Drivetrain noise / issues

[vetteboy79]

Hello everyone -

This is my first post here, so please bear with me if this question has already been asked/answer a million times...

I've got a '79 L-82 4-speed and it goes down the road really nice and smooth with hardly any noises or creaks. However, one day I was accelerating off the line and got halfway through 2nd gear when I heard this horrible 'clank' from the rear - it literally sounded like i threw a driveshaft and it was spinning around and hitting the body. I slowed down real quick and it went away before I came to a complete stop. Fearing something awful I crawled under the car (tight fit) to inspect the damage and saw nothing. Luckily I was near a friend's house so I managed to back down the road and into his driveway where we put the car up on some ramps.
The U-joints and shafts were all intact (the u-joints on the driveshaft are new). All shafts and joints were solid with the exception of a small amount of play in the tailshaft of the tranny (like the bushing may be worn a tiny bit). The only other problem area I could find was an excessive amount of play in the bushing that joins the differential mounting bracket to the chassis crossmember in the rear (a single, fat thick bushing in the middle of the vehicle about a foot in front of the rear diff). I could move this joint somewhat easily just by pulling on it - a new bushing kit is on order and I'll post what happens when I put it in.
So I put the car back on the ground and drove it a bit more, and it was good for like 10 minutes and then, the big ol' clank again. It seems to start off as a few small bangs, increase to one or two really good 'clunks' that you can feel through the whole car, and then disappear. And of course, it only does it intermittently, which means I can't make it happen, and every time I go to demonstrate it to someone it goes away...
If anyone out there knows what this could be, please let me know...or if you need more information about something specific I'll try to provide it. Thanks a lot!

 

[GerryLP]

Spider gear pin?

Vetteboy,

If it is internal to the third member (where the gears are actually located and sometimes called the pumpkin), then the spider pin in the spider gear area could be causing problems. This pin has two small retaining rings. If one retaining ring falls off, then the pin can work itself (slide from) its position and contact the spider gears or diffential ring.

I subscribe to the notion that most mechanical problems are caused by minor discrepancies; however, no matter how minor, the location of the failing part determines its complexity (more like tedious work than complex). If you can't find any other reason for this clunking, then try removing the differential cover and inspect for this pin being damaged or causing problems. I also have heard that a cracked cover can cause this, but I am not sure how true that is.

In the 79's one can remove the cover. On 81's one has to drop the whole differential to remove the bracket support/cover assembly to inspect the gears.

 

[78SilvAnniv]

Welcome vetteboy79

We are a bunch of friendly and helpful members at the Corvette Action Center Community, welcome. Come back often, and let us know how you make out with your 79's problem.
Heidi

 

[GerryLP]

Vetteboy,

I think that what I said about the 79 differential cover was misleading (unintentionally) :duh . There is a good deal of work involved to get that cover off. Just about as much as an 81's.

 

[JohnZ]

I'd bet on the bushings at the nose of the diff bracket - see this all the time on older Corvettes. Get the original-type rubber bushings from one of the Corvette suppliers, not poly - the poly bushings don't have the sleeve insert for the upper bushing, and they're too thin, which lets the nose of the diff rise up - screws up the operating angle of the rear U-joint, causes driveline vibration, and the U-joint can hit the floor pan under acceleration. Most poly stuff is junk, and these are a good example. When you install the bushings, you want the bolt/nut good and tight - 55-75 lb-ft.

 

[vetteboy79]

Thanks JohnZ

To JohnZ -

I had suspected that the bushing might have been the problem - when I had the car up on a lift I could move that joint around quite a bit just by pulling on it. I ordered the parts two days ago from a dealer and I should be able to install them sometime this weekend. I'll let you know if that solves the problem!

Chris

 

[vetteboy79]

Fixed! (almost)

After replacing the bushing on the nose of the rear differential mount (as per JohnZ's recommendation) the car accelerates and cruises much smoother...the noise I was hearing seems to have all but disappeared. The only time I can make it happen is if I drive the car really hard, which I usually don't do. The bushing itself was completely gone when I pulled it out - there should be an insulated gap of about an inch between the differential mount and the frame, and in this case the two metal pieces were touching because the bushing was so deteriorated.

So now that that's pretty much solved...I've still got the classic problem of miscellaneous creaks and groans off the line when starting out slow. I've heard that this could be the yokes for the U-joints...next time I've got it up on a lift I'll check this out. I also plan on changing the rear diff fluid & additive withing the next week or so.

Thanks to everyone who helped out! I'll keep ya posted on how things go.

Chris