How to Replace Transmission Lines

Written by chris stevenson
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How to Replace Transmission Lines
Broken or leaking transmission lines can cause severe damage to the transmission. (old gearbox dumped in a rest area on the side of the road image by Undy from

The transmission coolant lines route the transmission fluid from the radiator, under the main chassis frame and through the transmission housing. Transmission fluid enters one side of the bottom radiator core and returns to the transmission in a continuous loop. The lines, whether flexible or hard, can become damaged by impact under the vehicle by crushing forces or breakage. Transmission coolant lines can leak at their connection points, either at the radiator or at the transmission housing. A do-it-yourself mechanic can replace any or all of the transmission coolant lines with some special tools in a few steps.

Skill level:

Things you need

  • Line wrenches
  • Floor jack
  • Jack stands (4)
  • Shop rags
  • Penetrating oil
  • Screwdrivers (slotted and Phillips head)
  • Transmission fluid
  • Drain pan
  • 3/8-inch drive socket set and wrench

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  1. 1

    Shift the vehicle into park and set the emergency brake. Raise the hood. Lift the vehicle with a floor jack high enough to place two jack stands under the frame at the front end near each wheel. Lift the rear of the vehicle and place two more jack stands under the rear part of the frame near each wheel.

  2. 2

    Spray penetrating oil over all of the transmission line connection points. This includes any hose clamps holding flexible hoses to hard lines, such as the hoses to the radiator connections and also the hard line flared nuts attached to the transmission housing. Spray the oil several times and allow it to saturate the connection points for 10 to 15 minutes. Wipe off all excess oil with a shop rag.

  3. 3

    Locate the transmission coolant lines where they enter the bottom of the radiator. For flexible hose lines that fit onto a solid inlet nozzle, use a slotted screwdriver or small socket to loosen the hose clamps. Pull the lines free and let the fluid drain into a pan. If you want to remove the hard lines (if so equipped) that bolt onto the bottom of the radiator, use a six-point line wrench. Be very careful when using the line wrench--if the metal line begins to bend before the nut loosens, soak the connection with more penetrating oil.

  4. 4

    Identify the transmission lines where they enter the side of the transmission housing. These fittings will almost certainly be metal nuts that attach to threaded nipples. Use the line wrench on these nuts and be careful to remove them without excess force. Use penetrating oil if they appear to be frozen. When detached, drain the fluid into a pan.

  5. 5

    Locate all of the transmission line supports that follow the main part of the frame. Most will have single bolts holding a pressure flange. Some of the pressure flanges might have rubber (shock) inserts. Be careful not to lose these as they will fit back into the flange and support the new lines/hoses. Any of the rubber supports that show wear or are broken should be replaced. You can obtain these rubber inserts at your dealer's parts store or warehouse. After removing all of the line supports, simply drop the transmission lines down and pull them free of the vehicle.

  6. 6

    Position the new hoses, starting at the radiator bottom. To make it easier, you might want to install a few mounting flanges to hold up the weight of the transmission lines so they can be aligned properly. Reattach the front hoses to the radiator nozzles, securing the clamps with the slotted screwdriver. Move to the back of the vehicle and twist the new line connector nuts on by hand first then tighten them with the line wrench. Finish installing the rest of the mounting flanges, sandwiching the rubber inserts into their cradles.

  7. 7

    Run the engine and add fluid to the top of the transmission dipstick filler tube. Be careful not to overfill the transmission, bringing the level up to the "hot" line on the dipstick when the engine has warmed up. Check for leaks.

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