Engine and transmission mounts isolate vibration and noise from the passenger compartment of the vehicle. When the mounts wear out or fail, noises and vibration are normally the two first symptoms. If left unattended, other problems may also develop.
Inline mounted engines
An automotive engine produces vibration when it operates. They design the engine and transmission mounts as an insulator. Mounts that provide insulation from vibration hold the vehicle’s engine to the chassis.
Vehicles they built in the past were rear-wheel drive, with inline engines. On these models, the engine and transmission mounts were simple devices. Most often a rubber block, bounded between two pieces of metal, connected the engine and vehicle frame. A third mount attaches the transmission to a crossmember in a three-point system. Engine and transmission mounts of this type are inexpensive, durable and work well. The arrangement and weight of the engine and transmission prevented most movement and they did not require sophisticated mounting.
Transverse mounted engines
Today, most vehicles use transverse-mounted engines and transmissions. That means the engine is mounted perpendicular to the centerline of the vehicle. This style of powertrain mounting requires a more complex set of engine and transmission mounts. The engines are shorter, narrower and much lighter. Engine torque causes the lighter engine to lift more. Usually, they suspend the engine and transmission weight from a mount on each side. Mounts on the front and rear of the assembly control the torque of the engine.
When we place the transmission in a forward gear, torque causes the powertrain to rotate. This rotation lifts the front of the engine and pulls up on the front engine mount. If we place the transmission in reverse, we reverse the load. This causes a downward force on the front mount. They use between four and five engine and transmission mounts to control this motion.
Engine and transmission mount failure
Continual pressing and stretching of the engine and transmission mounts, eventually takes a toll. The mounts on the engine and transmission wear out. The rubber in the mounts may also tear and pull away from the metal attachments. When this occurs, the mounts no longer hold the powertrain firmly in place. The engine will lift up on acceleration and may slam down when we release the throttle. Often we will hear a bump or thud as the engine lifts and drops. This also causes the drive axles to lift out of their normal position. Running out of position causes a bind and eventually damage may result.
A collapsed engine and transmission mount
Failure of engine and transmission mounts also occur in other ways. Instead of tearing, some mounts collapse. These mounts fail internally and may not show external damage. When we compared an old mount with a new mount, the difference in height is apparent. With a collapsed mount, engine vibration passes to the vehicle. At an idle, the steering wheel and dash may shake. Often the vibration will be worse when we place the transmission in drive or in reverse.
When bad mounts cause the symptoms to become noticeable, we will need to replace one or more of the mounts. Some mechanics advocate replacing all mounts. Their reasoning is, new mounts will carry a disproportionate load when working with old mounts. Because of the expense involved, often this is not done. Instead, they replace only the broken or bad mounts. This may be a fair compromise, but life of the new mounts may suffer.
Hydraulic and active engine mounts
To improve the passenger's comfort, many vehicles today use fluid filled hydraulic mounts. Rather than a simple block of rubber, the mount contains chambers and fluid. Between the chambers is a valve that controls the flow, much like a shock absorber. More sophisticated designs employ a vacuum or electrically controlled valve. The power control module sends a command that controls the flow in the mount. This slightly improves vibration at idle, but drastically increases the cost of the mount. Most hydraulic mounts cost several hundred dollars each.
Newer designs may use electrically active fluid and magnetic fields. The power control module supplies a variable current to change the mount. These mounts can probably control vibration to an even higher degree. This type of complexity tends to be more fragile, and the life span of such mounts can be relatively short. Replacement costs can be shockingly expensive, costing into the thousands to replace.
Miles and time combine, and most engine and transmission mounts will eventually fail. With the standard mounts, the symptoms are often minor and replacement may not be needed. With more modern hydraulic and active mounts, the symptoms are normally far more severe.
How to diagnose bad engine and transmission mounts
An old method for testing powertrain mounts is to watch the engine, while shifting from drive to reverse, with the brakes applied. All engines will rise and dip slightly. Excessive rise of more than an inch suggests possible mount problems.
Vibration caused by mounts can be tested by slightly lifting the engine. A floor jack is used to lift the weight of the engine, a small amount. Cushion any contact between the engine and the jack with a block of wood. Care must be taken as most engine components such as oil pans are not designed for lifting. Do not attempt to raise the engine, only to support a slight amount of its weight. A change in the vibration means the mounts should be checked more closely.
Undamaged mounts may also vibrate if they are out of position. Hitting a curb, pothole or a collision can leave mounts in a bind and cause vibration. Bolts left out, from a previous repair can also allow mounts to move out of position. A vibration at idle, without bad engine mounts, may suggest such a problem. Specialty frame and chassis shops are often needed to diagnose such issues.