Do Bad Engine Mounts Cause the Transmission to Go Out?

The primary purpose behind any motor mount is to keep vibrations from the engine from snaking into the chassis and rattling your eyeballs or vibrating bolts loose. Most motor mounts consist of a rubber spacer sandwiched between a pair of metal plates. The rubber spacer has holes in it to allow the rubber to compress between the metal plates, one of which attaches to the engine and the other to the car's chassis. A front-wheel-drive car -- where the front mounts support both the engine and transmission -- typically has three mounts. Two support the weight of the engine, and a third, usually near the bottom-center of the transaxle, keeps the entire assembly from swinging back and forth under load.

When a motor mount "goes bad," it's generally a case of the rubber in the mount breaking down and the walls between the holes tearing. Concurrently, the rubber itself may dry out, harden and shrink, going from springy to something like the consistency of pine lumber. When these two effects combine, they'll allow the engine to rock as much as several inches every time you hit the gas pedal or brake. With the voids out of commission, more vibration will find its way from the engine and transmission to the chassis. So, generally speaking, most of the effects of a bad engine mount will have to do with the chassis, and not the engine or transmission.

The biggest problem with a bad motor mount is that it allows the engine to build up momentum before slamming to a stop on the flattened motor mount. The first components to suffer from this hammering effect will be the more delicate constant-velocity and universal joints, because they're designed to handle only a certain amount of force while bending and spinning. Front-drive cars are even more susceptible to damage to the CV joints, because the twisting of the transmission puts the CV joints at odd angles to the transmission. This can be particularly problematic for the inner CV joints, which aren't designed to cope with the high angularities that outer joints are.

Transmission damage via bad motor mounts isn't the most common type of failure, mostly because the U-joints and CV joints will usually -- by design -- fail before anything in the transmission will. However, with front-drivers, there's always the possibility that the motor will squirm so much that the CV-joint axle shafts will actually disengage from the transmission's differential. Many axle shafts use a type of C-clip or retainer near the tip, which slots into a groove in the differential unit. Engine twisting may alternately pull the retainers out of their grooves and shove them back in. While this might not damage the transmission (unless the retainers break and wind up in the differential gears), it will severely wear the differential's output splines and the axle shaft splines.

Transmission damage via bad motor mounts isn't likely to be sudden, unless the transmission happens to fall completely out of the car. However, bad motor mounts will eventually create a host of problems with your engine, chassis, transmission and drivetrain, owing to excess engine/transmission movement and -- over a long enough period of time -- excessive vibration. If you've got a bad motor mount, don't stick your head in the sand; get it dealt with before that $50 part winds up costing hundreds or thousands in ancillary repairs.