I have been having this issue for a while now and it has been getting worse and quite annoying.
My speedometer has been acting up only in 5th gear and under load. It becomes less prone to act up after the car has been driving in 5th gear for a little while, or if the weather outside is warm. In 5th gear under load my speedometer drops down to 0, and when I take my foot off the gas, it jumps back up again. This does not happen in any other gears, only in 5th. Other wise the transmission seems to shift fine and there are no strange noises or vibrations that are coming from the transmission or the drive train.

I have done some searching and one possible cause to this problem could be a bad extension housing bushing. However, when I spoke with a guy at a transmission shop, I was told that if the extension housing bushing had gone bad, I would be leaking a lot of oil out of the back of the transmission, even if the oil seal were new since the output shaft would push against the seal when it moves.

Does anybody have an idea about what I am dealing with? There have not been any leaks from my transmission. I know it isn't my speedometer gauge, because when my speedo acts up, the cruise control also doesn't work. Any help would be greatly appreciated.
Thanks

 

Is this a gear driven speedometer or does it have a speed sensor? I would guess that the pilot bearing on the main shaft is going bad and allowing the shaft to move around when it is in 5th gear. It also cold be that the main bearings have lost their preload and allowing the shaft to move. All of the other gears are on the other side of the rear main bearing, so they load the shaft in the other direction. This is all just a guess, but it is very common for the main shaft pilot bearing to go bad in the T-5. If all of the bearings, including the pilot bushing in the end of the crankshaft, are not good then it will allow the shaft to move around. I lost 5th gear in a T-5 because of a bad pilot bushing.

 

The trans has a speed sensor. How difficult is it to replace the pilot bearing? The transmission has no shift problems what so ever. It shifts fine, stays in gear, and doesn't make any strange noises or vibrations.

 

Did anyone find the cause of this problem? if so what is the fix? My 91 V6 Camaro just started doing the exact same thing last week. The speedometer works fine until you shift to 5th. As soon as you give the car gas the speedo falls to 0. When you let off the gas and it starts working again. I have stoped driving the car until i find out what is wrong. It took me forever to find a 20 tooth gear for the vss. I do not want to break/damage the plastic gear by continuing to drive the car.
Help and Thanks,
don

 

Yes, I did resolve the problem. The issue was a faulty speed sensor. I changed out the sensor and it has worked flawlessly ever since. Ordered the speed sensor from GM parts direct.

 

THANKS!!!