Thursday, May 25, 2017 Detailed Auto Topics
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Scanning the internet for transmission pan leaks, we find many complaints. We also find many suggestions to remedy the problem. Unfortunately, a huge amount of misinformation is present. Most transmission pan leaks result from improper service.

When does the problem start?

A spot on the driveway is the first symptom of a leaking transmission

In time, we need to service our transmission.  This is good maintenance and lowers our risk of transmission problems.  Proper transmission service requires us to remove the pan on those transmissions that have them. After the service, we need to reseal the transmission, and this is when problems often begin. A few days after replacing the transmission pan, we may find spots of fluid under the vehicle. This is a transmission leak and a common cause is the transmission pan.

Tightening the pan bolts is the most common advice given. This is usually wrong and can make things much worse. We should always tighten transmission pan bolts to the specified torque, when we install them.  This is done with a torque wrench. If we use this method, the torque on the bolts are correct. If the installer did not use a torque wrench, odds are the bolts are too tight already.

Pan gaskets do not shrink and pan bolts rarely come loose. This is why we see very few transmission pans leaking on vehicles that someone has not serviced. Over tightening the pan bolts will only damage the gasket or worse, strip the bolt threads.

How they seal the transmission pan

Manufacturers use several methods to seal the pan on their transmissions. High-quality reusable gaskets are very common. The technician inspects, cleans and reuses these gaskets. Many other transmissions today do not use a pan gasket at all. On these units, they seal the pan to the transmission case with an RTV sealer. Other transmission types use a fiber gasket or a traditional cork-style gasket.

Each method requires a specific procedure to seal properly. Recognizing the type of sealing method helps prevent leaks.

Reusable gaskets

A reusable transmission pan gasket

Reusable transmission pan gaskets may be heavy rubber or metal backed. These gaskets rely on raised ridges in the surface to seal. They design these gaskets for multiple uses. We inspect the gasket to be sure the ridges are not mashed flat. If the ridges are okay, they will seal the pan very well.

Avoid folded gaskets

Replacing this gasket with a cheap cork style gasket is a big mistake. Reusable gaskets are expensive but they work very well. Aftermarket filter kits often come with a cheap cork or paper gasket. Making this worse, they often fold these gaskets to fit them into the package. An unsuspecting enthusiast may throw away the high-quality factory gasket and install the inferior model. These leak and now we must purchase the expensive original style, along with losing the new transmission fluid.

A reusable gasket is cleaned of all fluid with an appropriate cleaner and installed. We also clean the pan surface and transmission case and do not use sealer. The dry gasket forms a seal against the transmission case, when we tighten the bolts to specifications.

RTV sealer

When manufacturers use RTV to seal the transmission pan, they are careful to apply the correct amount.  Many types of RTV are available and using the proper type is important.  Buying RTV from the vehicle manufacturer is the safest, but at least make sure it is suitable for use with automatic transmission fluid. Most problems occur when we use too much sealer or fail to remove the old sealer completely.

Some transmission pans are sealed with RTV

Removing a transmission pan, sealed with RTV, leaves the residue on the pan and transmission. We must very carefully scrape this away. Plastic razor blades are handy as they do not scratch the soft aluminum in the transmission case.

Clean any sealer off of pan bolts to avoid problems

Wire brushing removes any RTV sticking to the pan bolts. This may present a problem if not removed. Many threaded holes in a transmission case do not go all the way through. We must be careful to remove any sealer in the bolt holes.

Sealer causing bolt to not seal

Sealer that remains in the bolt holes takes up space. When we tighten the bolt, it may bottom out on the debris in the hole. Old sealer will not compress and the bolts will get tight, before they pull the pan into place. Tightening the bolts more will strip the transmission case or break the bolt.

With RTV sealer, the transmission pan and case should be clean and dry. We apply a thin bead around the center of the pan.  Make sure to also apply sealer around each bolt hole. When we tighten the bolts against the pan, the sealer should not ooze past the edge. Any RTV, which squeezes out, may clog passageways in the transmission and cause problems.

Other gasket types

Most other gaskets are cork-type material or a paper/fiber material. These gaskets should be flat and never creased. If the packager folds them into a tight crease, the gasket may not seal the pan properly. Quality vendors package their transmission-pan gaskets to remain flat. As with the other types, we first clean the pan and transmission and install the gaskets with no sealer.

Damaged pans

Over tightening the bolts can easily damage the pan of a transmission. The flange of the transmission pan should be flat and have no bulges. Over tightening the pan bolts causes the flange to distort.

Straighten any bulges in the pan rail

Inspect the pan and make sure the flange is flat. Most bent surfaces can be straightened. We should place a socket under the pan and gently tap the metal flat with a hammer. Check the surface with a straightedge if needed. We should replace a pan bent too badly to be straightened.

Tightening the bolts

Work out from center in a criss cross pattern

Some manufacturers give a specific pattern to tighten the transmission pan bolts. Service-data provides the pattern and the torque specification.  When we cannot find a specific tightening sequence, at least tighten from the center, working out and using a criss-cross patten as above. Tightening all bolts slightly, and then to the final torque, in the pattern specified, is best.

Transmission pans are easy to seal, if we follow the correct procedure. Play it safe and do the job right the first time.





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