There are many different possible causes for a transmission vibration. Wheels, tires, axles, driveshaft, transmission, clutch or torque converter, and engine components rotate at high speeds, and one or more of these components can create a vibration whether or not they are worn meet specifications. Worn or broken engine or transmission mounts can transmit normal vibrations that are usually never felt, and accidental body contact with the engine, transmission or exhaust can also be misinterpreted as driveline vibration. The first step in diagnosing vibration in a classic car is determining exactly when and under what conditions the vibration occurs.
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 while standing still) at a particular engine RPM or higher. This vibration can usually be attributed to the engine itself or to anything else that rotates at the same speed as the engine, such as the harmonic balancer, flywheel or flexplate, pilot bearing, pressure plate, torque converter, or gearbox. transmission input shaft. It is also possible that body contact with the engine, transmission, or exhaust causes vibration related to engine RPM. Worn or broken engine or transmission mounts can contribute to this problem. When driving the vehicle with vibrations present, maintain the vehicle speed and try to shift to a lower or higher gear. If the vibration changes or disappears 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 the speed of the vehicle, it will not be present until it reaches a certain speed, and then it will start gradually and then get worse as the speed increases. In some cases it will slow down at some point and then return to a higher speed. This type of vibration could be related to wheels, tires, axles, differential, driveshaft runout, balance or angles, u-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 into fourth gear at 50 MPH makes the vibration go away, then it is not related to vehicle speed and you can generally rule out any rotating components that are further back than the transmission output shaft. At a given MPH, your output shaft, driveshaft, axles, wheels, and tires rotate at a constant speed, no matter what gear the transmission is in.
3. Related to acceleration/deceleration/cruise – A vibration that changes depending on whether you are speeding up, slowing down or cruising at a constant speed could have several different causes. Usually this will be related to driveshaft angles or a worn or broken part, rather than something being out of balance. Think about what changes when the motor is under load. The motor mount and insulator loads are displaced; changes the load on the pinion bearing; your driveshaft angles change, possibly more than they should due to a broken engine or transmission mount; your exhaust, shifter, transmission, etc. could be in contact with the body only when accelerating or decelerating; if the car has been lowered (or raised), the suspension dampers could be in contact with the bodywork prematurely.
A vehicle works as a system, and you need to understand the relationships between all of the different parts when 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 prevent you from wasting time looking in the wrong places.