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964 C4 pdas slave cylinder pressure valve what does it do
I am wondering if anyone knows what part number 27 in the diagram below does and how it operates. It’s a pressure valve for the pdas slave cylinders part number 92831521102.
I am having an issue where my rear differential is slightly engaging when it’s not supposed to which causes an slight grinding noise around corners. If you know how the c4 diff operates it is an open diff unless it meets the perameters for the computer to envisage the diff plates. It is making this noise when it should not be engaging. It seems when the key is cycled the pdas solenoid block and slave tests itself. The slave should completely retract but I my rear diff it is not. The problem gets worse the more the key is turned on and off. If I crack the blender on the solinoid the pressure bleeds off and it retracts so I know it’s not a sticking slave. If I crack the bleader to remove the pressure after the car has started and the pdas self test has been done the noise stays away which eliminates the pdas valve block being an issue. So I have it narrowed down to the rubber hose being faulty and not allowing the pressure to bleed back. Or I was just looking at the parts diagram to order the hose and I noticed there is also something called the pressure valve. It seems like that could be a culprit as well
. If anyone knows how it works let me know. See the attached photos
For having replaced myself all the Gylon seals and discs in the two differentials of my gearbox last year, here is what I understood of the system:
the big difference between a brake caliper and the PDAS actuator system is that in the brake caliper, the elastic deformation of the joints around the pistons ( when braking )
allows the piston to return a little back after releasing the brake - thus leading the brake pad NOT to touch the brake disc anymore.
When these joints are tired, the brake pads retreats too much leading to this feeling of soft / dead zone in the brake pedal.
On the PDAS system, the receiving cylinders do not have this elastic join system ( it is specific joints, see the post of neinmeister for the remanufacture of these joints),
therefore have no slack and the piston of the slave cylinder does not "backs off".
I suppose it is necessary so that the discs (Gylon or Valeo) of the multi-disc clutch of the two differentials are always slightly in SLIDING contact.
The goal seems clear for me : when the PDAS box sends the order of "tightening" discs (which is multi-stage, it is not ON or OFF) there is no dead time or delay in the application of the pressure, because the discs are "already" in contact ...
Hence their wear ... EVEN if they are NEVER used to "straighten the trajectory"
that's why each time the starting key is cycled, the pdas brain is just checking there is this "minimal" pressure in the circuit .
the only way the release this minimal pressure is crack the blender on the solinoid.
FYI when reassembling new discs, the clamping ring around the differential must have some pre-stressing (see the workshop manual, I do not remember the numbers )
at first it surprised me, but it makes sense.
As the trimmings wear, the famous valves # 27 catch/compensate the operating clearance and ensure that this preload is still present.
There is no means therefore for the receiver cylinder to release "totaly" the pressure on the differentials ( when you remove the two slaves cylinders for example, they both are alway in pressure state .. ) .
the PDAS system needs this "zero tolerance".
the #27 valves act as a limited pressure valve ( like those on the rear brake circuit but in the reverse way ) -> thus maintaining only a minimal pressure on the disc.
The only way this valve could lead to a higher "minimal" pressure than expected is that the inside spring become stiffer ... not possible in my mind .
OR something is bloking the way of the fluid ... but you said you can bleed the system so ...
Now if your side differential makes a groaning noise when cornering, I think your discs could be dead (the gylon layer on each frictions discs has "peeled of" or disintegrated like on mine... )
and instead of having this normal sliping behavior , the discs stick .... ???? could it be ??
for me these two small valves # 27 are not to blame.
did you try a full bleeding of the system ( with the KTS ) ??
I'm soon to 'service' the transaxle slave cylinder (which with new seals which is connected to both pressure valves). Any useful related info here noted.
