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Transmission Mount Failures

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Transmission Mount Failures

Post by 69_RAG_TOP on Thu Jun 03, 2010 12:09 am

Here is a story that many of us may be able to identify with, or may be affected with sometime in the future. Considering that our engine, transmission and other driveline mounts are what holds our mega-horsepower components to the frame/chassis – and the implications of what can happen if they fail while under extreme load, it really pays to check them out – regularly.
We’ve all heard the horror stories about engine mounts that fail, causing the throttle to stick wide open. At best, you may be able to react quickly and shut down the motor before any damage occurs. The worst case scenario is the car careening out of control, causing severe property damage, and possible injuries or even death. All because a puck sized piece of rubber failed. It’s hard to believe that in most of our high output cars, the bulk of the engine’s support comes from two pieces of rubber, no more than 4-8 inches long.

One good point about engine mounts is, in most cases, they are relatively easy to diagnose as to whether they are breaking or already broken. The simplest way is to rev up your engine with the hood open, and see if the engine rocks excessively. To take it a step further, the transmission can be put into drive, hold the brakes VERY firmly, and revving the engine. A broken mount will make itself known when the driver’s side of the engine rotates, literally jumping off the mount. If a lift is available, or very strong jack stands, take your light and a crowbar, and find a good location to pry the engine up. A friend is a help here – while one is pushing up, the other can closely observe the movement present, as well as the presence of cracks and missing chunks – a sure sign that the mount is on its way out.

TRANSMISSION MOUNTS BREAK TOO! For a while, I was hearing a very strange noise from my car, most every time I would take off from a standing stop. The harder I took off, the louder the noise, accompanied by a soft banging or vibrating feeling.

Several times, we put the car up on stands, inspected all the usual items – u-joints, rear end gear play, control arm and torque arm bushings, anything and everything we could think of – including the transmission mount. We even started the car, engaged the tranny in drive & reverse – and could not replicate the noise in any way. The first time we snugged up all the attaching bolts on the arms, u-joints, anything we could see. Took it out – and the same noise appeared.

It got to a point where I would baby it off the line, just so as not to hear the noise. The few times I did get on it a little bit, the noise seemed to be getting louder, with more feeling of ‘movement’ each time. Up on the jacks again – searched thoroughly with a strong light, and found nothing, except frustration. When the tranny mount was looked at, while difficult to see fully due to the shape of the crossmember and hardware located in the area, it did look OK. We even pried up the transmission, but no movement was present, mostly due to the close quarters and the very small gap to the floor.

We were about to start pulling things apart in the rear (the noise sounded like it was coming from the right rear), but on a hunch I asked Mike to remove the tranny mount – just in case. Good thing we did – It was broken completely in two! When installed the crack was hidden by the curve in the crossmember, and its location was impossible to see. As shown, the safety brackets kept it from moving with simple pressure. In fact, when we bought it to the workbench, it was still intact, and appeared to be solid. It was only when we pulled sideways that the crack appeared, and more pressure led to it falling into two pieces. Evidently, when the 325 horses were coursing through the transmission, the rear of the tranny was actually moving side-to-side, and probably jumping a bit, too. Left unchecked, it could have led to much worse problems – driveshaft hitting the floor, or parts breaking. Best of all, now I have peace of mind when I want to ‘punch it’! Cool! Joe Genera

Get your motor runnin'. Head out on the highway Lookin' for adventure. And whatever comes our way. 1978 Pontiac Trans Am WS6, W72 Engine. bored .030 over Coded WC, 4-speed, Forged rods, pistons, 6X Heads Hays performance clutch, Flowmaster 40 Series 3" stainless dual exhaust, Hooker super competition, Ceramic coated long tube headers, 4 core HD radiator, Edelbrock Performer Series Carb
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Re: Transmission Mount Failures

Post by mr71transam on Thu Jun 03, 2010 7:49 am

Great Info, Thnaks for the Post.

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