ABS help?

[gardoni]

I've had my 964 for several weeks now. Within a couple of weeks of receiving it, while out driving, I had the ABS and all wheel drive warning lights come on coupled with the warning beep. Hitting the "!" button kills the beep, but the warning lights stay on. Restarting the car clears the lights for a while.

Brought it in to a local shop and they diagnosed a #15 fault and advised I replace the ABS control unit. I bought a known good, used control unit from someone here on the list and installed it. Drove the car less than 2 miles and got the same warning light/beep again. This seems to be happening more frequently now, and when it does I am not actively using the brakes or even driving "spiritedly".

I've gone back through what records I've got and found:

Jan '08- Fault codes 34 and 24. Relay (928 618 931 02) and Accumulator replaced. Note on receipt indicates left rear ABS sensor recently replaced and that there seemed to have been metal shavings on the old sensor (!?!?)

Other than that I know an ABS relay was replaced about a year ago, but don't know which one.

Before I go replacing other parts that aren't bad does anybody have a good guess of what's going on, or where to look next?

Many thanks!

[Hamptonsam]

I was having similar issue to you on my '90 C4. I cleaned the front wheel sensors (since I was changing the brakes and rotors anyway), however that did not help. What has helped so far was to top off the brake fluid. It appears that these cars are very sensitive to brake fluid levels. Since I added the brake fluid to the max level, the light has stopped coming on. Knock on wood.

There may be others that can chime in with more technical knowledge. Try the easy stuff first.

[kang]

I had ABS warning lights a while ago. I resolved it by cleaning and reseating all the electrical contacts to the pump and the control unit. There must have been at least one connection among all those that was intermittent. I suspect the ground wire on the ABS pump itself. It seemed loose and I was suspicious of it at the time. Regardless of which one it was, I recommend giving this a shot. Remove and replace and wiggle the plug to the control unit several times, that will clean the contacts. Loosen, wiggle, and then retighten all grounds or other contacts. This can’t hurt, and it might solve the problem. It solved it for me.

[john walker's workshop]

i second the check the brake fluid level recommendation.

[gardoni]

Thanks all. The brake fluid level looks fine (high actually), but I'll clean and reseat all the electrical connections I can find. Pretty sure I have a full schematic in pdf somewhere...
Again, many thanks for the input.

[JB]

I have a 1990 C4 and have been having the same problem, only now, the ABS pump is not running at all and the brakes are in the "non-powered" mode. By that, I mean the hydraulic booster is not working. They will still stop the car but it requires a lot of pedal pressure. I have un-plugged and replugged all of the electrical plugs and cleaned & tightened all of the grounds. In my diagnostics, I have determined that the ABS relay in the fuse box is good, but not energizing and it is the relay ground that is not present. I have traced the output from the relay directly to the ABS pump and the pump works when I jumper 12 volts to it. Does anyone have a schematic so I can see where the relay ground is supposed to come from?

By the way, I was getting the warning lights intermittently as well. When they increased in frequency, I topped up the brake fluid and they seemed to go away for a few weeks. Strange.

Thanks,
JB

[gardoni]

Cleaned all the electrical contacts I could find and reseated all the relays. It does take longer for the light to come on now, but it still comes on.

JB- have you thought about just running a new ground wire for your ABS realy?

[JB]

I did more diagnostics this afternoon and am 99% sure the problem is the pressure switch on the ABS pump. The pressure switch provides the ground that engages the relay that runs the ABS pump. With no pressure in the system, I have no ground signal from the switch. I can jumper a ground and the pump runs. I'll update when I get a replacement switch installed (964 606 904 00 - $301 from PP). (much better than a $4K control unit!)

According to the schematic, I believe the warning lights are tripped when the system pressure at the pressure switch falls below a certain level. That might explain the warning lights coming on before any indication of system failure. According to the manual, it takes 25 applications of the brake pedal to relieve the system pressure completely. For those of you experiencing this problem, the failure is not catastrophic as the brakes will still work if you press the brake pedal hard enough. In fact, I can still lock them up if I stand on them as hard as I can. Feels like an American car when the motor dies.

Fortunately, the switch is a component that after replacement, according to the factory manual, you can bleed without a Hammer.

JB

[JB]

Gardonis, It might be dangerous to run with a jumper-ed ground to the relay. If I did that, the pump would run continuously and would certainly overload something in the system. The pressure switch is supposed to provide the control for the ground thus turning off the pump when the pressure has reached the specified level (180 bar ~ 2700 psi i believe.) You can imagine what might happen!

