1982 Chevy Corvette Noise under load

CVANBEEK

MEMBER
1982 CHEVROLET CORVETTE
V8
2WD
AUTOMATIC
67,000 MILES

The transmission in my 82 Vette was rebuilt last year. I also replaced the engine and tranny mounts. When going up grade and car has shifted through all gears, there is a fairly loud noise resonating through the car. It usually occurs between 30-50MPH at lower Rpms, under load or climbing a hill. Let off the gas when it's groaning (reducing the load/lug) noise is immediately gone. Push the accelerator down to force downshift of transmission. Noise immediately goes away as tork is lessened and RPMs increased due to the downshifting under tranny. The noise is noticiable and I think it will be hard to duplicate in a shop as the need for load/lug is key in generating the noise. Push it hard from 0 - 100mph 100% of the shifts are smooth as silk and NO NOISE occurs at any point in time during a high torque run. Try baby it, you might be able to avoid the noise entirely on the level or going down hill, but going up hill, once the auto has shifted into 3rd or 4th and the RPMs are still low and the vehicle is still on a climb or underload and the noise is there and significant! Any ideas on where I might start would be greatly appreciated!

2 Replies

CARADIODOC

MECHANIC
33,738 POSTS

Hi cvanbeek. Welcome to the forum. Here's a copy / paste version of a reply I posted a few minutes ago for a very similar problem on a truck:

... Problems like this are real difficult to diagnose when we aren't there to feel it ourselves, but of all the noises and vibrations I can imagine, I can't think of an easy culprit to find. There are two tools I can think of that might help. The first one is called a Chassis Ear. It is used with noises you can hear. This tool consists of six microphones that you clip onto various suspect points, then you switch between them while driving and listening with headphones. You can move the microphones around to localize the source of the noise. Many mechanics have never heard of this tool but it is available from Mac Tools, (I think the model number is ET7600, or something like that), and from other tool truck guys. It was issued to Chrysler dealers too. That's where I became familiar with it and I used it a bunch of times. It costs $199.99 from Mac Tools. The microphones are connected with wires to the receiver box. There is a newer model that uses four wireless microphones and two with wires. That model costs twice as much.

If the grinding is something you feel rather than hear, Chrysler had a tool for measuring vibrations. We rarely needed to resort to using it so I'm not familiar with it other than what I was told at a training school. As I recall, it was connected to the scanner and the probe or sensor was placed somewhere that it could sense the vibrations. It measured the frequency of the vibration and compared it to wheel speed so you could look at items that matched that frequency in operation. A vibration that matched wheel speed would be related to an out-of-balance tire, broken tire belt, or bent wheel. A vibration three times faster than tire speed would indicate driveshaft balance problems. Six times faster than wheel speed would typically be a tight u-joint problem. My understanding is it could even identify bearing problems inside the transmission based on the frequency and when different clutch packs were applied and when different things were spinning. U-joint problems will also change depending on rear ride height because it changes driveshaft angle. Transmission bearing problems will often get worse when you let off the gas pedal to cruise. Under load they tend to stop vibrating if the bearings are sloppy. Tight bearings, such as in u-joints, tend to vibrate worse under load because they aren't free to move.

I know this doesn't help identify the cause of the problem but those are some ideas. You might also try shifting out of overdrive to see if the grinding comes back at a lower speed.

Caradiodoc