Transmission Mount

A transmission mount carries the back of the drivetrain, and where it holds that weight sets the angle the driveshaft runs at.

Which is why a collapsed one produces a vibration rather than just a clunk.

What a Transmission Mount Is

The engine and transmission bolt together and are supported as one assembly, which is why some catalogs list the parts together as engine transmission mount hardware. Engine mounts carry the front, and the transmission mount carries the rear.

On a rear or four wheel drive layout it sits under the tailshaft housing, bolted to a crossmember that spans the chassis rails. On a transverse layout it sits at one end of the transverse assembly, doing the same job in a different orientation.

Inside is rubber bonded to steel, specified to carry the load while isolating the cabin from gear noise and driveline vibration.

Some designs are hydraulic, filled with fluid to damp specific frequencies. Those fail differently, since a leaked mount has lost its damping rather than simply going soft.

The transmission mount bracket is the steel component the mount bolts into, and on some platforms that bracket is the part that corrodes or cracks rather than the mount itself.

Driveline Angle and the Vibration Nobody Traces

This is what separates a transmission mount from every other drivetrain mount.

The driveshaft runs from the transmission tailshaft to the differential, with a universal joint at each end. Those joints tolerate an operating angle, and they run smoothest when the angle at each end is close to equal.

The transmission mount sets the height of the tailshaft. Collapse it and the tailshaft drops, which steepens the angle at the front joint while the rear stays where it was.

Unequal joint angles produce a vibration that rises with road speed rather than engine speed, usually felt through the floor and the seat rather than the steering wheel.

People chase that vibration into driveshafts, balance weights and differentials for months. The cause is often a mount that has lost half an inch of height over a decade.

It also wears the universal joints faster, since a joint running at a steeper angle cycles further on every rotation.

Bad Transmission Mount Symptoms

The signs separate into noise, movement and vibration.

A clunk when shifting between drive and reverse, or when getting on and off the throttle, as the drivetrain moves and takes up the slack.

Vibration through the floor at idle that was not there before, since the isolation that was damping it has hardened or torn.

Driveline vibration at road speed, which points at the height change rather than at the isolation.

Shifts that feel vague on a manual, because the transmission is moving relative to the shifter while you are trying to select a gear.

Visible movement of the transmission when the engine is loaded against the brakes.

And the physical evidence, which is cracked or separated rubber, fluid residue on a hydraulic mount, or a transmission sitting visibly lower than it should.

Checking a Transmission Mount

Three checks between them will find most failures.

The load test is the one to start with. With the vehicle safely supported and the brakes firmly applied, have someone select drive and then reverse while you watch the transmission from a safe distance. Movement of an inch or more means significant play.

The visual check is the second one to do. Look for rubber separated from its steel, cracks running through the material, or fluid residue around a hydraulic mount.

The height measurement is the one most people skip and the most useful on a rear drive vehicle. Measure from a fixed chassis point to the tailshaft housing and compare against the other side or against specification, since a sagging mount shows there before it shows anywhere else.

A driveshaft angle gauge gives the same answer more directly, measuring the angle at the transmission output and at the differential and comparing the two.

Check the crossmember and the bracket at the same time. Corrosion, cracking or elongated bolt holes in either produce the same symptoms as a failed mount.

Sagging Rather Than Tearing

A transmission mount usually fails by compressing rather than by breaking, which is why it goes unnoticed.

The mount carries the weight of the drivetrain continuously. Over years the rubber takes a permanent set, losing height gradually rather than failing at a moment anyone notices.

Because it happens slowly, the driver adapts without noticing. The vibration that arrives at 60 becomes normal, the clunk into reverse becomes a characteristic, and nothing prompts an inspection.

That is why measuring is more useful than looking. A mount with no visible cracks can have lost enough height to change the driveline angle measurably.

Comparing the tailshaft height or the driveshaft angle against specification finds it where a visual check does not.

Adjustable Transmission Mounts

Where the driveline angle needs setting rather than restoring, an adjustable mount is the tool.

These allow the tailshaft height to be changed, usually with a threaded or shimmed arrangement, so the driveshaft angle can be dialed rather than accepted.

That matters on a lifted vehicle where the differential has moved, on a lowered car where the opposite has happened, and on anything with a transmission swapped from a different platform.

It also matters where a vibration persists after new mounts, since the correct angle may not be the standard one once other things have changed.

Setting it requires measuring the angles at both ends of the driveshaft rather than adjusting by feel, since the aim is matching them rather than hitting a particular number.

Mount, Bracket and Crossmember

Three components in the same assembly and the distinction matters when ordering.

The mount is the rubber and steel item carrying the load. The transmission mount bracket bolts it to the transmission or to the crossmember, depending on design. The crossmember spans the chassis and carries the whole arrangement.

Any of the three can be the fault. A sound mount on a cracked bracket still allows movement, and a corroded crossmember flexes regardless of what is bolted to it.

Check all three when the symptom is movement. The mount is the usual culprit and it is not always the one that has failed.

A transmission mount bushing or crossmember bushing exists on some designs as a fourth layer, isolating the crossmember from the chassis, and those add their own movement when worn.

Transmission Mount Replacement Cost

Parts are modest and the labor varies with what has to be supported underneath.

The transmission has to be held while the mount comes out, which means a transmission jack or a support bar rather than taking the weight on the sump.

On some vehicles the crossmember drops with two bolts and the job is straightforward. On others the exhaust, heat shielding or part of the driveline has to come out first.

Hydraulic mounts cost more as a part than conventional rubber ones, and polyurethane or complete performance mounts sit above both.

Where the mount is coming out anyway, doing the crossmember bushings at the same time costs parts rather than another labor charge.

Budget for checking the driveline angle afterward on a rear drive vehicle, since restoring the correct height is the point of the job.

Polyurethane and What Changes

Firming the mount changes the vehicle in ways worth expecting.

The drivetrain stops moving under torque, so power delivery feels more direct and the clunk on take up disappears.

Shift feel improves on a manual, because the transmission stays still relative to the shifter rather than moving with the torque.

Driveline angle stays constant under load rather than changing as the mount deflects, which helps the universal joints as well as the feel.

The cost of all that comes in noise. A transmission is a gearset, and the rubber being replaced was there partly to stop that reaching the cabin.

On a performance or working vehicle that trade is usually worth making. On a quiet daily driver it is worth weighing, and an insert rather than a complete mount is the middle ground.

Find Transmission Mounts for Your Vehicle

Lock down the engine end with polyurethane engine mount bushings, and find more driveline upgrades in our transmission parts lineup.

Select your vehicle to see the transmission mounts, brackets and bushings available for it. If the vehicle vibrates at road speed and the driveshaft has already been checked, measure the tailshaft height before looking any further down the driveline.