Transmission Cooler Lines

Replace or repair the automatic transmission cooler lines that run from the transmission to the radiator, preventing leaks and ensuring proper transmission fluid cooling.

Warnings

Tools required

Parts

• Transmission cooler line set (pre-bent or universal) × 1 — Use OEM specification or aftermarket equivalent for TH400/M40 transmission
• Transmission cooler line fittings (if not included) × 4 — Use OEM specification
• Rubber isolation grommets/clips × 1 — Use OEM specification

Fluids

• DEXRON automatic transmission fluid — 2 qt

Preparation

1. Park vehicle on level surface and engage parking brake
2. Place wheel chocks behind rear wheels
3. Allow engine and transmission to cool completely if recently operated
4. Raise front of vehicle with floor jack and support securely on jack stands at frame rails
5. Position drain pan beneath transmission cooler line connections at radiator
6. Inspect existing cooler lines to determine routing, mounting points, and line diameter for replacement

Procedure

1. 1

   Disconnect battery negative terminal
   Disconnect the negative battery cable to prevent accidental electrical shorts while working near the radiator and fan assembly. Store cable away from battery post.
2. 2

   Disconnect cooler lines at radiator
   Using a flare nut wrench, carefully loosen the transmission cooler line fittings at the radiator. The radiator has two fittings typically on the driver's side tank. Turn counterclockwise slowly to avoid rounding off the fittings. Allow transmission fluid to drain into the pan. If fittings are seized, apply penetrating oil and allow to soak for 15-20 minutes before attempting removal.
   ⚠Use a flare nut wrench, not a standard open-end wrench, to prevent rounding off the soft brass or steel fittings.
3. 3

   Trace and support cooler lines
   Follow the cooler lines from the radiator along the driver's side frame rail toward the transmission. Note all mounting clips, brackets, and routing points. Support the lines with one hand while removing fasteners to prevent them from falling and damaging brake lines or other components.
4. 4

   Disconnect cooler lines at transmission
   Position drain pan beneath the transmission cooler line connections on the driver's side of the transmission case. Using a flare nut wrench, loosen and remove the cooler line fittings from the transmission. The fittings thread directly into the transmission case on the TH400. Allow remaining fluid to drain completely. Plug the transmission ports with clean rags to prevent contamination.
5. 5

   Remove mounting clips and brackets
   Remove all rubber isolation grommets, metal clips, and brackets securing the cooler lines to the frame rail and crossmember. Note the location and orientation of each mounting point for proper reinstallation. The lines should be secured approximately every 12-18 inches to prevent vibration and chafing.
6. 6

   Remove old cooler lines
   Carefully extract the old cooler lines from the vehicle, working from front to rear. Inspect the routing path for any potential interference points or damaged brake lines, fuel lines, or wiring that may have been caused by failed cooler lines. Clean the routing area of any debris or old fluid residue.
7. 7

   Prepare new cooler lines
   If using pre-bent lines, verify correct fitment by test-fitting before final installation. If using universal lines, carefully bend to match the original routing using a tubing bender to avoid kinking. Maintain gradual bends with a minimum radius of 4 inches. Ensure lines do not contact exhaust components, steering linkage, or suspension parts throughout their full range of motion.
8. 8

   Install new mounting hardware
   Install new rubber isolation grommets and mounting clips at all original locations along the frame rail. Replace any damaged or missing brackets. The lines must be properly supported to prevent stress on the transmission and radiator fittings.
9. 9

   Route and secure new cooler lines
   Route the new cooler lines along the frame rail following the original path. Secure lines in all mounting clips and grommets, ensuring adequate clearance from all moving parts, exhaust components, and potential pinch points. Verify clearance with suspension at both full droop and compression if possible.
10. 10

    Connect cooler lines to transmission
    Remove the protective rags from the transmission ports. Thread the cooler line fittings into the transmission case by hand to ensure proper thread engagement. The upper line typically connects to the return port and the lower line to the outlet port. Hand-tighten fittings completely, then use a flare nut wrench to snug an additional 1/4 to 1/2 turn. Do not overtighten as this can strip the aluminum transmission case threads.
    ⚠Cross-threading can damage the transmission case. Start all fittings by hand and ensure they thread smoothly before using a wrench.
11. 11

    Connect cooler lines to radiator
    Thread the cooler line fittings into the radiator fittings by hand until fully seated. Use a flare nut wrench to snug an additional 1/4 to 1/2 turn. Do not overtighten. Verify both connections are secure and fittings are properly aligned without stress or binding on the lines.
12. 12

    Fill transmission with fluid
    Add approximately 2 quarts of DEXRON automatic transmission fluid through the transmission dipstick tube to replace fluid lost during line replacement. Do not overfill at this stage as additional fluid will be added during final verification.
13. 13

    Reconnect battery and lower vehicle
    Reconnect the negative battery cable. Remove jack stands and carefully lower the vehicle to the ground. Remove wheel chocks.

Reassembly

1. Verify all cooler line connections are properly tightened and no tools or rags are left in the engine compartment
2. Ensure all mounting clips and brackets are secure and lines are properly routed with adequate clearance
3. Confirm transmission dipstick is fully seated

Verification

• Start the engine and allow it to idle. Check all cooler line connections at the radiator and transmission for leaks while the engine is running
• With the engine running and parking brake engaged, shift through all transmission gears, pausing briefly in each position, then return to Park
• Check transmission fluid level with engine running at operating temperature and add fluid as necessary to bring to the full mark on the dipstick
• Re-inspect all connections for leaks after transmission reaches operating temperature
• Test drive the vehicle and verify proper transmission operation and no fluid leaks
• Perform a final inspection of all connections and fluid level after test drive, checking for any seepage or drops in fluid level

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