Thunk in my trans tunnel

HOLLYWOODSPIDER · Post by **HOLLYWOODSPIDER** » Sat Jul 15, 2006 1:33 pm

Hi, everyone. I've got a '78 Spider. When I am taking off from a stop, right at 10 mph there's a thuning/banging sound coming from the trans tunnel just behind the front seats. It only happens at 10-12mph and only in 1st gear. If I shift to 2nd before 10 mph, the noise doesn't happen. Any ideas? Propeller shaft bearing?

So Cal Mark · Post by **So Cal Mark** » Sat Jul 15, 2006 5:27 pm

probably a bad trans mount, but it could also be a bad mount for the driveshaft center support. Look at the driveline closely, a broken flex joint on the front of the driveshaft can cause a catastrophic failure if it fails completely at high speed. Also check the crossmembers that hold the tranny mount and center support. They should be mounted in rubber grommets that may have deteriorated

hughes6552 · Post by **hughes6552** » Mon Jul 17, 2006 10:09 am

similar sound in my 81 turned out to be a bad donut. As Mark says this can be catostropic. Check it at once, before driving.

kimu · Post by **kimu** » Tue Jul 18, 2006 9:39 am

Donut failure will destroy your tranny.
and the damange tunnel

An example of such a failure here:

http://fiat.nng.ca/images/79/IMG_2927.JPG

http://fiat.nng.ca/images/79/IMG_2930.JPG

http://fiat.nng.ca/images/79/IMG_2910.JPG

HOLLYWOODSPIDER · Post by **HOLLYWOODSPIDER** » Tue Jul 18, 2006 7:10 pm

I didn't think it would be the donut because it's so far back in the trans tunnel, behind the front seats. I'll check the donut immediately. I had one fail at 45mph on a spider I had back in high school. Shattered the trans casing, bent the trans tunnel up pretty bad and about ruined my knee when the shifter started flailing around fron hitting the spinning driveline. Cost me over $1200 to get her on the road again, with a new transmission.

perthling · Post by **perthling** » Tue Jul 18, 2006 7:49 pm

Sounds like your prop shaft bearing to me.

HOLLYWOODSPIDER · Post by **HOLLYWOODSPIDER** » Thu Jul 20, 2006 7:12 pm

I got under the car today and it seems like the donut is in good shape.
While I was down there, I noticed some scratching of the driveshaft that can only have been caused by the bolts securing the seat belt latches.
I couldn't get any play out of the u-joints in the rear section of the driveline, so I'm assuming they are still good.
I had a little bit of play in the center bearing/support. Is this normal, or is that where my problem is?

racydave · Post by **racydave** » Thu Jul 20, 2006 7:25 pm

take care of seat belt bolts, whaterer, they shouldnt touch d s, could easilly be the culprit

mbouse · Post by **mbouse** » Fri Jul 21, 2006 4:22 pm

In reading your original post, is it correct to assume that you have had the vehicle for a period of time, and it is just now making this noise?

Is the noise consistant above 10-12 mph, or does it leave as quickly as it appears? In otherwords, driving at 10-12 mph in first gear....does the noise cease after a period of time?

this thump is not normal. You have positively ruled out the rubber donut? It sounds like you have ruled out the u-joints which usually make a metal clink sound once or twice but not again until the shaft stops rotating.

I doubt it is the center support bearing - unless the rubber skirt is torn. Unfortunately, the only way to get that bearing off is to sacrifice the front u-joint.

However, if the center bearing support (the bracket that the center bearing bolts to) is not properly installed or is broken you will get this thumping noise when the drive shaft rears up and comes into contact with the tunnel. This happened to me this spring, in low gears as I was accelerating and shifting. It disappeared as soon as I leveled out on output rpm and the torque lessened when speed rose. My bracket snapped in two, but the four bolts kept it from being too noticable until I looked hard at that specific part. Double check this bracket. It is a safety hazard if not properly installed.

HOLLYWOODSPIDER · Post by **HOLLYWOODSPIDER** » Tue Jul 25, 2006 9:07 pm

It's the center support bearing/mount. The rubber supporting the bearing has dried out and is no longer supporting the bushing.

mbouse · Post by **mbouse** » Wed Jul 26, 2006 12:30 pm

Be sure to order at least the front U-joint, as getting that bearing replaced w/out sacrificing the u-joint doesn't usually happen. And, for the price, might as well replace the rear u-joint as well. that way all three wear items in this assembly have a good chance of lasting you for years to come, without the hassle of getting back down there soon.

HOLLYWOODSPIDER · Post by **HOLLYWOODSPIDER** » Tue Aug 01, 2006 12:47 am

When I replaced the bent panhard rod (that I mentioned in another post about ride height) the thunk went away. I'm still going to have to change that center bearing support, though. Thanks for the advice about the u-joints.