JB

[gardoni]

JB- Understand now. I was thinking the ground to the relay was constant, but it sounds like it only completes the circuit based on the pressure switch state. Yeah, that would be bad to jumper full time. Any chance you could post the procedure for replacing your pressure switch?

I reseated the ABS relay in the main fuse panel and also rechecked the wiring at the brake fluid reservoir, but haven't driven it yet so I don't know if that helped anything. The fluid in the reservoir is above the MAX level, but there's room for a bit more, so I may try adding some.

I did notice that there is some melted plastic on the brake booster below the pressure switch. It looks to be only on one small spot on the exterior- it doesn't look like anything got hot enough inside to melt. I'd guess that it is unrelated, but will post a pic when I get a chance.

Dyeing the new dash tonight...

[gardoni]

Reseated the relay in the main fuse panel and removed the brake fluid reservoir cap and shook it back and forth to make sure the valve was moving freely. Drove the car ~40 miles yesterday and got no ABS lights/horns. Maybe fixed? I'm skeptical.

[colo964]

Clean the ABS sensors, they tend to build up metal filings.

I solved my ABS/AWD warnings by cleaning nd recentering the "pendulum" in the longitudinal and transverse sensors. They are in the console - you'll find a cutout on the right side of the carpeted part, but likely need to remove the entire console to ensure you don't strip the phillips screw heads.

[dypascal]

Hi, I've Got The Same Problem On My 964 Turbo And The Light Come On More Often When I Drive On Bad Roads Or When I Pass In A Hole.i Don't Have Any Doc For My Car And I Think That's Come From The Abs Switch.another Thing,when I Make A Bridge On The Brake Wire Reservoir,the Light Doesn't Come On.strange!!can Someone Help Me?
Sorry For My Bad Knowledge In Your Language.

[JB]

Here is an update on my situation. I replaced the pressure switch and bled the system and the problem is solved. No more warning lights and everything seems to be working great.

Colo964, nice to hear that the acceleration sensors are somewhat servicable. Did the car give any indication other than the service lights that something was wrong? Does anyone have any insights on how you can tell if the 4-wheel drive is working properly?

JB

[colo964]

4WD is AWD. It works all the time. The hydraulics and sensors only act like an active limited slip diff - shifing the torque distribution.

If you let it idle, turn the knob to lock your diff. When you drive off, it should steer like a tank until you get about 20 mph. That means they are working - not neceassarily in real time driving, though.

Other than that, when there is a sensor fault, mine drove like I unplugged the ABS relay, i.e., on dry roads, it really didn't seem to matter - constant torque distribution.

I wouldn't experiment with it on a snowy S curve.

[dypascal]

HI JB,iS THE SWITCTH DIFFICULT TO REPLACE AND HOW DID YOU BLEED THE SYSTEM?
THANKS FOR REPLY

[dypascal]

Originally Posted by JB

Here is an update on my situation. I replaced the pressure switch and bled the system and the problem is solved. No more warning lights and everything seems to be working great.

Colo964, nice to hear that the acceleration sensors are somewhat servicable. Did the car give any indication other than the service lights that something was wrong? Does anyone have any insights on how you can tell if the 4-wheel drive is working properly?

JB

Hi,where can i buy a booster press switch and what is the part number.Thanks

[gardoni]

Cleaned the wheel sensors on the left side of the car (all I had time for) and went for a drive. ABS/AWD lights and warning buzzer going off repeatedly- more frequently than ever. Just cleaned the right side (much dirtier) last night, but no time to test drive.

The sensors on the left had what appeared to be a plastic housing, while the ones on the right were definitely metal. Also the (I'm guessing) magnetic sensor itself protrudes from the sensor body about 1.5mm on the right sensors, but is flush on the left. I'm guessing these are 2 different models of sensors? I do wonder if they behave somewhat differently and if that could be part of the problem... Of course the amount of grime on them certainly wasn't helping anything and I guess I'll know more once I test drive.

dypascal- I believe you can order the pressure switch through Pelican Parts. Part number 96460690400 for my car.

Thanks everyone for their continued interest and help on this annoying problem!

[dypascal]

Hi Gardoni.Thanks for your help.All the best

[gardoni]

pas de quoi