A bad transmission mount usually shows up as a clunk when you shift into drive or reverse, a thump when you get on or off the throttle, more vibration at idle, or visible movement of the transmission under load. The mount is a rubber or polyurethane cushion between the transmission and the crossmember. When the rubber cracks, separates or collapses, the drivetrain moves more than it should and the shock reaches the cabin.
Transmission mount symptoms
- Clunk shifting into gear. The drivetrain rocks as it takes up torque.
- Thump on acceleration or lift-off. Movement each time torque reverses.
- Vibration at idle or in gear at a stop. A hard, worn or collapsed mount transmits more vibration.
- Shifter or linkage misalignment. On some vehicles a sagging transmission pulls the shift linkage out of adjustment.
- Driveline vibration. A sagging tail changes the driveshaft angle on rear-drive vehicles.
How to check a transmission mount
With the vehicle safely supported, look for cracked, split or oil-soaked rubber and for a mount that has separated from its metal plates. Then have a helper hold the brake firmly and shift between drive and reverse at idle, keeping the engine speed low, while you watch from a safe position to the side. A little movement is normal; a lift or lurch of several inches, or metal-to-metal contact, is not. Engine mounts fail the same way and often at the same time, so check them too; see engine mount symptoms.
Mount or something else?
Clunks can also come from worn U-joints, a loose driveshaft, differential backlash or worn suspension bushings, and harsh engagement can come from the transmission itself. If the mount looks good, see is it the transmission or the torque converter? and transmission slipping: signs and causes.
Replacing a transmission mount
- Support the transmission with a jack and a block of wood before unbolting anything.
- Remove the mount-to-crossmember and mount-to-transmission fasteners, raise the transmission slightly and swap the mount.
- Lower it onto the new mount and torque the fasteners to the service manual figure.
- Check exhaust and shift linkage clearance afterwards.
On many rear-drive trucks this is a straightforward job; on some front-drive cars the mount is harder to reach and may share loads with the engine mounts.
Rubber vs polyurethane
Polyurethane mounts reduce drivetrain movement, which helps with modified or high-torque engines, but they pass more vibration and noise into the cabin. For a daily driver, a quality rubber mount is usually the more comfortable choice.
When the transmission is out anyway
If you are replacing the transmission, fit a new mount at the same time; an old mount can let a new unit move and clunk from day one. See how to install a transmission and the units by code on transmissions.
More guides: rebuilt vs used transmission · bad flex plate symptoms.
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