Remove and replace the automatic transmission oil cooler, including flushing cooler lines and refilling transmission fluid.

Warnings

Tools required

Parts

• Transmission oil cooler × 1 — Use OEM specification or aftermarket equivalent rated for TH350/TH400
• Transmission cooler line fittings (if corroded) × 2 — Use OEM specification
• Radiator-to-cooler line seals × 2 — Use OEM specification

Fluids

• Dexron III ATF — 11.5 qt

Preparation

1. Park vehicle on level surface and apply parking brake
2. Place wheel chocks behind rear wheels
3. If engine is warm, allow to cool for at least 30 minutes
4. Raise front of vehicle using floor jack at frame rail jack points
5. Support vehicle securely on jack stands rated for vehicle weight
6. Remove front wheels if additional clearance is needed for access
7. Place large drain pan beneath transmission cooler area

Procedure

1. 1

   Access transmission oil cooler
   From beneath the front of the vehicle, locate the transmission oil cooler mounted in front of the radiator. The cooler is a rectangular finned unit with two lines connecting to the transmission. On 1966 Corvettes, it is typically mounted to the front frame crossmember or attached to radiator support brackets.
2. 2

   Disconnect transmission cooler lines at cooler
   Using a line wrench or flare nut wrench, carefully loosen the transmission cooler line fittings at the cooler inlet and outlet. The fittings are typically 3/8" or 1/2" inverted flare type. Turn fittings counterclockwise while holding the cooler fitting body steady with a backup wrench to prevent twisting the cooler tubes. Allow fluid to drain into pan. Cap or plug the transmission lines temporarily to prevent contamination.
   ⚠Use proper line wrenches to avoid rounding off the soft brass or steel line fittings. Standard open-end wrenches will likely damage the fittings.
3. 3

   Remove cooler mounting hardware
   Remove the mounting bolts or brackets securing the transmission cooler to the frame or radiator support. There are typically 2 mounting brackets with 2 bolts each securing the cooler assembly. Remove these fasteners and carefully maneuver the cooler out of its mounting position, being careful not to damage radiator fins.
4. 4

   Inspect transmission cooler lines
   With cooler removed, inspect the transmission cooler lines from the transmission to where they connected to the cooler. Check for corrosion, kinks, damage, or leaks along the entire length. If lines are damaged or severely corroded at fittings, they should be replaced. Check rubber hoses (if equipped) for cracking or deterioration.
5. 5

   Flush transmission cooler lines
   Using a cooler line flushing tool or pressurized parts cleaner, flush the transmission cooler lines from the cooler end back toward the transmission. This removes any debris, old fluid, or contamination. Flush until solvent runs clear. Allow lines to drain completely and blow out with compressed air if available. If lines cannot be adequately flushed, they should be replaced.
   ⚠Do not allow flushing solvent to enter the transmission. Flush only from the cooler end with lines disconnected from cooler. Some solvent may flow back to transmission pan but should be minimal.
6. 6

   Prepare new transmission cooler
   Unpack new transmission cooler and verify it matches the original in dimensions and fitting locations. Inspect fittings for damage. Clean any protective oil or coatings from the cooler exterior. Verify that mounting brackets align with original mounting points. If new cooler has different mounting configuration, test-fit before proceeding.
7. 7

   Install new transmission cooler
   Position the new transmission cooler in the mounting location, ensuring adequate clearance from radiator and other components. Install mounting brackets and secure with bolts. Tighten mounting bolts snugly but do not over-torque, as cooler mounting tabs can be fragile. Ensure cooler is positioned to allow free airflow and is not rubbing against radiator or other components.
8. 8

   Connect transmission cooler lines
   Remove temporary plugs or caps from transmission lines. Inspect line ends and clean threads if necessary. Install new seals or O-rings if provided with new cooler. Thread cooler line fittings into new cooler inlet and outlet ports by hand first to ensure proper engagement. Using line wrenches, tighten fittings securely while holding cooler fitting body with backup wrench. Ensure fittings are snug but do not over-tighten and strip threads.
   ⚠Identify correct flow direction if cooler is directional. Typically, the line from the transmission outlet connects to cooler inlet, and cooler outlet returns to transmission. Consult cooler markings or instructions.
9. 9

   Check transmission fluid level
   Locate the transmission dipstick/fill tube (verify exact location for the 1966 Powerglide installation). With engine off, check current fluid level. The transmission will have lost fluid during cooler and line disconnection. Note the current level before adding fluid.
10. 10

    Add transmission fluid
    Using a transmission fluid pump or funnel with long tube inserted into the dipstick tube, add Dexron/Mercon-type automatic transmission fluid (Dexron III/VI is backward-compatible with the original Type A specification). Add approximately 2-3 quarts initially to replace fluid lost from cooler and lines. Do not overfill at this stage. Replace dipstick.
11. 11

    Start engine and circulate fluid
    Start the engine and allow it to idle. With parking brake applied and foot on brake pedal, shift transmission through all gear positions (P-R-N-D-L and back), pausing 2-3 seconds in each position. This circulates fluid through the valve body, cooler lines, and new cooler. Return to Park.
12. 12

    Check for leaks
    With engine still running, carefully inspect all cooler line connections at the cooler for leaks. Check fittings, cooler seams, and line routing. Also inspect connections at the transmission end if accessible. If any leaks are observed, shut off engine immediately and tighten fittings as needed.
    ⚠Keep hands and tools clear of moving engine components and hot exhaust while inspecting for leaks.
13. 13

    Final fluid level adjustment
    With engine at idle and transmission in Park, check transmission fluid level on dipstick. Fluid should be checked at operating temperature (after 5-10 minutes of running). Add fluid as necessary to bring level to the FULL mark on the dipstick. Do not overfill. Recheck after test drive.
14. 14

    Reinstall wheels and lower vehicle
    If front wheels were removed, reinstall and torque lug nuts to specification in a star pattern. Carefully raise vehicle slightly with floor jack, remove jack stands, and lower vehicle to ground. Remove wheel chocks.

Reassembly

1. Ensure all cooler line fittings are tight and leak-free
2. Verify transmission fluid is at proper level on dipstick with engine at operating temperature
3. Confirm cooler is securely mounted and not contacting radiator or other components
4. Dispose of old transmission fluid and cooler properly according to local regulations

Verification

• Test drive vehicle for 10-15 minutes, including highway speeds if possible, to bring transmission to full operating temperature
• After test drive, recheck transmission fluid level with engine idling in Park and adjust if necessary
• Perform final inspection under vehicle for any fluid leaks at cooler connections
• Monitor transmission temperature gauge (if equipped) to verify cooler is functioning properly
• Check fluid level again after 50-100 miles of driving and top off if needed

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