Ford F-150/F-250: What Causes Transmission Leaks?

Any number of things can cause your transmission to leak, and if left unfixed a damaged transmission can cost a ton of money to repair or replace. Locate the source of your leak with our handy guide.

By Pizzaman711 - October 23, 2014

This article applies to the Ford F-150 (2004-2014) and Ford F-250, F-350 Super Duty (2006-2014)

Any form of a transmission leak is a serious concern. Without proper lubrication from transmission fluid, your transmission can kill itself in a matter of miles. There're several places that a leak can occur so it'll take a little investigating on your part to find the source. If you're unsure of doing the repair work yourself I highly suggest taking it to a shop and having it done there instead. Just whatever you do, don't use stop-leak products. These try to stop the leak by swelling gaskets and seals, but they can actually end up causing more damage than they fix.

Tools Needed

  • Transmission Fluid
  • Paper towels
  • Cardboard
  • Socket set
  • Transmission seals/gaskets

Step 1 - Find the leak

This step might be the most time consuming part of the process due to the amount of places a leak can form. If your unsure of whether it leaks when the transmission is pressurized or not (the engine is running), this process can take even longer.

I suggest parking the truck over some cardboard or paper. This can make a leak easier to spot because the fluid will show up more clearly than if it were to just drip into the driveway. Also, it'll help protect your garage floor from stains. A spot on the cardboard doesn't necessarily mean that the leak is directly above it. Fluid could easily be running along your truck's undercarriage until it drips off. Get around this by dragging paper towels along the drip path until you hit/see the spot where the fluid is actually coming out. The possible areas for a leak include:

Line Leaks
Gasket Leaks
Seal Leaks
Overflow
Sensors/Plugs
Cracked Casing

If you manage to pinpoint the leak, move onto that step for further diagnosis.

Step 2 - Line leaks

Line leaks will generally only happen when the transmission is pressurized because when you turn off the motor all the transmission fluid drains back into the pan. Line leaks can be caused by rusted, cracked, or punctured lines. A accident can also cause a leak to form from the lines as it can slightly bend them out of the fittings. If the leak is caused by rusted out line you'll want to do a thorough inspection on the rest of your lines as well because the pressure in the lines can cause it to start leaking at the next rusted and weak point.

Depending on the extent of the damage, you can splice in a patch line for pin sized holes. For bigger damage you may need or want to replace the entire line to be on the safe side.

  • Figure 1. 6R140 Transmsission Cooler Line Diagram
  • Figure 2. 4R75E/W Transmsission Cooler Line Diagram
  • Figure 3. 5R110W Transmsission Cooler Line Diagram

Step 3 - Gasket leaks

Over time the transmission pan gasket will wear out and can begin to leak. If you find the leak coming from the gasket, whatever you do, do not try to tighten down the bolts more to make it seal. Over tightening the bolts can not only make the leak worse, but can bend the transmission pan and also strip the threads holding it on. Gasket leaks from the pan will normally occur when the engine is off due to all the fluid draining back into it. The fix isn't too hard, all you'll need to do is:

  • Drop the transmission pan
  • Dispose of the old fluid
  • Carefully remove the old gasket
  • Put on the new gasket and reattach the pan
  • Top off the fluid
Figure 4. A transmission that is leaking from the pan gasket will have tell tale red drips like so.

Step 4 - Seal leaks

A leak from a seal can be hard to diagnose as the majority of the seals are internal. On top of that, a seal leak can be hard to distinguish from a cracked casing. Seals can leak anytime, but the leak will be stronger when pressurized. If you see no visible signs of damage to the transmission and/or the leak is coming from a spot where two pieces are bolted together but there is no gasket, chances are it's a seal that's gone bad. Unfortunately, almost all seals will require pulling most of the transmission to replace them.

  • Figure 5. This transmission is leaking from it's rear seal. You can tell from the dampness around the rear area and the color of the fluid.
  • Figure 6. It's clear from this angle that transmission fluid is leaking. From where, exactly, is more difficult to determine.

Step 5 - Overflow

Definitely the easiest to both check and fix, an overflowed transmission will leak from various places when pressurized until the level has returned to a normal amount, and then will hopefully stop. However, depending on how overfilled it is, the leak could cause damage to both seals and gaskets from the fluid trying to find a way to escape. To fix it you'll simply need to drain some fluid by partially un-bolting the transmission pan until the fluid is within the proper range. Afterwards, you'll want to check and make sure the leak is persisting due to it causing another problem elsewhere.

Step 6 - Sensors/Plugs

Some sensors in the transmission have fluid going into them to monitor things like the level, temperature, etc. Inside the sensors o-rings are used to seal where the fluid travels and over time they wear out. If you see fluid coming out a sensor or its plug, there's a good chance it needs to be replaced or rebuilt if possible.

Step 7 - Cracked casing

This is definitely the worst case scenario for a leak and the least likely. The casing can get damaged in a couple different ways but most of the time it will be from something physically impacting it and cracking or puncturing it. Unless the damage is to the transmission pan, this is far from an easy fix. At the very least it will require pulling the transmission and the worst case scenario will have you replacing the transmission.

Figure 7. A transmission case with a crack in it will need to be replaced or fixed.

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