Shake, rattle and roll. That’s what the drivetrain can feel like if a motor or transmission mount is broken. The mounts are there to not only support the engine and transmission but to also isolate engine vibrations from the rest of the vehicle.
Engine vibration is usually most noticeable at idle, especially with four-cylinder engines, odd-fire V6 engines and diesels. Vibrations tend to worsen when the A/C compressor is engaged, or when there is a heavy electrical load on the alternator.
Motor mounts are fairly rugged, and typically consist of a molded rubber donut or spacer laminated between a pair of stamped steel plates. On in-line rear-wheel drive applications, the mounts are usually located on both sides of the engine block and under the transmission tailshaft. On front-wheel drive applications, the mounts are usually more intricate (and expensive). The mounts are located at both ends of the engine (right and left), with a third mount located near the top of the engine or under the transaxle to prevent the engine from rolling.
Over time, the rubber portion of a mount may delaminate from its support plates (broken mount), or it may crack or deform (collapsed mount). Either way, you customer is going to need one or more replacement mounts. On a high-mileage vehicle, replacing ALL of the mounts at the same time can eliminate annoying vibrations and reduce the risk the other mounts failing at some point later on.
Although the motor mounts on many vehicles are the relatively simple solid rubber and steel mounts, many import cars and luxury vehicles use liquid-filled “hydromounts” to minimize NVH (Noise, Vibration & Harshness). The liquid is contained within a hollow cavity inside the mount. Over time, leaks can occur causing the mount to deflate and lose its ability to dampen vibrations. Hydromounts should be replaced with the same type of mount to maintain the original dampening characteristics that were designed into the vehicle. But if a customer is looking for a less expensive repair alternative, solid mounts are available for many of these applications.
The latest technology includes “active” motor mounts that can change their dampening characteristics in response to external inputs. Active mounts can be relatively soft at idle to absorb the unwanted shakes produced by widely or unevenly spaced cylinder firings, then stiffen up at higher engine speeds and loads to limit unwanted engine motions. Applications include certain late model Honda, Hyundai, Jaguar, Lexus Toyota and others.
Some active mount systems use a pulse width modulated control signal from the PCM to vary the stiffness of the mount. The control signal opens and closes a solenoid that allows engine vacuum to pull air out of hollow cavity inside the mount. A bad solenoid or vacuum leaks in the hose connections can prevent the mount from functioning normally.