Classic Hydramatic Rebuild and Resurrection Service
Classic HydramaticRebuild and Resurrection Service

Transmission Refill and Cooler Flush / Repair Tips

Transmission Refill and Initial Run Instructions




Once the transmission is installed back into the chassis, a refill and initial run procedure is required:

  • Hydramatics generally require about 10 quarts for a RWD and 12 quarts for a FWD when the unit is dry and the convertor is new.  Check a spec book.
  • BLOCK ALL 4 WHEELS!!!  FRONT AND BACK!!  To refill, begin with the engine OFF and transmission in PARK.  Be sure to BLOCK THE WHEELS in case the shifter is misaligned and the transmission might be in gear.
  • Install 5 quarts of quality transmission fluid into the sump through the dip stick tube (regardless of transmission style).
  • Start the engine, do not shift the transmission into gear.  You may hear some buzzing sounds as the system attempts to purge air.  Run engine at idle for 60 seconds, then shut the engine off.
  • Add fluid to the transmission sump until fluid shows on the dip stick slightly above the normal fill mark.
  • Restart the engine and allow to IDLE.  Place foot firmly on the brake.  Shift to Drive, wait 5 seconds, then shift to Reverse, wait 5 seconds, then shift to Manual 1, wait 5 seconds.  You should feel the transmission attempt to engage into the gear.  DO NOT REV THE ENGINE.  Place shifter back into Park and leave engine idling.  NOTE: BEFORE YOU REMOVE YOUR FOOT FROM THE BRAKE, MAKE SURE THAT THE VEHICLE IS IN PARK AND NOT ABLE TO MOVE IN GEAR.  IF YOUR SHIFT LEVER IS NOT CORRECTLY ADJUSTED, THE TRANSMISSION MAY BE IN GEAR AND WILL MOVE ONCE ENOUGH FLUID IS ADDED AND WILL CREATE ENGAGEMENT.  USE CAUTION AND COMMON SENSE.  BLOCK ALL 4 WHEELS!!!  FRONT AND BACK!!
  • While the engine idles, check transmission fluid level and slowly add fluid until the level is within the normal zone on the dip stick.  Be sure to allow the fluid that has been added to clear the dipstick before checking the level.  Again, with foot on brake, shift transmission into each gear range, neutral and park.  You should feel good engagement and feel the engine load down in each gear.  DO NOT BRAKE TORQUE, DO NOT REV THE ENGINE.
  • IMPORTANT: Before the test drive, recheck for any signs of leakage and that the throttle linkage is correctly adjusted, the shift linkage is correct adjusted, and everything is attached and in place.  Be sure to check cooler lines for leakage, front pump area (convertor cover shield area), and pan gasket.  No leaks should be noted.  If leaks are noted, determine the leak origin and correct as required.
  • If leaks are NOT noted, test drive the vehicle.  Be sure to check that the vehicle upshifts normally, down shifts normally, shifts into manual gears, reverse works correctly, Park holds the vehicle on an incline, neutral is neutral, and all gear train sounds are normal.
  • After the test drive, recheck the fluid level and adjust if required.  DO NOT OVER FILL!  Recheck for any noted leakage.


Flushing Transmission Cooler and Lines

The transmission cooling system is comprised of a small radiator inside the engine cooling system radiator.  The transmission cooler transfers heat to the engine radiator as the transmission fluid travels from the transmission convertor, to the transmission cooler, and then returns to the transmission and feeds the transmission lubrication system thus lubricating the bearings, thrust washers, clutch discs and gears.  Early Hydramatics do not use a cooler.  Jetaway began using a cooler.  Some Turbo Hydramatics did not use a cooler.


If the engine radiator is in “questionable” condition, it is recommended that you replace the engine radiator which will in turn also replace the transmission cooler.  Here are reasons to flush or replace the transmission cooler:

  • If the cooler is restricted the transmission will not receive sufficient lube flow (lubrication and cooling) and this may result in premature transmission failure.
  • If the cooler is contaminated with metal debris the transmission bearings, gears and thrust washers will receive this contamination since the fresh transmission fluid will act to clean the debris and this contamination may cause premature transmission failure.

When flushing a transmission cooler, always use ultra-low pressure fluid or air pressure.  The cooler is easily damaged by high pressure shop air or high pressure fluid flushing procedures.


WEAR SAFETY GOGGLES!  To flush a cooler, use a low air pressure (no more than 5-8 PSI) to first purge all old fluid from the lines and cooler.  Once purged, flush the lines and cooler with suitable cleaning fluid (connect the Safety Clean tank to one end of the line and connect a return line to the tank basin.  Flush until clear then dry with low pressure shop air.  If you do not have a cleaning tank, spray Brake Cleaner (USE A SUITABLE RESPIRATOR) into the line at cooler top and capture the return flow into a container at the other line exit point.  Follow up this cleaning process with low pressure air to purge and dry the system of cleaner.  NOTE: Be sure that you do not detect any flow restrictions while cleaning / purging.  When in doubt, replace the radiator and flush the lines.  A plugged or restricted cooler will damage the transmission.


Metal Cooler Line Repair

Metal transmission cooler lines move transmission fluid from the transmission convertor to the transmission cooler inside the radiator and back the transmission where the oil lubricates the bearings, gears, clutch packs, and thrust washers.  The cooler line system must be leak free and allow fluid free-flow. Sometimes you may have noted that repairs of cooler lines may use a rubber or neoprene hose slipped over the two metal lines and secured with a worm clamp.  This is a suitable temporary repair, but not a recommended long-term repair.  


To correctly repair a section of damaged line, use the appropriate sized metal tubing bent to the correct configuration, and secured at appropriate retention locations.  Splice metal lines to each other using brass double compression fittings.  Be sure to securely tighten the compression fitting to ensure a leak free connection that will not come apart over time.  Use the compression fittings over clean rust free metal lines.  In some cases it may be better to simply remake a new metal line that will be connected to the transmission and cooler.  This is easily accomplished by using the original line as a “bend guide” and adding the flare nuts and flares to each end. 


Be sure the cooler line does not contact any frame members, engine or transmission housing, and related.  The line should be securely fastened at the OEM recommended locations using the appropriate OEM style bracket.  If line contact does occur, you may hear a “buzzing sound” in the passenger compartment while driving or idling.  Plus, a line that contacts will eventually rub through and create a leak.




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