Front left brake applying itself on its own (C4)
 

I have a 94 C4 that I recently vacuum bled the brakes on. I now have problems with my front left brake applying itself without me even touching the brakes. I will be driving for several miles (car completely warmed up) and while going 50 mph or so and without doing anything the front left brake will gradually start applying itself. The steering wheel will shake and the braking pressure eventually becomes significant enough over the next minute that the car would come to a stop from 45 mph in about 500 feet if I let my foot off the gas. I have to stop when it happens. When I do the brake will slowly loosen its grip from the caliper and the car will roll freely. The front left wheel will be very hot to the touch while the other three are cold. The whole process is about 3 minutes and happens about every 50-100 miles. I have now read that on the
C4 (964) the proper way to bleed the system is with the *system
tester 9288* which opens the solenoid valves, lock cylinders and
master cylinder circuits. Do you think it is possible that my
front left brake is applying itself because I didn't use the
9288 tester? Or could it just be a coincidence and a proportioning valve or even the
ABS brain went faulty on me the same time I bled the brakes? Any suggestions? Should I have the dealer do a bleed to see if it goes away? Anyone else ever run into this or bled their brakes on a C4 without incident? Please help, I'm afraid if I take it to the dealer they will do two hours of checking and come back with a "could not dupicate" but thanks for the $150.

Took the car to Stoddard yesterday for a bleed. Told the service advisor about the mysterious front left braking while cruising and not touching the brake. He bled the system and said there wasn;t any air in the ABS. He suggested I have both front calipers rebuilt for $154 plus 6 hours labor, or about $575. So this mysterious self braking happens to me again today and this time it was the left rear! So I'm figuring the problem can't be mechanical in nature, I'm sure it has something to do with a computer control. What might cause a single brake to apply itself and vary from wheel to wheel? Only one wheel will be hot and the other three cold.

George
94 C4

That kind of bothers me that Stoddard would be so 'normal' in the sense of treating your problem just like the local Ford or Chevy dealer does, i.e. 'shotgun it' till the customer goes broke! If PCNA doesn't have an answer better than what Stoddard suggests, how about just pulling the relay or fuse to the ABS 'brain' ... and use 'normal' disk brakes the way all 911s did prior to 1990? Me thinks 'HAL' has got it 'in' for you, one way or another!!!

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Warren Hall
1973 911S Targa

According to the shop manual the system tester is not required to bleed the brake master cylinder circuits or the clutch hydraulics. It is required to bleed the high pressure circuits that control the 4WD solenoid controlled locks and the brake booster circuits. The summary being that as long as the only bleed valves opened were the ones on the calipers - theoreticaly there is no mechanism that would affect the ABS system.

The prior comment that the shop (Stoddard?) bled the ABS would need the system tester as this is the high pressure circuit, but you did not mention whether they used one or not.

Rgds: Bob Ellis
90 C4

Robert, yes, Stoddard (A Porsche dealer) did bleed the ABS unit with the system tester. No air was found. Early S Man, you have a great idea on pulling the fuse, that would help isolate the problem. But 2 things: Does the all-wheel-drive rely on info from the ABS, such as wheel speed? And two, I'm thinking I might get the loud warning buzzer every time I start the car with the ABS disconnected. I have a call into Stoddard letting the service advisor know that a different brake caliper did it this time. I asked him to talk with the Porsche techs and possible call Porsche to see what the fix might be. I will certainly keep the group posted.

G

I think I am beginning to understand why the race drivers DEMANDED a big toggle switch to disable ABS on the dash of the 908s and 917s that had experimental ABS systems at Le Mans in the early '70s!!! I should have known there would be a stupid warning buzzer for the ABS, but I some how EXPECTED the switch to disable, just as a courtesy! I would suggest a test drive to see if all of your calipers are working properly, and highly recommend the following procedure on a lightly-travelled, two-lane road not likely to be patrolled by state troopers: A couple of moderate stops from 60 and 80 mph just to warm-up and 'exercise' your brake system, followed by a full-pressure, maximum effort, panic stop from 100 mph, but do NOT lock-up any wheels! With, or without ABS, if your car stops without any pulling to one side or the other, and without locking up any particular wheel, and if the stopping distance is satisfactory, I think you can tell the Stoddard service advisor, and what ever mechanic will be working on your car ... that the calipers are NOT the problem, it really IS an ABS 'brain' problem causing this undesired caliper application of an unexpected nature, every 100 miles!!!

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Warren Hall
1973 911S Targa

George, with reference to your question regarding whether the wheel speed sensors are a required input to the 4WD system. The manual is not much help here as there is no circuit diagram for the computer only pit out information. I would speculate that the lateral and longitudinal accelerometers are the primary inputs to the 4WD system not the wheel speed sensors.

Does anyone out there have access to the actual schematics for the computer systems on the C4?

Rgds: Bob Ellis

George, with reference to your question regarding whether the wheel speed sensors are a required input to the 4WD system. The manual is not much help here as there is no circuit diagram for the computer only pit out information. I would speculate that the lateral and longitudinal accelerometers are the primary inputs to the 4WD system not the wheel speed sensors.

Does anyone out there have access to the actual schematics for the computer systems on the C4?

Rgds: Bob Ellis

George, with reference to your question regarding whether the wheel speed sensors are a required input to the 4WD system. The manual is not much help here as there is no circuit diagram for the computer only pit out information. I would speculate that the lateral and longitudinal accelerometers are the primary inputs to the 4WD system not the wheel speed sensors.

Does anyone out there have access to the actual schematics for the computer systems on the C4?

Rgds: Bob Ellis