mounts are not a very glamourous product, but they do have an important
function: They support the engine and transmission, and dampen noise
and vibration. The mounts isolate the engine and transmission from the
chassis so vibrations and noise are not transmitted to the rest of the
vehicle. On some front-wheel drive cars, upper mounts (torque struts)
also control the fore and aft movements of the engine during
Most motor mounts are relatively simple in design and
consist only of metal attachment plates and large rubber insulator
blocks. But some vehicles have “hydraulic” or “hydro-mounts” with
hollow chambers filled with glycol or hydraulic fluid. Hydro-mounts act
like a jelly-filled donut to absorb vibrations that would otherwise be
transmitted to the chassis. Hydro-mounts are often used with
four-cylinder and V6 engines that don’t idle as smoothly as a V8, and
in luxury vehicles where motorists expect less noise and vibration.
Some hydro-mounts even have internal valving and/or a solenoid to
change the dampening characteristics at different RPM to better tune
out unwanted vibrations. These are called “switchable” hydraulic mounts
or “electronic” mounts.
WHEN MOUNTS GO BAD
When a motor or
transmission mount fails, one of several things can happen. If the
rubber separates or delaminates from the steel, the mount can break.
The design of the mount usually prevents the engine from falling out of
the car, but it can’t keep the engine from twisting or rocking when the
vehicle accelerates or is under load.
This can produce thumping and
rattling noises, as well as overstressing components such as radiator
and heater hoses, wiring connectors and the exhaust system. In
rear-wheel drive applications that have an engine-driven fan, a broken
mount may allow the fan to hit the radiator or shroud. Drive belts or
pulleys may also be forced to rub against other components if
clearances are tight.
A broken or loose motor mount in a front-wheel
drive application can be even more serious because it may allow engine
movements that interfere with the throttle or shift linkage. Excessive
fore and aft rocking of a transverse-mounted engine can also lead to
exhaust leaks where the head pipe joins the manifold, or cause the head
pipe itself to fail. If the broken mount is an end mount, it may also
contribute to a torque steer condition and cause accelerated wear or
separation of the inner CV joints on one or both halfshafts.
motor mounts are seldom checked unless there is an obvious problem, and
they may even be overlooked if the engine or transmission is being
replaced. You should remind customers to check their motor mounts if
the engines seem noisier than usual or they can feel engine vibrations
inside their vehicles. The condition of the mounts should also be
inspected when any major engine or transmission work is done, or when
replacing a clutch, halfshafts or a driveshaft.
Mounts can be
visually inspected for cracked, loose or broken brackets, loose or
missing bolts, collapsed rubber or fluid leaks (hydro-mounts). A pry
bar can be used to check for separated or broken mounts.
to check mounts is to put the transmission into drive and lightly load
the engine while keeping the other foot on the brake. Excessive engine
movement may indicate loose or broken mounts that need to be replaced.
mounts may or may not have the same construction as the original.
Fluid-filled hydro-mounts are expensive, so a more affordable
alternative may be a solid replacement mount. But a solid mount
obviously can’t provide the same level of dampening as the original
hydro-mount. Consequently, the vehicle owner may not be happy with the
way his car feels if a less-expensive solid mount is substituted for a