Last year I received a phone call from a gentleman who owned a classic 1995 Chevrolet Camaro Z28 with a 6-speed manual transmission; his car had a vibration that “nobody could fix”.
He told me the story of the vehicle being taken from local shop to local shop and even to a local Chevrolet dealership; nobody could figure out what the problem was. This gentleman contact me because he saw some of the vibration diagnosis videos on my WeberAuto Youtube Channel and realized we were in the same city of Ogden, Utah. He wanted me and my students to take a look at his car and see if we could diagnose it. Normally I do not allow vehicles to be brought into my classroom environment like that, but we were actually at the vibration diagnosis portion of my manual drivetrain class and I thought it would be great to diagnose a real vibration rather than one I created.
The Z28 owner told be that he has had the tires balanced multiple times, he had the driveshaft balanced and both universal joints replaced on two separate occasions. The vibration he was experiencing occurred at higher vehicle speeds of 50 mph and higher.
Using the same diagnostic procedure outlined in our new NVH smartphone app, we performed a quick road test of the Camaro. The results of the road test indicated that the vehicle had a first order driveshaft speed related vibration. As the NVH app’s own diagnostic help file indicates, first order driveshaft speed related vibrations are only caused by a few select problems on the vehicle.
- A part rotating the same speed as the driveshaft that is out-of-round
- A part rotating the same speed as the driveshaft that is out-of-balance
- Drive axle problems
I thought this diagnosis could be a little more challenging than usual because the driveshaft had already been balanced, it even had a shiny new black paint job, from a local driveshaft repair shop.
My students and I started down the list of things to check. The first thing to check in the driveshaft runout. The FIRST STEP of checking driveshaft runout is to check for excessive vertical endplay in the slip yoke splines where it slides into the rear of the Getrag 6-speed manual transmission. The vertical endplay itself was 0.015” (normal endplay on a new vehicle is 0.000”). This endplay was a problem. We checked the runout of the driveshaft just before the weld at the front universal joint , it was 0.025” (which is within specifications). I have seen this problem before, the combined driveshaft runout (0.025”) and the vertical spline endplay (0.015”) combine to create an overall driveshaft runout (0.040”) that is excessive. This results in the driveshaft whipping around at high speeds rather than just rotating.
I told the Z28 owner what we found and recommended that he get a new slip yoke. He tried to purchase a slip yoke, but they were no longer available from the Chevrolet dealer. The Z28 owner decided to have a whole new driveshaft built using a custom new slip yoke. It took about a week to get the new driveshaft built. We installed it and rechecked the spline endplay and the runout, the new values were greatly reduced. We gave the car back to the owner and he took it out for an extended road test, the vibration was gone! The worn slip yoke splines was the cause of the vibration.
There were bad parts on this vehicle, but nobody detected them. One very unhappy customer was the result. Hundreds of dollars were wasted on attempted repairs, many hours of labor were wasted because nobody really knew how to diagnose a vibration problem; they were just guessing! The NVH app eliminates the guesswork in vibration diagnosis and guides you step-by-step through the entire diagnostic and repair procedure. Get the NVH app today by clicking on the link below.