There are three basic types of vibrations:
1. Engine RPM related – If the vibration is related to engine RPM, it will occur in all gears (and possibly even sitting still) at a particular engine RPM or above. This vibration usually can be attributed to the engine itself or anything else that turns at the same speed as the engine, such as the harmonic balancer, flywheel or flexplate, pilot bearing, pressure plate, torque converter, or transmission input shaft. It is also possible for body contact with the engine, transmission, or exhaust to cause an engine RPM related vibration. Worn or broken engine or transmission mounts can contribute to this problem. When driving the vehicle with the vibration present, maintain vehicle speed and try shifting to a lower or higher gear. If the vibration changes or goes away while maintaining the same vehicle speed, then the problem is not related to engine RPM.
2. Vehicle Speed related – If the vibration is related to vehicle speed, it will not be present until you reach a certain speed, and then it will usually start gradually and then become worse as speed increases. In some cases, it will decrease at some point, and then come back again at a higher speed. This type of vibration could be related to your wheels, tires, axles, differential, driveshaft runout, balance, or angles, universal joints, or transmission output shaft. Try the same driving test as above. If the vibration is present in third gear at 50 mph, but shifting to fourth gear at 50 MPH makes the vibration go away, then it is not going to be vehicle speed related and you can usually rule out any rotating component that is further back than the transmission output shaft. At a given MPH, your output shaft, driveshaft, axles, wheels and tires are all turning at a constant speed, no matter what gear the transmission is in.
3. Accel/Decel/Cruise related – A vibration that changes depending on whether you are accelerating, decelerating, or cruising at a steady speed could have quite a few different causes. Generally, this will be related to driveshaft angles or a worn or broken part, instead of something being out of balance. Think about what changes when the engine is under load. The engine and isolator mount loads shift; the load on the pinion bearing changes; your driveshaft angles change, possibly more than they should due to a broken engine or transmission mount; your exhaust, shifter, transmission, etc. could be contacting the body only on accel or decel; if the car has been lowered (or raised), your suspension snubbers could be contacting the body prematurely.
A vehicle works as a system, and you have to understand the relationships between all the different parts when you are trying to diagnose a driveline vibration. Determining if the vibration is related to engine speed, vehicle speed, or engine load will help you narrow down the list of possible culprits, and keep you from wasting your time looking in the wrong places.
Keith Farren is an ASE Certified Master Technician with a BS in Business Administration, an Associate degree in Automotive Technology, and over twenty years experience in the automotive industry. He also operates two websites dedicated to classic cars and automotive repairs, Classic Car Financing and Loans and Auto Window Repairs.