Motor Mount Faults

4-03 Identify motor mount failures.

Motor mounts support the vehicle’s powertrain while insulating the passengers from noises and vibrations produced while the engine is running. All engines and transmissions generate both noise and vibration due to reciprocating components. Typically, the engine’s vibration characteristics are most noticeable at idle. Shifting gears causes the most noticeable movement. The powertrain’s mounts act as an insulator to control both of these issues. When a mount fails, it typically transmits the noise and vibration to the passengers. A mount that fails can also induce problems other than vibration and noise due to excessive powertrain movement.

Transverse-mounted engines (sit sideways in the engine compartment) also usually incorporate strut rods to control engine movement when placing the vehicle in gear and during acceleration or deceleration (FIGURE 4-7). When placing the transmission in gear, torque passes through the engine to the transmission, causing the powertrain to rotate. When putting the transmission into drive, the front of the engine lifts, pulling on the front mount while placing downward pressure on the rear mount. When shifting the transmission into reverse, the load changes, and now torque lifts the rear mount while pushing down on the front mount. The addition of torque or strut rods limit this movement and help prevent damage to the motor mounts. Worn or damaged motor mounts allow excessive engine movement, creating abnormal noises or a loud bang and harsh engagement when placing the transmission in gear. Contact can also occur between moving and stationary components while driving the vehicle, due to worn mounts. Tearing of exhaust flex pipes or the fresh air inlet tubing is also often a result of excessive engine movement.

FIGURE 4-7 This is a torque mount that has failed and will need to be replaced. This failure can cause the engine to jump around as the vehicle is accelerated. As the engine moves, it causes vibrations and noises inside the vehicle.

Motor mount technology has evolved and continues to evolve. Older, rear-wheel-drive vehicles with in-line engines commonly used a simple three-point mounting system of a rubber block mounted between two pieces of steel. A mount was located on each side of the engine, with a third mount in the center of the transmission tail shaft. Newer vehicles may use hydraulic mounts, hydroelastic mounts, hydromounts, or active motor mounts. Hydro-style mounts have a hollow chamber that is typically filled with hydraulic fluid (some may use a glycol fluid) to absorb and reduce vibrations, preventing vibrations from being transmitted to the chassis and passengers. These mounts are found most often on four-cylinder and V6 engines, along with some diesel engines, due to their inherent rough idle qualities. They may also appear, however, on luxury vehicles, which use larger engines, to increase the level of satisfaction associated with those vehicles. Some higher-end mounts use an internal valve similar to a shock absorber or an electronic solenoid to change the mount’s ability to dampen vibrations that vary with engine rpm.

Diagnosing Standard Motor Mounts

Several methods can be used to test and verify motor mount operation. Before testing, perform a visual inspection. Check for loose or cracked brackets, missing bolts and nuts, and collapsed or torn rubber in the mounts. Inspect for fluid leaks; oil and power steering fluid may degrade the rubber over time, tearing or collapsing under engine torque. Check hydromounts for fluid leaking from the mount itself, indicating failure. Use a pry bar to check the mounts by applying pressure while watching for movement; separation of the rubber from the metal or physically broken motor mounts are also unacceptable.

After performing the visual inspection, additional testing may be required. One test involves two technicians: one watches the engine while other slowly and repeatedly shifts the vehicle from drive to reverse (FIGURE 4-8). While performing this test, ensure the parking brake is applied, chock the drive wheels, and apply the service brakes. Another test involves brake torqueing the engine in gear while watching for excessive movement. Remember that the engine’s mount was designed to allow slight up-and-down movement; this does not indicate a failed mount. In the case of excessive movement, however, suspect a probable mount failure.

FIGURE 4-8 The technician must verify which motor mount has failed so that it can be replaced.

Note: Follow all safety precautions when performing these procedures. Apply the emergency brake and have an assistant keep a foot on the brake at all times. Stand to the side of the vehicle, not directly in front of it, while watching the engine. Due to cramped engine bays with limited visibility, perform the transmission engagement test with the vehicle on a rack, in the air, without loading the engine. To view and monitor mounts that are difficult to see from the top of the engine compartment, take position underneath them. While looking for excessive movement, have an assistant move the shifter with their foot on the brake.

Another method for detecting a broken motor mount entails slightly lifting the engine by using either a floor jack or a large pry bar to eliminate or change an engine vibration. With the engine running, apply slight upward pressure on the engine, using the floor jack. Install a block of wood between the floor jack and the engine to prevent damage to the engine. Use care when lifting the engine. The idea is not to raise the engine and lift it out of its mounts. Apply only slight upward pressure, supporting a small amount of the engine’s weight, checking to determine whether the vibration has altered. If the vibration and shake have changed, closely inspect the mounts. Moving the block of wood and jack to different locations and performing this procedure may together help to isolate the faulty mount. Use a pry bar to apply slight pressure to the engine from the top side. Before prying, verify that location of the pry bar will not cause damage to the engine or vehicle, at the prying point. Placing a block of wood under the prying point will also help protect the vehicle. Do not pry on plastic or composite engine parts; exercise extreme caution around fluid lines, and keep the pry bar away from moving components and belts when prying.

