So what gives? my 87' Trans Am has a 700R4 when your at a dead stop and you smash on the throttle and give it a good hard launch a loud thump happens in the transmission right under where the radio is but will disappear after that. it only happens from a hard launch. So what could be the problem?

 

Check your longitudinal brace bushing where it attaches to the tailshaft of the trans. Mine has the same "thunk" under hard braking or hard launch, and it took me FOREVER to figure out the source. The rubber had deteriorated just enough that the brace is shifting up and down slightly as the rear end rotates with the torque. I could feel it in the floorboard, but as it's something you can't visibly see it took a while of checking every nut, bolt, bushing, and linkage in both my front and rear ends before I came across it during a trans swap.

 

Aka the transmission mount.

JamesC

 

Actually no, NOT the trans mount. The trans mount is attached to the transmission crossmember, this is the rubber clamshell mount on the side of the trans tailshaft that attaches to the longitudinal torque brace that runs parallel to the driveshaft. If the rubber is worn it can shift up and down as torque acts on the rear axle.

 

Aka, the torque arm mount/bushing. Either mount, the trans or torque arm, could be the issue.

JamesC

 

James do you know if anyone makes a direct OEM replacement? My car still needs one, but all I can find is the polyurethane, which is a special order item and there are 2 versions available. Unfortunately, it's not very clear which I need...It's an "M" shape, and it only says "Lips towards driveshaft" and "Lips away from driveshaft", without specifying just what part of the "M" they consider to be the aforementioned "lips". As it has to be paid for in advance to special order, I'ld like to know which I need.

 

No OE that I know of.

Scroll down a bit in the following link for a diagram of the "M" and which you need:

http://www.energysuspensionparts.com...?prod=3%2E1111

JamesC

 

That helps tremendously. Now I can get my replacement and get rid of my own torque arm "thunk".

 

I had the same problem and it was my trans mount. It took me about 15 minutes to replace.

 

i looked under the car today the rubber looks pretty chewed up and needs replacing

 

I know I'm a bit late, but please, use the poly bushings and mounts at the very least. I burned through three factory tranny mounts in a month before I found the poly stuff.

Trans mount: http://www.summitracing.com/parts/ENS-3-1108G/


This picture shows a torque arm bushing clamshell with lips facing away:



If that's what you have, use this bushing, or similar: http://www.summitracing.com/parts/ENS-3-1111G/

Or, use this if you have the other: http://www.summitracing.com/parts/ENS-3-1112G/

Hope this helps someone, even if you've already got a handle on this.

 

I had a 1987 Formula with the same issue. Trans mount was the culprit. Torque arm mount was still good. Quick change of the trans mount and no more clunk or thump on hard take off. Never heard it during hard braking though.

 

I agree, sounds like the tranmission mount is the culprit.. That would be the 1st thing I would look at.. They are fairly cheap and very easy to install