flamefront

Guys,

I've read all I can find on this great site and have tried many things to eliminate the warning alarm for ABS and the differential on my '91 C4 - I need some input from the group.

Symptoms:
Warning (!) light in the cluster stays on after engine start, all other lights go out except the light on the console highlighting the bypass switch. When I drive past ~12 mph, the warning tone goes off and the ABS and AWD warning lights illuminate. Hitting the bypass switch kills the warning tone, but the lights stay on. ABS does not seem to work on extreme braking - the left front wheel will lock up first (haven't pushed it harder to see what the other wheels will do. Strange additional behavior: The brake pedal in normal stopping does the ABS judder only during the last 5 feet of a stop, almost every stop. This may be a clue to something.


Here's what I've accomplished so far with NO success:
1. Confirmed that brake fluid level is correct
2. Confirmed that brake fluid level switch works with a DVM
3. Checked the R34 relay for activation. Using a jumper wire in the correct slots of the fuse box changes nothing. Relay clicks when energized with 12VDC, switched contacts show 0 Ohm when relay is energized.
4. All four ABS sensors show 1,100 Ohms. Connections to ABS leads are clean, pins are straight and clean.
5. All four ABS sensors have been cleaned of iron filings and debris
6. Removed both inertia sensors from front console, pulled off their covers and inspected them. They look good as new: no debris, arm moves freely, contacts and pins look good. Re-installed with no benefit.
7. Pulled the large connector off the front trunk's ABS computer. Saw some corrosion (slight) at the base of a few pins. Blew out debris with air gun, used a brass brush to clean up the pins, then blew it all clean again. Reinserted harness connector carefully and cycled the installation/removal a few times to scrape clean the pins. Re-attached, no change.
8. Cleaned main power lead to battery's positive terminal for small ABS wire.
9. Cleaned gang ground just behind battery and re-tightened.
10. Cleaned gang ground on passenger side front inner fender (brown wires bunched into two eyelets).
11. When the key is turned on and I insert the R34 relay, I hear a "clunk" from somewhere in the front tray/ABS equipment area. I am not sure if I hear a pump winding up. If I disconnect the car's battery (with the key "off"), I do hear a pump wind up when I re-attach the positive lead to the battery, and it stops after ~10 seconds. I assume this is the ABS pump pressurizing the system.

Still, symptoms persist. I guess the next step is to get the car scanned for codes on the ABS and AWD system. This is my tenth Porsche, my fifth 911, but my first C4, so whatever you can offer as advice, I will give it a try.

Thanks for whatever ideas you guys come up with -

Gordon52

Had all the symptoms you describe as far as the lighting and alarms,abs and brake lights would be on at start up with alarm...Alarm would stop after about a minute but lights remained on,at about 20 mph the lights would go out,however as the problem progressed the lights began staying on all the time. No noticeable braking issues however...In my case replacing the pressure control switch was the fix, part# 964-606-904-00. Good luck hope this helps !

flamefront

Gordon52

Thanks for the tip. Are there any pin-out tests to perform on a pressure control switch to determine if it is functioning? They are now $400, so I'd like to be sure before I fire that gun.

Gordon52

Flamefront

Sorry I have no specific test that determined the problem, I chased this issue down for over a year and nothing seemed to solve it,finally when the lights stayed on continually I broke down and took it in to a local Porsche shop.Been over a year now and all seems ok.

This is the Quote from my repair ticket.
"found warning light for brakes and traction control on-traced wiring diagram from instrument cluster warning light to abs/traction control module to brake pressure warning switch.found switch was leaking into connector and developed a pin hole leak in side of switch-replaced brake pressure sensor-system now operates as designed". $$$ Yup ! They also charged me for brake fluid so bleeding may also be needed. hope this info helps...

flamefront

Gordon52

That's excellent information. My pressure sensor is dry and the connector is clean as a whistle, but it may still be bad.

One encouraging thing is that folks seem to be driving with these warning lights on for years, so I guess I'm doing no harm in taking my time to diagnose it.

I'll do some more digging - the last thing I have not tried from the boards is to change the distributor cap and rotor, which are worn.

John McM

I had this alarm on my car. Turned out to be the pressure switch and the accumulator that were bad in my system. The shop tested the parts operation with a high pressure gauge.

John McM

In my case the pressures were all over the place, making the pump work very hard. I was also taking the car to the track. For safety sake and also to avoid spreading the repair bill I'd get it diagnosed quickly. The pressure switch is expensive but there is no other source. Just a part of 964 life.

flamefront

This all very good input - thanks. One additional diagnostic clue: When I first start the car the central warning light comes on, and the reset button's light illuminates. Once I start driving, the Diff warning light comes on, with the associated alarm horn. Sometime later (~30 seconds) the ABS light comes on. I have read that a failure in the AWD system will defeat the ABS system by design, so both systems go off if a problem is encountered in either. So, maybe my ABS system is fine and the fault is only in the AWD system?

At a rest with the key on (engine off) I can hear the diff lock actuating when I manipulate the diff switch in the console: A single click is heard when I turn it off, and a cascading triple click when I activate it. Does this shed any light on the subject?