I was really hoping it’s not the discs. I would rather spend that money elsewhere on the car. I have not been able to do a bleed for the pdas valve block and solenoids. I have a guy that is selling me his durametric 6 that can bleed the pdas. My car is getting some coil overs installed and it has a mega squirt engine computer installed on it. I’m not sure if the durametric will hook up to my abs/pdas computer without the stock engine computer installed. The guy said I could try hooking it up before I buy it to make sure. But my car isn’t mobile right now to take to his place to try it. As soon as it is I’ll take it there and if it can connect to the pdas/abs computer I’ll do the bleed. Maybe that will solve it. I am doubtful about that though. It’s sounding like the discs from your explanation. Potentially the flex hose. The issue does seem to get worse the more you cycle the key. Maybe it’s allowing fluid through the hose but not back causing the plates to be more engaged every time you cycle the key. Like a bad brake flex hose. I will get my hands on a scanner and see if the bleed does anything first.
My money is on the slave being no good. Pull it out and inspect.
I had same problem with the diff slightly engaging.
Wouldn’t retract properly unless all pressure was released.
I had an issue once where my diff slave was sticking - i rebuilt it and haven't had a problem since. Honestly it sounds in your case that the issue is in the diff...but if you want to try rebuilding the slave w new seals, i have them available still. It's an easy rebuild.
Have you checked your trans fluid? Over time we lose a fair bit of fluid through the old flange seals that inevitably let some out...
Also, there is a drain hole in the bottom of the diff slave- put a pin up trough there and make sure that is clean, quite possible it's a vacuum issue the piston presses in, and then if that hole isn't open, maybe it's having a problem retracting? Just a thought there. Never seen that issue, but always good to hit the free fixes before the expensive ones...
I’m picking up the durametric on Thursday. I’ll bleed the system first/road test. If that doesn’t fix it I’ll clean out the drain hole on the bottom of the slave/roar test. If that doesn’t work I’ll replace slave hose/road test( they are 30 years old I may aswell put new ones even if they are not an issue now). If that doesn’t fix it I’ll rebuild the slave/road test. If that still doesn’t fix it I spoke to Roger at California Motorsports who deals with a lot of Porsche transmissions. He seems very knowledgeable and will rebuild the diff for a very reasonable price if I take it out and send it to him.
Goughary how how do I get a slave rebuild kit from you and do you happen to have any pictures of the drain hole. Or is it very obvious?
The drain hole is a pin hole directly on the bottom very close to where it mounts- shine a light from underneath and you'll see it- literally sticking a pin through the hole is all you'll need to do, just dig out the crap in the hole...
As to the diff seals, just pm me and we can chat. You PayPal i send- it's very loose and disorganized. But gets the job done...
I was just at my shop. I got my durametric a couple days ago. I ran into 2 issues.
1. The durametric cable is showing as connected by the durametric. But there is no communication between the abs/pdas computer. I’ll call them on Monday to see if I can figure out why.
2. I went to clean out the drain hole on the slave and it was filled and covered with undercoat which I think was put on when the car was new. I tried using a pick to get it out and it’s like concrete, I was unable to clean it out. Attached are pictures. Maybe someone can verify that is the correct hole.
In the first photo, the white powdery substance is crystallized brake fluid from lack of use and maintenance.
If you can disassemble the cylinder and the walls are still smooth without pitting, and the piston is not too corroded then all you need are the seals for rebuild.
I have rebuilt a few of these with the exact symptoms you describe.
I had an issue once where my diff slave was sticking - i rebuilt it and haven't had a problem since. Honestly it sounds in your case that the issue is in the diff...but if you want to try rebuilding the slave w new seals, i have them available still. It's an easy rebuild.
Have you checked your trans fluid? Over time we lose a fair bit of fluid through the old flange seals that inevitably let some out...
Also, there is a drain hole in the bottom of the diff slave- put a pin up trough there and make sure that is clean, quite possible it's a vacuum issue the piston presses in, and then if that hole isn't open, maybe it's having a problem retracting? Just a thought there. Never seen that issue, but always good to hit the free fixes before the expensive ones...
Goughary any chance you still have some of these seal kits?
01-28-2019, 02:28 AM
964 C4 pdas slave cylinder pressure valve what does it do
with interest...
