Hey all,

I just bought a 1999 Mustang Cobra with 85k on it. Sweet ride but I'm worried about a clunking I hear when shifting into 1st gear or from 1st to 2nd gear. It also will happen when I left off the acceleration. My mechanic seems to think it’s in the differential but said the bushing looked good. Anyone have any ideas?

Thanks!

 

hmm.. there was a TSB for something of the like.. I dunno if it was for that tranny or not

 

ya i have a 02 and i was told from the dealer that it was okay .. i have some kind of sound like you're describing in my trans also... i would say have the dealer check it out just to be safe...

 

i test drove an 03 cobra the other day that did this and i was told that it might be a driveshaft u joint going.......

 

I found the TSB. here it is:

Defect: If the ball joint assembly where the lower control arm attaches in the rear knuckle on certainof these cars moves forward or backward from its installed position, greater stress will be placed on the knuckle casting. If this occurs, the casting could fracture, allowing the corner of the vehicle to drop, and the lower control arm could contact the inside of the rear wheel. In some cases, steering of the vehicle could be reduced.

I'll call tomorrow but I'm thinking that the service was done. The vehicle isn't under warrenty so I hope they can help me. Humm, wonder what the problem is....

 

I checked with Ford today about the ball joint recall. My Cobra had it fixed in 91. So does anyone thing it might be my syncro's going out? It seems to clunk when im letting out the clutch and acellerating. The whole car is stock except for the Cobra emblem...maybe that's the clunk

 

I've had 2 heavily modded IRS's. Usually what you're hearing is the loading and unloading of the diff causing it to pitch on its crappy mounts.

In the back the diff cover is also the rear support bracket for the diff and that bolts laterally to a bushing dealie that's a totaly piece of criznap. you also don't have a vibration damper at the front of the diff like the later models came with. That rubber bushing and the soft rubber bushings on the front of the diff are all the time bouncing back and forth. They're great for keeping vibration down but jeebus. Anyway, the basic principle of IRS is that the differential is fixed to the car firmly and the wheels and halfshafts do all the moving around. Ford's implementation didn't work quite so well in that respect since it would have made too much NVH for it to pass and hit the market with the 99's. Part of making it quiet seems to have materialized in looser gear setups as well. I don't know for sure but it was a concern of my diff guy when he set up my gears.

Fixing the diff case to the subframe in as firm a way as possible takes care of most of the bounce that leads to clunk and allows you to run a tighter setup on your gears for less clunk there. There'll always be some clunk I'm afraid. There's just too many bushings and replacing them all with delrin is expensive.

The ball joint issue would most likely cause the clunk on every accel/decel cycle or every time that side of the suspension articulated a little (I'd think)

Here's how to fix this long term at the expense of some minor increase in the sound of the gears (provided that the issue you're having is "cobra clunk"):

MM Aluminum Differential Bushing Kit
MM IRS Subframe bushings (Poly Urethane)
03 cobra front IRS crossmember.
high speed Balanced aluminum drive shaft.

That'll fix your diff in place and help keep the clunk from getting a place to start. It'll also reduce the rotating mass/energy and reduce wheel hop a good bit.

I did those things and was solid as a rock till I found another way to break my IRS. After a while I said screw it and went back to a live axle. they're just lighter and more durable.

 

Thanks Redneck,

This is great information. Is it safe to say that the car is running fine and the clunk is part of the car design? Or will the clunk get progressively worse causing more damage?

Also, how much was it to upgrade to the equipment you've listed?

Thanks again....I gotta get this baby on the road it's my day off!

 

if it's cobra clunk it's just fine. If it's something more serious, only 1 way to find out.

Mine clunked from the day I got it both times, so I'd get out on the road and enjoy. Don't shift into 1st unless you're crawling or stopped.

250 for the diff bushings plus a couple hours to install, 50 for the cradle bushings and a couple hours to install, find a used 03 brace or make one, drive shaft could be pricey... You should be able to find a good one for 200ish.

If you go to do the cradle bushings, remember torch, drill, dremel with sanding roll, done.

 

Welll...guess it's not the cobra clunk. I was driving the freeway after replacing my old sparkplugs (weeee, sa-weeeet engine!) and "Bam!" I here the ultimate "Clunk" and at that point I lost all my fluid from my rear differential. Ah man, it was a sunny day and a freshly waxed cobra....what a mess. Towed it and now I gotta big hole in the rear differential case. Well at least I got 3 weeks outta the rear end....


:censored:

 

uhhh...yeah it was cobra clunk. that's the ultimate effect of leaving it to get worse. I've done it a couple times. normally you'll get a signal that it's getting THAT bad before you drop the diff on the ground. eeek. Usually you'll feel the diff slap the cradle when you go into reverse.

You can get a new rear cover from a 92-94 Tbird SC at any junk yard hella cheap or from ford for a grip of cash. If that broke there's a good chance you snapped one of the ears off the front of the diff too. If so, you can use the T-bird case, and even the gears but it'll probably be an open diff. T-Bird cases are also iron for the front half so they'll be kinda heavy but really really strong.

Alternatively, you can swap to a live axle. It weighs a gob less and if you go with something hotter than stock for suspension (simple tubular arms and coil over will help a LOT) it handles just as well as an IRS if a little more firm of a ride. If you want to keep the IRS, start with the parts list I spec'd above. It was designed to prevent just that sort of failure.

 

Ok, I think ill stay somewhat with the original IRS design. Thanks again Red, always good to get more info from veteran mustang owners.