in all my trawling through different websites and forums i have never come across two accelerometer failures occuring simultaneously.

The only thing to do is go through a session of replacing parts with a known good C4..thats what Steve Bull did to diagnose my ecu failure.

give me a shout if you need a C4

Doesnt sound to me like they opened up the sensors and cleaned them especially considering the price of them if they broke one whilst doing it....

 

Thanks for your offer Ade. I'll certainly bear it in mind.

Dave

 

The Axial is in front of the shifter, Lateral is all the way forward.

From the Shop Manual, Volume IV, Chassis. Pg 45-1.

Dave, if you need more info from the manuals let me know and I'll see if I can't pass along the pertinent bits.

 

For what it's worth - when my accelerometer was jammed, it did cause me ABS problems - and it didn't result in any warning lights. I found that if I braked hard, it felt like the brakes got totally released rather than ABS kicking in - and when it was finally fixed, I discovered that had been happening way eariler than the ABS would have cut in anyway.

But I agree with the general view - it doesn't seem likely both of yours are stuck.

 

Dave (F) - Much appreciated. I DO have the manuals and will be studying them over the weekend.

Dave (K) - I agree.

Dave (Boy, there are a lot of Daves around here!!!)

 

About 10 years ago I had an ABS failure of the type you described and it turned out to be caused by a bad ignition switch. I do not recall the exact failure mechanism nor do I have the circuit diagrams handy to check this out now, however, I would check this (and all other simple fixes) first. Accelerometers are generally quite robust sensors.

 

Ditto to nearly everything above! My first warning lights came on when I was forced over rumble strips via a lane shift in a construction zone. I nearly passed out when I saw the $1,000-$1,200 price tag. Then removed both, opened (they looked spotless) cleaned using electric motor cleaner. No problem for two years, until a week ago .

Shop hooked up the hammer, ditto above, longitudinal. No good solutions to this point.

Three Daves, thanks for great posted info .

Please keep us all posted if a solution is located.

 

Just a quick update. Having collected the car today I managed to establish that the shop HAD cleaned the accelerometers inside but a test performed using the Porsche test equipment revealed that both units, although functioning, were outside of the specification. This still surprises me that it should render the ABS competely non-functional but I've got no other option than to accept it.

I might see if any units are available from the usual breaker suspects but I'm reluctant to pay too much for units which may be no better than mine.

I wonder how many other C4s are out there with the same problem??

ETA: I also found out today that, contrary to what I read into Adrian Streathers description, both the axial AND lateral accelerometers are ONLY fitted to the C4. On page 221 of his book Adrian implies that it's only the lateral that fitted only to the C4, implying that the axial is also fitted to the C2. It isn't, it just ain't there. We checked a C2 and a Turbo (with the high pressure system) and neither is there.

Dave

 

What he said!

It worked for me as well. Just be careful breaking the seals on the cps (cheap plastic s#&@), clean the contacts of debris if any, I also gave the contacts a shot of tuner cleaner for good measure, reassemble and test. I cleaned both of mine and haven't had a warning light since. Been over a year ago.

 

Dave,

Have you tried measuring the electrical signals coming from the accelerometers your self? It would be interesting to see how they compare to the Shop Manual reference values.

If I find time to check mine I'll post the values.

Best of luck. Hope you get it sorted.

 

I am highly dubious of anyone who says two units have failed at the same time with the same symptoms......Lightning striking twice does happen but...........

Kevin

 

+1

and we would have seen lots of threads on here about failed accelerometers. Therefore I think if you try some used ones from a breakers (and clean them first) you've a good chance of getting some good ones for a fraction of the price of the new ones.

 

To be clear, I cleaned the lateral accelerometer and tested again and still had all the ABS/PDAS warning lights and horn. I then cleaned the second one and the problem went away. Only one accelerometer was causing the problem.

Interestingly enough, I went back to my written records and found that the lateral acc had metal shavings in between the swing arm and sensor housing. I cleaned the debris, retested on the car, but it did not solve the problem.

I then cleaned the longitudinal acc, but I found it was designed differently and the swing arm was not accessible, in my opinion, without destruction. So, I cleaned the pcb, reassembled and tapped the housing gently on the work bench. If there was debris in between the arm and the sensor, I dislodged it , because the problem promptly went away and has not returned.

 

The PET shows the lateral and longitudinal accelerometers with the same part number : PET 901-03 #4 993 606 153 00

However, I've also found 928.606.153.00 referenced as the part number. This is, in fact, the number on my spare unit. I had assumed this was an upgraded part.

Do you know what part numbers were stamped on your units?

 

Unfortunately, I did not write them down. I could be the only one to have run across a mix of parts. Most posts I found did indeed describe the parts to be identical in design, with exposed swing arms or pendulums. My car is an 89 model.

The longitudinal acc did not have the swing arm passing between two sensor coils like the lateral one. Thats not to say it isn't there, but I did not see how to go any further without things becoming destructive. An Xray would tell whats in there, though.