Hi guys,

I have problems with my PSD unit.

It gives me "Differential Control Off" warning every time (or almost) I start the car. Many times even if the car is not rolling. After maybe about 10 to 15 seconds after the car is started.

No ABS warning at all. So I guess all my ABS sensors work fine.

Also, and that's a bit weird, there's a "tock-tock-tock" sound in the area of the PSD unit every time I turn the key on.
It starts off with maybe 2 or 3 "tock’s" per second, then the frequency increases to maybe 4-6 while it fades away. The whole thing lasts for about 5 seconds.
Exactly like describe in this thread:
http://forums.rennlist.com/rennforums/928-forum/294532-91-gt-knocking-sound-from-psd.html
I also can hear the pump building pressure and stops. Like it should be.

Last year I changed my pressure reservoir because it was leaking. I bled the system then but did not did it right.
I'm saying that because I did it again last week to try to fix my problem and there was a lot of air in the system. Both in the pressure reservoir and in lock slave cylinder and pressure line.
I did not have any problem (Warning message) last year. This time I did it right - I think - and made sure that there was no more air in the system.

Last night I did few things to try to know what cause that "tock-tock" sound.
1) I removed the fuse and the relay of the PSD unit (in the spare wheel well) and turn the key on. No PSD pump working - absolutely normal - but the "tock" sound was there.
2) I then removed the ABS fuse and relay (in the fuse panel) with the PSD fuse and relay still removed and no more "tock" sound!
So it looks like the ABS system is sending something to the PSD unit and cause that sound. At the same time it put the PSD unit off.

Before this problem occurred, I did a lot of work on my suspension.
I removed everything in the four wheel wells. I did a lot of cleaning and some painting there. I changed all my wheel bearings too.
I didn't touch the PSD unit though. I removed all the wiring connections to do the cleaning and put everything back except the PDK sensors.
As my system is bypassed, I can't see how it could cause any problem.
And like I said, I don't have any problem with the ABS system so I guess that my ABS sensors are ok.
I also cleaned all four ground on the wiring harness. Put it back there with new bolts with anti seized.

I did not have the system tested with a hammer yet. The only place around here -I think - with a hammer is the dealer.
Not a place I like to go! If I don't find anything, I won't have much choice. Won't have any repairs done there though.

There is a relation for sure between the "tock-tock" sound and the PSD unit down.

So, any idea what is happening there?

By the way, it was fun driving the car with the system off!
But as it is a fine piece of equipment and don't want to see that warning all the times, I would like to fix that for sure.

And yes I did a lot of search but did not find anything for that particular problem.

Any advices are welcome.
Thank you very much.
__________________
Bertrand
Cobalt Blue 1991 S4


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I checked continuity on the four sensor and found that the rear left is bad.
There is continuity between the two sensor wire (about 984) witch is good but some too from one wire to ground (about 984) witch I think is not good.
All other sensors have continuity between the two wires of about 1024. And nothing from wires to ground.
Also, when I plug all three good sensors but leave the bad one disconnected, I don't get the "tock" sound anymore. I can hear the PSD pump running and stop like it should.
But when I plug the bad sensor, the "tock" reappears. The PSD pump still working like it should.

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The tock tock that you are hearing is the PSD solenoid releasing pressurized fluid to the slave cylinder thinking one of your wheels is slipping. Your problem is with the faulty ABS sensor.

The ABS system does two tests on startup - one internal - ie pump and relays and then a secondary on all sensor signals once the car has reached more than 5mph/8km/h. If your car is up in the air, the secondary test cannot be performed, thus you are not getting the ABS Off message. Swap the sensor for a good new one and I guarantee you both your PSD and ABS will work well.
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Thank you.
Now it's getting clearer.
But when I was driving the car few days ago I got the PSD warning but no ABS warning.
That is strange!!!
__________________
Bertrand

I found something else this morning.

I checked continuity on the four sensor and found that the rear left is bad.
There is continuity between the two sensor wire (about 984) witch is good but some too from one wire to ground (about 984) which I think is not good.
All other sensors have continuity between the two wires of about 1024. And nothing from wires to ground.
Also, when I plug all three good sensors but leave the bad one disconnected, I don't get the "tock" sound anymore. I can hear the PSD pump running and stop like it should. But when I plug the bad sensor, the "tock" reappears. The PSD pump still working like it should.

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Per the Porsche 'ABS Test Program' manual, with your multimeter, you should see...

800 - 1,800 ohms between the two speed sensor terminals (disconnected) of each sensor, and 20,000 ohms between ground and any one terminal (also disconnected).