How to Repair a Truck Transmission Cooler Line

January 27, 2012

There are some simple procedures to use when you repair truck transmission problems. Unfortunately, transmissions are very complicated, and your repairs should not be made to internal components. However, some of the more common transmission repairs are leak repairs, and they are readily assessable to the do-it-yourselfer. Transmission cooler line leaks are very common, and are easy to repair.

Materials List

  • Wrench set/Socket set
  • Shop towels or rags
  • Drain pan
  • Vehicle jack
  • Replacement transmission cooler line
  • Transmission cooler line bender (optional)
  • New transmission fluid
  • Teflon tape (used in plumbing applications)

Step 1: Find the Leak

If your truck is high, enough for you to move comfortably under it, there is no need to jack up the vehicle. The transmission cooler line will be a solid metal line, typically made of steel or aluminum. There will be a pair of lines that connect to the transmission, at one end, and the cooler at the other end. If your vehicle has an external cooler, it will be mounted below the engine so that air will pass through it as the vehicle moves. An external cooler looks like a miniature radiator. If an external cooler is not found, then your transmission cooler is part of your radiator. Inspect along the transmission cooler lines checking for leaks. Otherwise, the leak will be either in one of the transmission cooler lines or in the cooler itself. For coolers that are built into the radiator, leaking is evidenced by finding either transmission fluid in the radiator coolant, or coolant in the transmission fluid. For external coolers, a leak will show itself as transmission fluid drips from it.

Step 2: Repair the Leak

If you were lucky, the problem was just a loose compression fitting. In that case, tighten the fitting, and recheck for leaks. However, this is generally not the case. If the leak was found to be in the cooler itself, then it should be replaced. External coolers can be fixed by soldering, but this is a job that should be performed by a radiator professional. Leaking coolers contained within the radiator cannot be repaired, and the entire radiator must be replaced. As this is an expensive fix, most people will opt to add an external cooler instead. If the leak was on the transmission cooler line, the line should be replaced.

Step 3: Replacing the Transmission Cooler Line

Simply loosen the compression fitting on both ends of the cooler line. Allow one end to drop into the drain pan, while the other remains in the air. This will allow the transmission fluid to drain from the line. Apply some Teflon tape to the threads of the compression fittings, and hand tighten them into their positions. Using a wrench, fully tighten the fittings. Start your vehicle and allow it to reach normal operating temperature. Check and fill the transmission fluid until it reaches its correct level.

Most external leaks can be fixed by the do-it-yourselfer. Transmission repair for the cooler line is straightforward, and a cost effective way to ensure your transmission is not damaged by excessive heat due to cooler leaks.

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