1991 Transmission Downshift Clunk

[Genna1]

Hi everyone: Just a quick question on this topic; on my 1991 L98 automatic convertible, if I slow down and come to a stop in either Overdrive, or 2, the transmission will downshift smoothly with no noticable clunk, however if I have the car in "D", when I get down to around 5 mph or so, the trans will exhibit a noticable clunk when downshifting to 1st gear. Car has 68,000 miles on it and is a 2-owner car; I had the trans serviced earlier this year, and the mechanic said everything looked good, with no metal junk in the pan; the fluid is still a nice pink color with no burned smell to the fluid; car will upshift perfectly in any gear position; any thoughts on this one?? Possible bad/sticking solenoids etc???
Thanks, Chris.

 

[MBDiagMan]

Most likely the problem is not in the transmission itself. It is likely something in the remainder of the drivetrain including; u-joints, half shaft joints, transmission mount, differential mount, motor mount or some link or member of the rear suspension.

The way to find it is to raise the car SAFELY with the rear wheels suspended and the emergency brake released, then apply the "Four P's," Push, Pull, Pry and Prod on everything until you find the loose part.

Good luck,

 

[JrRifleCoach]

A couple of tips. Inspect the U-joints closely. Look for signs of rust dust coming from behind the grease seals. If your shaking it down you'll need to apply pressure with a long bar to make a bad joint "move". Be careful and don't start bending things. You might even have someone apply the brake and place the trans in D then R to look for movement. You should also look at the trans output shaft and try to move it up or down. Again, alot of pressure will be required. Try to turn the C-Beam bolts. If they're loose, thats not good.

One last thing. After completely overhauling my drivetrain there was still some clunking in there that wasn't loud but kinda annoying. Many other Vette owners have told me some of these noises are normal. I've reinspected mine and have learned to ignore, but keep the ear tuned if the noises change.

BTW the C4 does not have a transmission or differential mount. Instead there is a C-Beam the bolts the tail of the trans directly to the nose of the diff.. Makes inspecting the main drive shaft joints very difficult....

 

[MBDiagMan]

Yes, you're right there is no trans mount, but the diff does mount to the subframe.

Regardless of how things mount in the vehicle, you need to be looking for something that is loose.

Good luck,

 

[Genna1]

Thanks for all the info; my guess was it was not really a big deal; I don't think the previous owner installed a shift kit, but I couldn't say for sure without dropping the pan and checking it out, although this car shifts smoothly; I've been in cars with shift kits installed and the shifts are much smoother in my car; maybe I'll inspect underneath carefully to make sure everything is tight and in order........

 

[Wegone]

Sounds like you may have a bit of play in the rear end my 95 does the same thing if the down shift is not under a load.

 

[compyelc4]

Yep. Got the same thing with my babied '95. Had the noise since I picked her up at 12,000 mi. I live with it!

 

[vetteboy86]

I have had a similiar noise for a couple years now, I have lived with it. However I am either going to find it in the spring, or wait until it breaks with the new motor, which should accelerate the situation.

 

[Vettefan87]

Mine will sometimes make somewhat of a noise going from 1-2, it is not very loud and hardly noticable unless you are listening to it. There was a thread around here talking about that noise a while back with many other owners having the same problem. How loud of a noise is it??

 

[compyelc4]

"How loud of a noise is it?"

Answer: Mine sounds like someone is striking the driveshaft with a ball peen hammer. I've noticed it does it sometimes too on the 1-2 upshift. It actually does it more after its warmed up. Does not do it cold. I have the auto 3:07 HiPo. I was at a Vette get-together the other nite. A few C4 guys were sitting at the same table. 75% of the guys have this noise! Noise seems to be slightly more intrusive with the high performance rear end. All of us have drained and refilled the diff with that purple "slickery lube" stuff. Absolutely no help. Nice knowing I'm not the only one.