1991 GMC C3500 Trans Oil Pan Gasket Replacement Cost

The reservoir for transmission fluid is below the transmission in what is called the transmission pan. This pan is strikingly similar to a misshapen metal cake-pan, and is secured by several bolts all the way around the top edge of the pan. It bolts directly to the transmission, but has a thick gasket between the transmission pan and transmission to prevent the transmission fluid from splashing out of the seam at every turn. This gasket is aptly called the transmission pan gasket, and is typically made of a synthetic rubberized material.

Gaskets of all types work by removing the gap between sealed components, therefore eliminating the ability of air and liquids, like transmission fluid, from escaping from those gaps. The transmission pan gasket is applied with even pressure around the entire mating surface of the transmission housing and the transmission pan gasket, so it maintains its integrity and lies completely flat against both components. The light torque required of the transmission pan bolts is sufficient to hold the gasket in place, allow the gasket to seal, yet does not squeeze the gasket to deformity.

A leaking transmission pan gasket will drip transmission fluid on the ground, even while the vehicle is not running. This is because of the large amount of transmission fluid needed to protect the transmission and maintain internal operating temperatures at a reasonable level. Also, in a worst case scenario transmission fluid can drain onto the transmission wiring harness causing electrical issues for the transmission as the transmission fluid wicks up the wiring harness into electrical connections.

Transmission pan gasket seepage is not a large concern, in fact, it may not warrant a trip to the shop, however, droplets forming on the transmission pan, or leaks on the ground are sure signs that the transmission needs service. A few days with a leaking transmission pan gasket is not a dire situation, so long as the transmission fluid level is checked daily with the engine warm and running. Though it is important to have the transmission pan gasket replaced without undue delay.

Transmission pan gaskets are replaced once every 40,000-60,000 miles on vehicles with service intervals call for regular transmission fluid changes. Some automatic transmissions and CVT transmissions may be considered "sealed-for-life", but as these transmissions are gaining miles in the field, they are experiencing more failures due to lack of maintenance. Even these transmissions are being recommended for fluid and filter changes at normal intervals.

A leaky transmission pan gasket will be diagnosed at a glance, but replacing the transmission pan gasket can be for many reasons. There is no diagnosis needed when performing scheduled maintenance, repairing the transmission, diagnosing the transmission, or replacing the oil pan or filter.

The transmission pan is removed by removing all bolts surrounding the transmission pan. Before removing the final few bolts, any obstructions will be removed, and the pan will be allowed to tilt into a drain pan to lower the transmission fluid level. The transmission pan gasket will be peeled away as the pan drains, and the transmission fluid filter will be removed simultaneously, using the leaning transmission pan to catch the filter, gasket and all the fluid that will be falling from the transmission. Once the transmission is done draining fluid, the technician will clean the pan, the lower valve body, inspect for obvious signs of failure or future trouble, and checked the fluid, magnets, and filter for debris. If everything is satisfactory the technician will clean the mating surfaces of the transmission and transmission pan, replace or reuse the gasket, instal a new filter, and instal the transmission pan with locking compound on the threads of the transmission pan bolts. Once complete, the transmission will be filled, warmed by running the engine, and topped off with transmission fluid

We recommend using a one piece synthetic gasket where silicone is not recommended. Even on models which originally used a cork gasket, the rubberized synthetic gasket has far superior sealing and adhesion properties, and may even be re-useable.

Modern vehicles with drain plugs in the transmission drain pan should be referenced before engaging in servicing the transmission at home. More frequently, manufacturers are using transmissions that must be filled from underneath the vehicle with a pump. This means that a leaking transmission pan gasket change may require the vehicle be taken to a service center.

This job requires a little bit of finesse, and most DIYers will split their first transmission pan gasket when installing the transmission pan. That is because the transmission pan should be installed with a small torque wrench, all bolts tightened in sequence, and by placing trust in the manufacturers light torque specifications. Again, if the pan is filled through a combination drain and fill plug in the bottom of the transmission pan, only the advanced DIYer capable of monitoring the transmission temperature sensor via data link should attempt this repair.