Reverse gear engagement

[2BLACKZS]

Recently bought my first vette, a 75 coupe and now starting to work on it. The car has a TH400 trans in it. It drives forward okay, no tranny or rear end noise, but when put into reverse gear it seems like the driveshaft makes about a 1/2 rotation before making the gear engagement, makes an audible clunk then will move the car. Any tips on where to start pinpointing the problem areas?
Thanks!
Wade

 

[1995 RedVette]

The clunk sound is almost normal. There are 6 universal joints between the drive shaft and tires(two in the drive shaft and two in each halfshaft). Each of those universals have some degree of clearence in order to opperate properly and that will give you a bit of noise when all of the backlash is taken up. It is much more noticable in the late model cars with aluminium drive shafts (they tend to ring a little more than the steel).

That said, if you are having any vibration while going down the highway or the shift from drive to reverse and reverse back to drive is violent, then you may want to have things checked. However, the some noise is to be expected.

Brett

 

[wishuwerehere82]

Put the rear of the car up on jackstands and rotate the driveshaft back and forth to listen where the clunk is and to see where the loose points are. If the driveshaft rotates and the halfshafts don't, then the problem is probably in the carrier bearings in the differential. If the half shafts move, but the tires don't, it's in the outer U-joints on the halfshafts. In any case, you should be able to see what's moving and what's not. You can rotate the wheels to see where the looseness is back to the differential.

The clunk in my '82 was all in the driveshaft U-joints. The halfshafts were and still are fine.

 

[2BLACKZS]

I hope it'll be a simple case of failing u-joints. Thanks for the feedback!

 

[74bigblock]

Ehhh.... just put some tape on it.... it will be fine.

 

[black_81_vette]

well if its ok other than in reverse then id say the u joints are fine.I recall reading a tech bulleton on how some th400 trannys of that era had harsh engagement going into reverse.I can remember what all was done.Something to do with a waved sping in the clutch pack and or valve body mod.Id find a trans shop thats been doing work that for years or find one of those all service data books. good place to find em is the library.

 

[2BLACKZS]

Thanks for the tip. Harsh rear gear engagement is a good description! The whole drivetrain is rudely shocked when this gear only is selected. I'll get that checked, rebuild or replace the trans if that's the problem. Not something I can live with, much less put some tape on.. ha ha.

I did check the u-joints. One of the 1/2 shaft u-joints has some slack in it. Good time to replace them all.

Thanks for the advice.