tbennett017

In the C4, the driveline has a pair of hydraulic slaves that actuate the clutch packs at both differentials. One locks the rear axles and one locks the longitudinal driveshaft. These work when the PDAS system requires a change in traction. These are driven by the same high pressure brake pump/system.

I might have been calling them the wrong thing in referring to them as PDAS slaves, but they are the hydraulic slave units either way.

tgage

Seems to happen to only RED C4's.

No issues like that on my C4

tom

ozmatt964

Hey Tom......mine's a Grand Prix White C4.......

I have it in getting a full bare metal respray at the moment, so when it comes out it will be straight to the shop to get their opinion.

It could be the hydraulic system, because the last time it was in the shop there was a comment made that it looked like the hydraulic system was contaminated.......at the time I put this on the "to do" list for fixing after the respray had been completed.

amr89c4

I had a clunk in the front diff area and it turned out to be the front diff mount bolt had come loose and backed out several turns. Its easy to see once you remove the windage tray that covers it. As I recall it has about a 17mm head. Mine came loose and I could hear it on hard acceleration-decelleration. Before I figured it out, the diff jumped so hi it nicked the power steering return hose which promptly laid down all the ps oil and caused me to find the leak, thus finding the bolt. It is a real real chore to replace the hose as it is a convoluted formed steel and rubber hose and cost about $150 for the part alone as I recall. I did the work and a shop would charge a bundle.

I strongly recommend you check that bolt for security before you drive again. It may not be the problem, but it certainly sounds like what I experienced, which is a very solid thunk (caused by the diff torquing upwards due to the loose mount bolt).

Computamedic

Trying to diagnose a "clunk" is pretty difficult even if you're in the car. Over the Internet is a tad more of a challenge!! It could be a multitude of things, but I would certainly check the engine mounts. I know Ian has had his engine mounts changed only recently and I wonder if anything has changed since?? There's a lot of weight hanging on those mounts and any sloppiness will generate a considerable jolt when driving over a bump.

The PDAS slaves are generally very quiet - unless you engage manually (at a stand still). I often notice the light flicker when driving over bumps (I've got the dash mounted light) but never any associated noise.

Regards

Dave

ozmatt964

I appreciate feed back guys, it is always good to have a short list of things to check.

I want to get the car out on a few track days, but want to be sure that every thing is as close to 100% correct as possible before I venture out.

The car is booked in for a service so if they can locate the issue I will be sure to post the findings.

Matt

tokoro

I have this same problem but when turning like in parking only sometimes it stucks when im completely stop and when I accelerate it make some effort to move at first. dont know what to do.

Rocket Rob

I would first check the basics that all of your suspension bolts are tight and that there is no play in the sway bar, drop links, strut top & bottom, steering rack, tie rods and wheel lug nuts.

tokoro

Thanks Rob the car is in the shop im bringing all possible issues im fighting for solving this
Is that normal when you activate the awd turn **** to the right you cant turn to the right or left? is just for going straight? If i turn with the awd activated it makes that clunk sound but all the time and strong. but i think thats normal
But when I deactivate it should be normal acting not making that sound and you feel it it just happend when you are parking that you are turning the wheel to both sides like eliminating the differential. DRIVING ME CRAZY!!!!

Rocket Rob

Yes, its normal when you active the AWD ****, it locks the front and center differentials This makes steering very difficult. I have only used this mode a few times in 20+ years. Once to try it and the other to help me get myself out of a snow bank. Otherwise, I leave it in the normal mode.

Does your mechanic have a computer that can connect to the PDAS to test all of the locks and check for error codes?

tokoro

Hi Rob
Yes he has the hammer and i it was bleed it was with a lot of air and dirty now every time i turn in the car you hear them "clunk" is that normal?
But the thing is that before the bleeding when I'm turning the wheel for parking it acts like if it has it activated not that rough but occasionally and the wheel is a little hard when itis at the completely right or left.
Thanks rob

Rocket Rob

I don't remember hearing the slave locks but its been a long time. I can test it tonight when I get home. I didn't drive my C4 today.

tokoro

Ok let me know appreciate your help!!
Thanks

-nick

The locks only make noise when I twist the PDAS switch on/off. Otherwise they're silent. The PDAS light (next to the switch, also in the dash if you put a bulb in it) comes on over big potholes, but no accompanying sounds.

I couldn't tell from the previous info- is the "clunk" over bumps the same as the "clunk" when you twist the switch on/off? Possibly just a short in the switch while driving?

tokoro

When you turn the **** thats normal to sound, mine is making it wile you turn the wheel parking the car going slowly in reverse or forward.