Another rear end "thumping" mystery

[GTR1999]

Tgreen,
I just emailed you about some tech info. Let me know if you're interested.

 

[C3RVETTE]

I agree with John,look over those bushings again and use rubber. I never use poly except for motor mounts-had good luck with them.

The tach is driven by a cable from the distributor. May be something simple as a loose cabel but don't count on it.Remove teh distributor end of the cable and spin it the tach should move. If it doesn't the cable may be broken in the casing. If it does then you have to look into the distributor for a worn side gear and main shaft gear. If the side gear is worn then the main shaft is most likely too. The after market gears are a crap shoot for quaility. I had rebuild a couple of distributors last year and had a problem with the available parts. Look into High tech in Fl they make their own gears and do it right.

Lastly, realize you have a 32 year old car that may have been worked on by any number of "mechanics" that in conjunction with age will cause things to stop working. Believe me you're not alone!

I just started getting tach fluctuation when I hit my window swiches,but I hear that may be a tach filter.

 

[GTR1999]

CRvette- I believe he has a 72 which runs a mechanical tach drive,starting in '75 they went to the electronic tach and distributors.

 

[tgreen]

Thanks for all the info. What is involved with inspecting and changing the differential bushings? From what I can see it looks pretty difficult to get to it. If the noise is not getting any worse, will I do damage to it by getting a few miles in this fall before the snow flies. I don't drive it much anyways but I'd like to give her some excersize before parking her for the winter.

Last night I disconnected the tach cable from the distributor and turning it does result in the tach needle moving so it is not a broken cable. I added a little grease to the end of the cable and reinstalled it. Still didn't work. I guess that narrows it down to the tach drive gear and coupling? Is there a trick to getting that out or does the whole distributor need to be removed?

 

[windrider50]

One way to isolate the thumping you describe is to jack up rear of car, remove wheels, put jack stands under the trailing arms on both sides, lower car onto jackstands, and start car. While a buddy slowly cycles the transmission from drive & reverse, you can begin to search for the source. Be careful about getting too far under the car, even with jackstands.


Good Luck

NJ BB Ken

 

[GTR1999]

You can remove the cross gear from the tranny but you you won;t be able to inspect the bore too well or the main shaft. I would pull the distributor and disassemble it. You get still drive the car if you have access to a stock chevy distributor. The biggest hassle of this job is getting good parts.

You can replace the front differential bushings with basic hand tools from under the car.

 

[tgreen]

I went to the local corvette shop this morning and picked up a new front differential cushion/bushing. The job looks easy enough, however I can't seem to get the old bushing out. I have the nut and bolt out. I can't imagine that you are just supposed to pry it out from there. It looks like I could lower the whole support bracket if I could get the two bolts out of that. The castle nuts came off with no problem. Is there some obvious trick that I am just missing?

 

[dwilson50]

I went to the local corvette shop this morning and picked up a new front differential cushion/bushing. The job looks easy enough, however I can't seem to get the old bushing out. I have the nut and bolt out. I can't imagine that you are just supposed to pry it out from there. It looks like I could lower the whole support bracket if I could get the two bolts out of that. The castle nuts came off with no problem. Is there some obvious trick that I am just missing?

After removing the bolt the old cushon should fall out...theres nothing else holding it in...unless its just stuck to the bracket. If that's the case you can just use something to scrape/pry it loose. I have the diagram from the assembly manual. I can fax it to you if you want it.

 

[tgreen]

I still haven't gotten the bottom cushion out. It is wedged in between the bracket and the frame. I am starting to think that you guys are on to something here though. I found an exploded diagram in the Mid America catalog. Looking at that gave me a good idea of how these cushions should be setup. My top cushion appears to be kind of old and I am not sure if it was even in the bracket hole. It looks like the mounting bolt should come down from the top and the washer and nut should be fastened from underneath. Mine was the opposite of that and the washer was not in the right spot. I am sure I haven't explained this well at all but the bottom line is I will keep working on getting the bottom cushion out and I will head back to the corvette barn tomorrow and hopefully get a new top cushion and the proper mounting hardware.

Thanks again for all the advice. I'll let you know if this solves the problem as soon as I can.

Ted

 

[tgreen]

I finally got the new upper differential cushion today. Got the new cushions installed and took her out for a test. NO BUMPING NOISE!!! You guys were right on the nose. Thank you all very much for the help. You guys are an invaluable resource for someone like myself with limited mechanical experience but a bunch of enthusiasm for doing my own work on my baby. As I learn more about working on these cars I hope to be able to help others like you guys do.

Next I will turn my attention to the recently non-functional tachometer. I am sure I will need more advice as I get into that.

Thanks again.
Ted

 

[dwilson50]

I finally got the new upper differential cushion today. Got the new cushions installed and took her out for a test. NO BUMPING NOISE!!! You guys were right on the nose. Thank you all very much for the help. You guys are an invaluable resource for someone like myself with limited mechanical experience but a bunch of enthusiasm for doing my own work on my baby. As I learn more about working on these cars I hope to be able to help others like you guys do.

Next I will turn my attention to the recently non-functional tachometer. I am sure I will need more advice as I get into that.

Thanks again.
Ted

Glad you got it fixed...