Thanks to all -

LittleBirdPilot

Did you ever solve this issue? I have the exact same problem and will probably end up going to the pros soon. Thanks for any help. Graham

jlmartin94

hi ,
excuse my poor english
I had the same kind of trouble last year, and after ( very) long search I finally zeroed in on a very painful aspect of the ABS module ( regardless C2 or C4 )

when you switch the key in running position but BEFORE you start the engine , the ABS module is powered ( so is the high pressure pump too ) .
you can ear the pump clearly ( how long? ..it depend the pump , the accumulator etc.. normaly less than 60sec ) IF the time goes more than 60sec --> warning gond etc .. that point is well know i mean ..
you can hear the "clonk" when the ABS module activate the valves for testing purpose ..

BUT next step is to start the engine .. and the starter motor consume a HUGE current so the battery level ( "volts" i mean ) drop near a very low level ..
mainly if the high pressure pump is yet pumping ... !!

as SOON as the engin is started ( first cylinder/ first ignition ) the motornic know the engine is runing and "tell it" to the ABS module
The ABS module then start a Built In Test Equipement .... ( testing each ABs sensor etc.. ) and ..........test the battery level .

If the battery level is below ( or near ) 9 volt .. you have the same type of warning gong + light tha you can see ....
the KTS can read a failure " 43 - ABS valve Relay fautl ".... ( don't remenber the exact label ) but it is NOT not necessarily linked to the relay ... ( the one 928 615 124 01 )

It seem that the ABS module should wait some time before starting the BuiltIn Equipement Test but if your battery + starterMotor + pump motor are in ( very ? ) good condition ( don't miss the GROUND CABLE on the starter motor ..!!.. ) the voltage shouldn't drop below 9V ...
so should not be a problem ...

THAT is the point ...

you need a NON-digital voltmeter ( with a needle . ) to see the drop in the voltage level ..... the ABS module only measure the voltage once ... and then .....

this sort of trouble doesn't raise on a C2 ... because mainly there is no high pressure pump .. and the battery doesn suffer the 60sec pressure build .....

At least that's how I solved my problem for my 91 C4 ...

hope it help ..

flamefront

jlmartin94

That's an excellent observation - I will take a look at my voltage drop during engine cranking.

I have not been able to solve this issue, but have come to understand it a bit better. My central warning light comes on immediately upon engine start, and will go away with the override button on the console. Once I drive over ~30 mph it will come back on again showing the AWD warning light. Sometimes the ABS light comes on at this point and sometimes it does not.

I have checked all my ABS sensors and they ohm-out at the correct level. Since my ABS light does not illuminate at first, I have become a bit comfortable with the idea that the sensors are okay...Something is triggering the central warning light at engine start but not triggering the ABS warning light until I drive.

I will disconnect an ABS sensor and start the car and see if the ABS light comes on to confirm that a faulty ABS sensor will throw an ABS code on engine start and not only after driving.

On a related note, I went to measure the ABS sensor resistances at the ABS module connector, and my connector does not match the factory wiring diagram for a '91 C4. I am missing a terminal in my connector at a few spots...I have to dig around and see what this means. The terminals have not retracted, they are just not there, but they are called out in the wiring diagram. Mysterious...I could blame the connector but there are a few times when my car runs fine and the warning lights do not illuminate, so it must be some other intermittent fault, not a permanent wiring harness problem.

Related to this issue is that I have had a bit of the Christmas Tree effect on my warning lights due to a bad ignition coil. I have isolated the bad coil and have a new unit on its way. There is a long shot probability that my central warning light is caused by this bad coil's influence, but I am not so sure.

I have bled my transverse diff lock slave cylinder and it was in fine shape - no air bubbles, etc.

The search continues....

jlmartin94

yep .. there IS a second BITE ( BuildIn Equipement Test ) after the car reach approx 30mph .. but this second BITE is about testing the "active" parameters ..

first "BITE" : testing voltage , presence of ABS sensor , resistnace of ABS sensor, resistance of solenoid, etc ..

second "BITE" : testing the "waveform" of the ABS sensor .. ( for example )

I suppose you have to clean well the 4 sensor ... the electrics contacts ...

flamefront

I had heard about the BITEs - thanks for the more complete description.

I have cleaned all four ABS sensors, and they all measured out fine, but now am going to check that there is continuity to the ABS control module from each sensor. The next step would be to look at the waveform for each sensor at the control module connector with an oscilloscope, which is one next step.

I can report more once this step (and the new ignition coil installation) is complete.

Does anyone know which AWD sensors would trigger the Central Alarm upon start up? Something like the center diff lock or similar must be malfunctioning. I performed the test suggested by Adrian where you engage the diff lock and drive up to 40 mph and look for it to disengage on its own. On the one time I tried this it worked as it should, but the light and alarm still persist.

I have ordered a diagnostic harness from Bergville and we'll see if it can read the ABS/PDAS/AWD trouble codes for me...

flamefront

Update: I measured the ABS sensor resistance values at the ABS Module connector and found that my right rear sensor "signal" is not reaching the module due to some harness issue (connector, etc.). The sensor itself measured out at 1kOhm, so I will look at the wiring and find the fault. Once this is corrected, perhaps my AWD warning light(s) will go away. I will update soon.

sonett43

interested in what you find, I have a similar issue with my left front sensor