1998 GMC Sonoma Trans Mount Replacement Cost

Depending on the orientation of the engine compared the the vehicle, a transmission mount will either be located near one of the front wheels, between the transmission and front floorboard, or underneath the vehicle somewhere between where the front seats would be. These mounts provide not only a mounting point for the transmission, but they provide a barrier to vibration from the engine and transmission. This vibration damper allows the vehicle to stay quiet and feel smooth, especially when the transmission is being shifted to driver or reverse and under heavy acceleration. Transmission mounts will typically have a metal casing which surrounds a tough rubberized material, which surrounds another metal component. Let's examine the functionality of this form.

The transmission mounts outer casing provides a strong mounting point to the vehicle. This is made of metal because it will support the entire weight of the transmission, allow bolts to be tightened into it, and must be machined perfectly flat to avoid vibrations. Inside this metal housing, the rubberized damper is tightly fit to the outer casing and molded around the other metal portion which will be described momentarily. This prevents metal contact from the transmission side of the mount to the chassis side of the mount. Sometimes, for additional vibration prevention, the rubberized portion is filled with liquid as well. The metallic piece in the center of the rubberized insert is rigid and strong, and provides a way for the transmission to attach to the rubberized portion, which, again, is locked into the outer housing. When the transmission side metal bracket is bolted to the transmission and the chassis side is bolted to the chassis, the transmission is actually supported by the rubberized insert, and linked to it by the metal bracket and housing discussed.

Since vibration prevention is the reason for the design of the transmission mount, you can guess that excessive vibration from the drivetrain is likely due to engine or transmission mount failure. Also, when the transmission mount is completely worn out, meaning the rubber insert has worn and failed, the transmission may begin to make clunking noises against the chassis. This is the noise of the metal bracket and housing contacting very harshly with very little padding. This would be noticed the most when the vehicle is being shifted into reverse or drive, when braking quick and hard, or when accelerating rapidly. In some cases, the transmissions movement may even cause the check engine light to illuminate or the engine to stall. This is due to excessive movement of the engine and transmission, which allows components to be damaged or loosened. Lastly, front wheel drive vehicles may have clicking upon turns due to the sagging transmission mount causing the CV axle to bind while turning and accelerating.

Leaving a transmission mount to further decay is a bad idea. The vehicle can be driven, but the transmission mount, as discussed earlier, can cause many issues if left uncorrected. The CV axles and wheel bearings for front wheel drive vehicles may fail repeatedly, and these will render the vehicle useless until repaired. For rear wheel drive and four wheel drive issues, the transmission output shaft seal and bearing or bushing is likely to suffer from sporadic lateral pressure from the driveshaft slip yoke. This will lead to leaks, driveline noise, and likely an extension housing replacement if the issue continues. The best recourse is swift action to correct the issue before the problem gets larger.

Transmission mounts, like engine mounts, are after specified for inspection on factory scheduled maintenance after 50,000 or 60,000 miles. The expectation is not failure at these points, but enough failures occur that most manufacturers decide to inspect this early. In general a set of motor and transmission mounts on a normally driven vehicle should be expected to last around 100,000 miles or more.

There are a couple of things that will be checked when a transmission mount is suspected of failure. First, with the engine on, the technician will have a helper shift the transmission from reverse to drive several times to search for excess movement, noise, and to measure the vibration in the cabin of the vehicle. This is done with the understanding that a bad transmission mount will make the most vibration in gear with the vehicle stopped. Next, if excess vibration or movement is detected. The technician will visually inspect all mounts for signs of past leakage. This is only done for liquid filled mounts. Afterwards, even if no leakage is detected, the mounts will be manipulated with a prying tool. When the mounts are moved, they will give the technician an idea of the firmness of the mount, and allow any cracks or deterioration to show. If the mounts fail any of these checks, they will be replaced.

The engine and transmission must be supported, and this may be done with a transmission jack or engine brace, depending on the location of the mount and the orientation of the drivetrain. Once weight is removed from the transmission mount, the bracket will be disconnected from the transmission, then the housing will be removed from the chassis. Note that some subframe components or chassis crossmembers may need to be removed. The new mount is placed into the vehicle, attached first to the chassis, then the transmission is positioned in line with the new mount before bolting the transmission to the metal bracket extending from the rubberized insert. In some cases it may be necessary to adjust the other mounts for proper fitment, as the failing transmission mount can cause small movements to occur for the other mounts.

We recommend having the motor mounts inspected at major services. This should already be on the maintenance schedule, so follow the owner's manual guidance concerning the inspection and repair intervals. In general, transmissions require fluid and filter changes at the same interval they require transmission mount inspections.

Keep in mind that motor mounts and transmission mounts fail gradually under normal circumstances. With every blow they absorb from the drivetrain they become unnoticably weaker. This will cause a long term owner of a vehicle to gradually adapt to the feel of the vehicle with bad mounts, and leads to increased repairs of the CV axle and wheel bearing assemblies. Also, some motor and transmission mounts are required to be adjusted for exact positioning. This will eliminate the possibility of excessive vibration from the new mounts.

For most transmission mounts, the experienced DIYer can take on replacement with no problems. However, when precision alignment of the mount is required, even experienced technicians may have issues eliminating vibrations after installation. In theory, there should be no trial and error to adjusting the mount, but since aftermarket parts are likely to be used, the mount may not align as the factory mounts once were. If supporting the engine and transmission is not possible for the exchange, or excessive vibration is noticed after the repair, consult a professional.