ozmatt964

Hi all,

I have a 1990 C4 that on a few occasions develops a "clunk" sound from what I think is the 4WD system.

It tends to occur if I loose traction over a bump or pot hole (and the wheels scramble for traction), it's like the system slip & then "thuds"....

Question to fellow forum members, is this normal (probably not), does any one else have similar experiences and does it sound like the diff or transfer case needs repairing? Any thoughts on this would be greatly appreciated.

I just wonder now if the 4WD system is actually an Achilles heel of the C4.

Thanks

Matt

newsboy

I also have a 1990 C4, but have never experienced (wheels scrambling for traction) that you describe. Although the all wheel drive system is complex, it has proven to be a very reliable. You won't see many posts on this forum about failures.

klr10

Oh boy.... sure hope it's not your planet gears. Hopefully it's just something simple. I ended up having to get another gearbox when mine failed. I have also heard from several others this has happened to.

wellcraft290

Are you talking the normal clunk as if you switched it to lock the diff? I tried it and you hear a clunk when it locks or something else?

Rocket Rob

+1 on the C4 all wheel drive system is very reliable. You didn't mention how many miles are on the car and what you have checked. I would look at the basics. Verify your suspension bushings (wishbones & sway bars), shocks, wheels lug nuts are all in good shape. If those don't yield anything, you can also verify that your gear oil in both the tranny and front diff are full.

ianellison

I have exactly the same clunk on occaisions. It seems to occur when I have been coasting at say 10 - 15 mph over speed bumps in a 3rd or 4th and I then apply a little gas to pickup the drive.

I have mentioned it to several specialists but no one seems to know what it could be - it does sound like some slack being taken up in the transmission somewhere.

I've had the car for 5 years and it hasn't got worse. Be interested to know if there is anyone who does NOT get this slight clunk on their C4?

dfinnegan

I've not noticed any clunk in my 91 C4 with 85k miles that I've owned for 5 yrs. I do experience a clunk when manually shifting into 4 wheel locked mode when stationary.

Although, I have never tried accelerating from 10-15 mph in 3rd or 4th gear. That would be 1st mostly, though I may occasionally go that slow in 2nd.

Cheers

Rocket Rob

I've not noticed any clunks either. I've had my C4 for 18 years and 200K miles.

DaveK

I've not noticed any clunks after 8.5 years / 60k miles (with 180k mile total now). I do feel like an amateur compared to Rob!

tbennett017

Have you bled your PDAS slave units? I wonder if the clunk is related to either of those?

drjsuh

No clunks on my 1990 c4. I did have a similar clunk on my 993 C4S though and it was the exhaust (had an aftermarket exhaust on it that would hit the bumper cutout sometimes when going over a bump/accelerate really hard, etc.). As someone else noted above, I would start with the simple stuff that usually clunks over bumps on older cars -- i.e. suspension pieces/bushings, exhaust parts/hangars, motor mounts, etc.

ian89C4

I have a 'clunk' every once and a while when going slow over bumps, but I have since traced the sound to the flaps in the heater connectors. The ones that connect to body and the tubing runs down to the heat exchangers. See if you can replicate the sounds when you shut the door hard, that is how I found out what it was. Other than that, had noise from the system when I first got the car, but it had wrong size tires on and the system couldn't figure out what the h*ll was going on. So with that, check to make sure you have the correct tire size as well.

Good luck

ozmatt964

Thanks for the feed back guys, the car has 120k on the clock and the suspension bushes etc are due to be replaced..... Again this is not a common occurrence, it happens on the odd occasion. I believe most of the time it happens in the same situation as Ian has described above. If the gears are under load or I am accelerating then no problem, but under light accelerating (and a speed hump or pot hole included) then I get the clunk when I reapply the accelerator.

The car is due for it's 120k service so I will quiz my mechanic, but I agree it does sound like "slack being taken up in the transmission somewhere".

Fingers crossed it is not an expensive fix but I will report back if they find the issue.

ozmatt964

Sorry for my ignorance, but what is a PDAS slave unit?

JDD

Failed engine mounts?