Thumping noise when turning left or right

[TorqueHead]

Hoping someone can help me out.

My 92 C4 has a consistent thumping noise coming from rear wheels when turning left or right, even in a very slight turn.
The thump frequency is tied to speed. Faster speed, faster thumping. If going in a straight line, no thumping is heard.

Backstory:
Car is new to me and was stored for about 15 years. I have replaced all fluids except transmission and front differential oil. Those are next on the list. Diff locks have been flushed with UDT999.

Car is now back on a lift with all 4 wheels hanging free. When I rotate a front wheel, the other front wheel will counter-rotate. When I rotate a rear wheel the other one always spins in the same direction. Is this normal?

When I rotate the traction control knob clockwise, I get one fairly quiet knock from the diff lock slave but when I rotate the knob counter-clockwise, I get a series of 3 or 4 knocks that start loud and then get progressively quieter. Regardless of knob position, the yellow light stays on. This sounds messed up to me, but this is my first Porsche so I turn to the Rennlist community for guidance. It seems like maybe the transverse diff lock is stuck in the fully locked position??

I have checked for play in wheel bearings and CV joints and all looks good. I can slide the half axles in and out a small amount and they slide smoothly. CV joint boots look ok.

(also posted on Rennlist)

[TorqueHead]

Nobody wants to weigh in on the problem?

Further testing has shown that both rear wheels are locked together. This may or may not be normal. I know it is normal on a C2 but this is a C4.

With all 4 wheels off the ground and a man on each back wheel turning in opposite directions we cannot make it slip. It is as if the diff lock is on.

I have been reading up on breakaway torque for the C2 limited slip differential, but I cannot find any mention of breakaway torque for the C4. Does anyone know if this applies to the C4 and if so, what is the torque value?

Hope someone can offer some guidance here.

[bazar01]

Do you have an option code sticker somewhere that you can decode?

[TorqueHead]

Yes I have decoded all options for my car. As far as the AWD and ABS are concerned there are no options. It is a standard PDAS.

[TorqueHead]

I believe I have identified the problem.

The slave cylinder for the lateral differential lock was stuck in an extended position which was exerting force onto the clutch packs which in turn, locked the wheels together.

By loosening the nuts on the slave and allowing the slave to move out by 1/4" the force was released and the rear wheels can now rotate independently.

Hopefully this was the source of the noise. I will report back after I rebuild the slave.

[bazar01]

My C2 LSD has a preset spring pressure on the clutch plates.
So the C4 rear diff engagement is achieved thru a hydraulic cylinder pushing on the clutch plates?
That's nice!

So hydraulic cylinder is activated and rear diff clutches is staying engaged all the time and ramps are making thumping noises due to difference in inner and outer wheel speeds?
Must be a control issue.

[TorqueHead]

Adrian has written a nice article on the C4 PDAS that is available for free. I will try to post a link here.
https://www.adrianstreather.com/docs/PDAS%20Technical%20overviewR2.pdf

I believe my issue is a mechanical sticking of the slave cylinder itself, as opposed to a control issue. I will know more when I remove the slave this week.

[bazar01]

What's commanding the slave cylinder to extend?
Hydraulic pressure
Where is the hydraulic pressure coming from?
Hydraulic pump.
Where is the hydraulic pump located?
What turns on the pump?

[TorqueHead]

Originally Posted by bazar01

What's commanding the slave cylinder to extend?
Hydraulic pressure
Where is the hydraulic pressure coming from?
Hydraulic pump.
Where is the hydraulic pump located?
What turns on the pump?

There is a PDAS computer in the frunk on the passenger side that controls the slaves and the ABS. There are two slave cylinders; One slave is for rear wheels and the other is for front wheels. The inputs to the computer are from the 4 wheel speed sensors and the two accelerometers.

Hydraulic pressure comes from a frunk-mounted pump and is held in the accumulator. Hydraulic pressure is held at around 180 bar (2600 PSI)

The pump turns on with ignition key and builds pressure in the accumulator.
The PDAS computer decides when to activate the slaves and by how much to activate the slaves, thus controlling the amount of torque going to the rear wheels and can also vary the power transfer ratio between front and rear wheels which is normally 69/31 with 69% going to rear wheels.

I suspect after 15 years of storage with old brake fluid in the slave, the slave stuck after the first extension. The slaves are cycled as part of a safety check each time the car is started. I can post photos of what I find when I take it apart.

[bazar01]

Well that explains the extension.
Rust build up inside the slave will not keep it extended but will damage the seal and cause a leak. I suspect, a solenoid valve is getting stuck keeping the pressure applied or not released.
A fluid flush and cycling the valves might help.

[TorqueHead]

As I understand it, the only mechanism for return of the extension is spring pressure, after the hydraulic pressure is released. So to me, it would seem that rust could prevent the return stroke.

Fluids were flushed and valves have been cycled. But you have a point that if the control solenoid got stuck after the flushing/bleeding, then it could maintain pressure in that line. I will know if that is the case, as soon as I crack the hard line that feeds the slave.

New seals are on the way. At 30 years old, they are likely due anyway.

Thanks for your input.

[bazar01]

Originally Posted by TorqueHead

As I understand it, the only mechanism for return of the extension is spring pressure, after the hydraulic pressure is released. So to me, it would seem that rust could prevent the return stroke.


Thanks for your input.

Spring pressure to retract the slave cylinder under 180 bars does not make sense.
There has to be a solenoid valve to release fluid back to reservoir.
Do you have a hydraulic circuit diagram?

[TorqueHead]

Exactly! Which is why I said "after the hydraulic pressure is released".

[TorqueHead]

I was probably not being clear when I said "after the hydraulic pressure is released"

I should have said "after the hydraulic pressure is released by the solenoid valve. This is done by a command from the PDAS computer to the solenoid valve after the PDAS has determined that the differential lock is no longer needed.

Sorry for any confusion.

[bazar01]

Here is the PDAS pinout that may help test if the valve is getting activated.

[TorqueHead]

Thanks bazar01,
I will post back here with any results. The C4 is a wonderfully complicated piece of machinery, eh?

[bazar01]

Originally Posted by TorqueHead

Thanks bazar01,
I will post back here with any results. The C4 is a wonderfully complicated piece of machinery, eh?

It is, up in your neck of the woods.

Here in the south, nah. Too many components to go bad.

[TorqueHead]

I guess my attempt at sarcasm failed. LOL.

Its a wonderful system for rain, ice, snow or any slippery conditions but it is also supposed to be very good at preventing the back end from passing the front end under hard cornering on dry pavement.

But yes, wonderful when it all works as it should. Not so wonderful when it needs repairs.

I have still not had a chance to dive deeper into the problem. Soon, I hope.

[bazar01]

Get on it and so you can start driving the s#$t out of it.

The C2 is also a magnificent machine. It just keeps going, and going, and going. The last work I did was the shifter and coupler bushings.

[TorqueHead]

I will get on it as soon and I get my motorcycle put back together so I can drive the s#$t out of it before the snow flies.