Bertrand Daoust

Hi guys,

I have problems with my PSD unit.

It gives me "Differential Control Off" warning everytime (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 everytime 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:
https://rennlist.com/forums/928-foru...-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 bleeded 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 alot 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.
I used this method to do it and everything went very well this time:
http://928oc.org/journal/psd1.pdf
It's also interesting that I don't get any "tock-tock" sound while doing the bleeding procedure.

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 wheell well) and turn the key on. No PSD pump working - absolutly 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 occured, I did a lot of work on my suspension.
I removed everything in the four wheell wells. I did a lot of cleaning and some painting there. I changed all my wheel bearings too.
I did'nt touch the PSD unit though. I removed all the wirring 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 wirring harness. Put it back there with new bolts with antiseized.

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 mutch 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 alot of search but did not find anything for that particular problem.

Any advices are welcome.
Thank you very mutch.

john gill

You did not say whether the pump is running or not ?

THe system works by when the ignition goes on the pump starts and pressurises the system , when it reaches it preset pressure , ot switches off ( this is 80 bar by the way , so do not get in the way of this without serious protective gear , as at thet pressure the fluid will inject into the skin eyes etc , permanent damage) . WHen on standby its for signal from psd controller to apply locking pressure to the differential plates .

If this is not happening then therein lies the problem , if the pump runs but wont swich off , the pump will be faulty as it will be unable to reach its shutoff pressure . Then will require replacement .

borland

John, He said the pump was cycling, so apparently the pump and limit switches are working properly. What he is hearing clicking is either the relay in the spare well, or the solenoid valve.

Bernard, What brand of brake fluid are you using? The ATE brand typically has a higher viscosity than other brands, so it helps to maintain the system pressure when the car sits for very long. That can extend the life of the pump by eliminating the number of repump cycles.

Bertrand Daoust

Borland,
I used some Prestone Synthetic Dot 4 Brake fluid.
As mentionned in my first post, I can hear the pump cycling and stop as it should.
The tock sound happens at the same time and even a bit longer.
What I find strange is I don't get it when I bleed the PSD activating the pump and the solenoid. Only when the ABS system's working. I mean when I switch the key on with ABS fuse and relay in place.
I don't fully understand the relation between the ABS and PSD.
There is something happening between the two when I switch the key on that makes that sound and shutting the PSD unit down.
I have to mention that I hear that sound most of the time but sometimes the PSD works fine. At least, it did that two days ago.

Any advices are welcome.
Thank you.

borland

I see. Both ABS and PSD are controlled by the same controller. PSD gets wheel spin data signals from the ABS side. So, if the problem doesn't go away by itself, you might want to try another PSD/ABS controller.

Bertrand Daoust

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) 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.

All this as been done with car on jack stands, so not rolling.

There is good chance I think that the PSD shut down because of that sensor. At least I hope!

Now, what I find strange is why I don't get any ABS warning with one bad sensor?
Could it be good enought for the ABS but not the PSD???

Let me know what you think.
Thank you.

Cheburator

The tock tock that you are hearing is the PSD solenoid releasing pressurised 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.

Bertrand Daoust

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!!!

Any thought?

borland

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).

Not sure why your ABS light is not functioning, unless there is no fault code stored for the sensor in question. One of the 928 diagnostic testers would be a good tool to have with your problem.

Bertrand Daoust

Good news!
Like I said, the left rear sensor is bad - grounded.

I find out that only removing the sensor bolt fixed everything. Not grounded anymore and no more PSD problem!
Weird but true!
Of course I will need another sensor soon but for now everything works like it should. Tried the ABS on hard braking and the PSD on gravel and everything is like it should.
And yes, it would be VERY nice to have the diagnostic tester indeed! Quite expensive though.

Thank you very mutch for your help.