Auto Trans "Thump"

[Bob S.]

Just purchased a great 1968 427/390 Auto. I'm told that the loud "Thump" I hear when shifting from Park, to either Drive or Reverse is normal for a car of this age. Once in gear, all is fine and the car shifts smoothly through all gears.

I have the idle at 600 RPM as instructed, and at times the transfer is much lighter. More like a car of that era shifting into Reverse or Drive, and at other times, the "Thump"

My mechanic told me that over time the ring and pinion will develop slack, and to fix this, would, most likely cost plenty, with very little in the way of appreciable results.

This is a sweet, well cared for, numbers matching Corvette, but this ONE thing really makes me nuts at times

Any suggestions would be helpful....Oh, yes...it's not the Snubber Cushion

Thanks,

B

[William P.]

Re: Auto Trans "Thump"

You may want to have a look at the snubber bushing on the lower front of the rear end housing. If it is loose , or worn or someone may have put a polly bushing in place of the origonal rubber . I try to look for the inexpensive fixes first .
Bill

[Terry B.]

Re: Auto Trans "Thump"

Hi Bob,

I have the same '68 427/390 Auto you are speaking of. I have the same problem even with having the transmission rebuilt about 6 years ago. It is more noticeable after it has been driven however. It runs and shifts well. I would like to know if there is a way to fix it without having to remove the transmission or is this just the way it happens to be. I can and have lived with the thump.

Terry

[Paul J.]

Re: Auto Trans "Thump"

Bob,

I assume by "snubber cushoin", you're talking about what Bill said, which a common problem. This can also be caused by a bad u-joint, although there is usually a distinctive clank associated with the thump.

However, my vote is the transmission. I am not an expert on these but it could be related to the clutches or the modulator, although if it is shifting well it's probably not the latter.

It could also be a motor or transmission mount.

Paul

[Bob S.]

Re: Auto Trans "Thump"

You may want to have a look at the snubber bushing on the lower front of the rear end housing. If it is loose , or worn or someone may have put a polly bushing in place of the origonal rubber . I try to look for the inexpensive fixes first .
Bill

Thanks, Bill....

Snubber appears to be in good shape, and the entire undercarriage has been gone over...New U-Joints, bushing, bearings, shocks, sway bar, trailing arms...everything.

[Bob S.]

Re: Auto Trans "Thump"

Bob,

I assume by "snubber cushoin", you're talking about what Bill said, which a common problem. This can also be caused by a bad u-joint, although there is usually a distinctive clank associated with the thump.

However, my vote is the transmission. I am not an expert on these but it could be related to the clutches or the modulator, although if it is shifting well it's probably not the latter.

It could also be a motor or transmission mount.

Paul

Hi Paul,

Undercarriage is in great, recently gone over, condition....all new, everything..Including U-Joints. Had my mechanic go over everything. Still tells me, that's the way it is. Urgh

[Bob S.]

Re: Auto Trans "Thump"

Hi Bob,

I have the same '68 427/390 Auto you are speaking of. I have the same problem even with having the transmission rebuilt about 6 years ago. It is more noticeable after it has been driven however. It runs and shifts well. I would like to know if there is a way to fix it without having to remove the transmission or is this just the way it happens to be. I can and have lived with the thump.

Terry

Wish I knew...Like my mechanic said, "I could fix this, and it would be 'Somewhat' better, and you'd say...'Gee...I paid Jeff all this money, and it's still not all that great' "

WORST thing? Filling her up...."Hey..Nice car!...etc"...Little chat.....hood opens, gasps of pleasure....Then, when you're leaving, "WHAM" into gear....Nice....