Silk

This week the ABS / PSM check lights popped up together with the 'drive to workshop' message while driving.

After a while I found out that when the engine is running everything works fine as long as I keep the lights switched off. As soon as I turn on the headlights the ABS / PSM check light goes on and my brake lights stay permanently on (not touching the brake pedal).

I quickly checked the wiring in front and back light units. All seems normal.
Also replaced this week the battery as it was loosing its power faster every week.

I ordered a new brake pedal switch to replace it as a precaution as it seems to be the culprit in many of the ABS / PSM warning lights. Did not have the chance to measure the current switch.

Find it strange that as long the lights stay off everything works fine. Any knowledge here where I have to look If the switch is ok?

I am running aftermarket LED units for the back lights.

I will be be able to read the OBD with durametric only by Monday.

AWDGuy

1st thing I'd try is the stock lights and see what happens

Silk

Also when it went fine for more then 10 months?

Macster

Reads like the alternator is not putting out enough electricity or maybe it does for a while after an engine start but then its output falls off.

Turning on the lights ups the car's electricity needs and the alternator can't meet those needs and the voltage level falls enough the ABS/PSM controllers detect a low power condition and both issue a warning.

Silk

Yes, that sounds also like a possibility.
Would also explain the fact the battery not being charged fully and cause the symptom of it draining quickly.

What should be the output voltage with the engine running?

I noticed when switching on the lights there is a strong electric whine when the xenons starting up, gradually becoming weaker and disappearing. Never paid attention to it. So do not know if it is normal or not.

Another thing I noticed with the lights off when actuating the brakes the parking lights in front would start to glow.

Silk

Will test alternator with multimeter on Monday. Any special procedure? Where to measure best. Is the battery a good location. Or better to use the terminals at the engine in the back?

When running the voltage should be between 13.5-14.5V all time while revving the car above 2k rpm or with all consumers (airco, blower max, lights, ...) on. Is that correct?
Als will see if there is an unusual voltage drop between the alternator and terminals.

Ahsai

What's the voltmeter reading in the instrument cluster with low beam and a/c ON at cold start at idle and when car is moving?

That voltmeter should read between 13.5 to 14.5v whenever the engine is running, at all revs at all car speeds with low beam and a/c ON.

Silk

Ok here are some results from my testing

I connected the car to Durametric and got the a P0571 code pointing towards the brake switch. I already ordered a new one last week so replaced the switch with the new one. The problem persists.
As soon as I switch on park lights or low beam The PSM ABS warning pops up and the P0571 code reappears after erasing all pending codes. As long as the lights stay off everything seems to be ok.

I used Durametric also to monitor two values which seems relevant te me: 'power supply' and 'supply voltage'. During a short trip the values were mostly around 13.5-13.7V irrespective of rpm. The lowest value I saw was around 13.4 the highest 13.9V. The in dash voltmeter showed a value very close to 14V.

Only the first time switching on the low beam I saw the voltage dropping for a split second, to 12.5V, but it then recovered almost instantly to 13.5-13.7V. After this first ignition switching on the lights did not influence anymore significantly the voltage (probably it is the 'cold start' of the bulbs which draws more juice, I hear also a buzz from the headlights). However no voltage drop is present it still triggered the ABS/PSM and P0571 code.

Tomorrow I will put back the original tail lights to see if this makes any difference.

Ahsai

My guess is your LED lights.

Silk

I also discovered that in the headlights the park lights were replaced by LED equivalents by the previous owner. Could they also interfere with the monitoring of the brake light circuit?

Ahsai

That's unlikely as the headlights don't have detection circuits like the rear lights. The car needs to see the load it expects from the rear lights, otherwise abs warning will light up. The LED lights have resistors in parralell to mimic the correct load.

Silk

Had the bright idea to disconnect both tail lights. No warning. Then connected the right tail light, no warnings, connected the left tail light, ABS/PSM pops on. As I move the left tail light I see the rear light flicker
then it stays off.

Swapped wiring harness between left and right. Left tail light functions properly, right one not. So after all the fuzz above is most orobably caused by a bad wiring harness.

5CHN3LL

Brake pedal switch? Or MAF? Oh, I see you had the brake pedal switch in your original post. Doh.

I have a decent-quality set of LED tails on my 996. When the car was stock, I never had the ABS/PSM light issue. Once I added the LED tails, it happened once in a great while (once or twice a month) - always when the battery charge was on the low side.

Upon installing an underdrive pulley, ABS/PSM became a daily occurrence. Probably every second/third drive the damn light would come on. The tails DO have resistor packs.

Since the tails plus the pulley seemed to be the cause, and I really like the tail lights, the pulley came off - and the problem immediately went back into semi-remission. Maybe once or twice a month, I got the ABS/PSM lights. Always when the voltage is low; NEVER the day after I pull the car off the maintainer.

After replacing my battery, the problem is completely gone. I expect it to return intermittently as the battery ages and starts to have a wider charge swing.

For peace of mind, I determined (in a safe place) that the ABS pump WAS still functioning even though the ABS/PSM lights were lit. I would test this myself if I were OP. Annoying lights that don't mean anything important is one thing. Annoying lights that are telling you your ABS isn't going to be there if you need it should be addressed.

Silk

omg a new wiring harness for the back lights costs 180$!
Found a second hand on e-bay, waiting for it to arrive and install.

Plenty of space here to test the ABS ... good idea

Silk

Found it. One of the flexible poles of the rear/brake light fitting of the wiring harness broke off.


I feel a bit stupid now. So once again it is true. Check the simple things first before looking for some more complex causes. I ordered a new loom via ebay.