Physical damage to the mounts does not have to be present to create a vibration. Improper installation of the mounts can create a binding in the rubber of the mount, transferring engine vibrations to the chassis and then to the driver. Incorrect installation of the mounts, attaching hardware, or not allowing the engine to neutralize before tightening the bolts to the specification can each lead to vibration transferring to the chassis.

When replacing a mount, lift the weight from it before removing it from the engine or transmission. Exercise caution when lifting the engine or transmission with an engine support bar, transmission jack, floor jack, etc. Use an insulator to protect the lifting point, and visually inspect the area to verify that no damage will result during the process. Verify the proper procedures in the service information. Some engine and vehicle combinations need to have their intake plenum, radiator shroud, or cooling fan removed to allow the engine to be lifted enough to gain the clearance needed to access and replace the mount, for example.

Whenever replacing or tightening a mount after service, hand-tighten the attaching bolt and nuts. Start the engine, and move the shift lever through the gears, pausing briefly in drive, neutral, and reverse. This procedure neutralizes the engine, transmission, and exhaust—preventing the rubber mounts from binding. Place the transmission back in neutral or park, shut the engine off, and torque the bolts and nuts to specification. Always draw the bolts and nuts down evenly, to prevent binding. If the exhaust system is being serviced, follow the same procedure. Install and route the exhaust correctly, using the equipped hangers. Hand-tighten the attaching bolts and nuts, start the engine, and operate the shifter through the gears to allow the exhaust to neutralize. Exhaust vibrations can transfer through the chassis similar to engine vibrations. After any mount or exhaust repair, drive the vehicle through a broad rpm range, varying the load on the engine, to verify that a vibration or shake is not present.

High-Tech Motor Mounts

To dampen and control engine vibrations, manufacturers are installing motor mounts that change stiffness based on engine speed. Controlled electronically by the powertrain control module (PCM), “active” or “smart” mounts are switchable either hydraulically or electronically. The PCM activates a solenoid to control the mount’s movement before placing the transmission in gear, during acceleration or deceleration. By applying a vacuum through the solenoid, the mount becomes softer, usually done at idle, further dampening the powertrain vibrations (FIGURE 4-9). At higher rpm, the mount stiffens, controlling unwanted engine movement.

FIGURE 4-9 Adjustable motor mounts can be used to compensate for engine noises, vibrations, or running conditions. The PCM can control how the mount reacts to the running condition of the engine.

When a hydromount fails, it loses its fluid, resulting in a significant loss of the amount of vibration movement it can control. Fluid seen leaking from a mount indicates it has failed and thus needs to be replaced (FIGURE 4-10).

FIGURE 4-10 Using hydraulic fluid to help with vibration absorption is an easy way to create a high-performance engine mount.

So-called smart mounts can use vacuum to control engine shake based on engine rpm. Another option for the smart mount is an electronically controlled motor mount. The electronic motor mount uses a combination of an acceleration sensor to monitor movement/shake and an electric actuator to control the amount of shake or movement in the engine. Vacuum-controlled mounts use a solenoid that regulates the amount of vacuum to the mount, changing its stiffness based on rpm. Applying vacuum to the motor mount at idle makes it more flexible, absorbing more of the engine’s pulses and vibrations. As rpm increases, the PCM removes vacuum, increasing the rigidity (stiffness) of the mount and thus preventing excessive movement. Any vacuum leak that occurs at the mount, vacuum line(s), or vacuum storage canister frequently results in an increase in the engine’s vibration and shake at idle. Use a handheld vacuum pump to diagnose the system. Check to verify that vacuum reaches the mount from the control solenoid. Verify that vacuum holds at the mount, the supply line(s), and reservoir. Any leak indicates that the failed component needs to be repaired or replaced. If vacuum is not available at the motor mount, check the control voltage, ground, and vacuum supply to the control solenoid. The solenoid is typically tested with an ohmmeter to verify whether the solenoid windings are not open or closed.

An electronically controlled engine mount uses several inputs, a module, and an actuator to offset movement. The control module manages an actuator inside the mount, producing its own countershake or movement. The movement created is exactly opposite of what the sensor is receiving, canceling out the vibration, or shake, created by the engine. Other inputs are used to control the mount, including transmission selector range, coolant and air temperatures, engine load, and vehicle speed. Use a scan tool to check for codes. Diagnosing these systems requires a high-level aftermarket or dealer-level scan tool to check for codes and perform the active motor mount actuator test.