a new wire harness, a new alternator and also a new battery. The lights come on and stay on untill I turn the car off after which sometimes they go away. Any suggestions???

 

Sandman,

You have a new alternator and battery? That’s bad news for me, as I was hoping that would be the fix. How new are they? Did you have the lights before you replaced them? If not, how soon after the replacement did the lights appear? If the lights came immediately after replacing them, the parts might be bad off the shelf.

I’m kind of short on suggestions for now. Take it to a dealer and have them read the pooter codes with the hammer when the lights are on. Have them check the entire charging circuit, the ABS and ABD, the wheels speed sensors, etc. Other than that, you could ignore the lights or remove the bulbs. Sorry, I wish I had a better answer. If you do find an answer, please let me know as well.

Tom