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Hydramatic ID & Diagnosis Tips

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Basic Automatic Transmission Diagnosis

Preliminary Checks

Always begin ALL transmission diagnosis with the following initial steps:

  • Check transmission fluid level and condition.  Fluid level should be at correct range hot, be free of bubbles on the dip stick, and not be excessively dark, watery or appear contaminated.
  • Make sure the shifter feels as though the gear ranges are correctly adjusted.  Drive should engage drive, reverse should engage reverse, and the shifter should be free to move.
  • Ensure the engine is running correctly.  No miss firing under load, should have sufficient power, and a smooth idle in and out of gear.  If the engine is not running correctly, fix any concerns that are engine performance related.  Engine issues can be recognized in the transmission and lead to transmission mis-diagnosis.

Fluid Diagnosis

Transmission fluid, the life blood of any transmission, should be checked for proper level, but also evaluated for condition.  The fluid should not exhibit the following:

  • Small metallic particles (metal) or dark colored particles (friction material).  Check this by wiping the dip stick fluid into a clean white paper towel or piece of white paper.  If material is noted in the fluid, the transmission will likely require overhaul to correct the condition.  A further check point would be to remove the transmission pan and inspect for excessive debris in the pan.
  • A watery fluid condition coupled with very dark color and a strong burnt odor.  This is an indication of overheating.  The transmission will likely require overhaul to correct the condition. Change fluid and filter and reassess transmission operation.
  • When the fluid is dark, yet does not have a strong burnt smell, this may be a normal condition.  High mileage units and age can darken the fluid.  If the transmission is operating normally, service the transmission by replacing the filter and exchange as much fluid as possible.  Exchange the fluid after the initial filter / fluid change to freshen as much transmission oil as possible.

Shift and Driving Diagnosis

Transmission shifting should be smooth and exhibit normal application (no delays, abrupt engagement, drag, or slide).  When driving with normal acceleration the transmission should be expected to shift out of 1st at about 15 MPH, and out of 2nd at about 25-35 MPH.  This will depend on how hard the accelerator is pressed.  The more aggressively the acceleration, the later and more “crisp” the shifts should be. Some basic shift diagnosis include:

  • Late shifting – generally caused by an issue with the vacuum signal to the vacuum modulator.  Check for engine vacuum leaks, collapsed vacuum hoses, misrouted vacuum lines at the engine.  A plugged vacuum manifold port may also cause the issue. Leaking booster.  When in doubt, install a vacuum gauge at the vacuum modulator and check the vacuum signal and response. The gauge should follow "crisply" the engine throttle changes without delay.  If all looks good, try another modulator. Also, if the modulator is leaking fluid at the vacuum port, replace it.
  • No Shifting – be careful with this one.  Can be tricky.  No shift may be caused by a stuck or inoperative governor, or may act like a no-shift when in fact the modulator is holding back the shift since the vacuum signal to the transmission may be “wide open throttle”.  The best test is to accelerate the vehicle to the last 1st-to-2nd shift point (generally at or near 35 MPH).  If the vehicle shifts hard into 2nd, check the modulator and vacuum signal.  If the vehicle does not shift, it is likely governor pressure problems and deeper diagnosis or repair may be needed.  If you are able, check for a stripped governor gear or stuck govenor valve by removing the governor and inspecting.  Stuck valve may be an indication of metal in oil.
  • Late bump shift from 1st to 2nd – This may be caused by second gear clutch / band issues, a damaged 1-2 accumulator servo or second gear band adjustment (if equipped).  In any case, the transmission will require inspection, adjustment and / or repair.
  • Early Shifts – This is not common but may be caused by a partially stuck open governor valve, or a restriction in the vacuum signal to the modulator caused by a plugged port or kinked vacuum line.
  • Starting in 2nd or 3rd gear – Begin by checking Reverse gear operation.  If the transmission locks up (or drags) in Reverse, then an internal clutch or band or one-way is seized in the engage position.  That is why it starts in a gear other than first.  Internal transmission inspection and repair is likely required.  If Reverse engagement is normal, then the likely cause is a stuck open governor valve.  In any case, an inspection and repair of the governor and potentially the transmission will be required.
  • Slipping – slip in one or more gears, with or without load.  Generally will also have burnt fluid.  Check the transmission fluid level, and if low, correct and retest.  The damage may already have occurred.  Internal inspection and repair may be necessary. 
  • Will Not Move Any Gear – If it will not move in any gear, then a major failure may have occurred.  Check transmission fluid level.  If empty, correct and retest.  If fluid is at correct level, then an inspection and repair is required.
  • Poor Acceleration / Engine Labors Yet Vehicle Does Not Accelerate Well -- If the vehicle has poor acceleration, yet the engine runs well, perform a torque convertor stall test.  This will test the convertor stator one-way for failure.  If the convertor stator one-way fails, the vehicle will exhibit poor acceleration up to about 35 MPH, then like a switch being tripped, suddenly accelerate normally above 35 MPH.

Leak Diagnosis

Leaks may be minor or severe.  Older transmissions may leak from more than one location since the seals have aged and hardened.  If the leak is minor, try resealing the leak (such as the pan gasket, speedometer O-ring, tail shaft seal).  If the leak is major (such as front pump seal) or the leaks are too numerous, then a complete internal inspection, repair and reseal will be required.

Sound Diagnosis

Generally, gear or buzzing sounds may be caused by transmission gear failure or air in hydraulics, cooler line contact with body or frame, exhaust contacting body or frame, or an engine problem.  If the gear sound follows the transmission shifting (such as growls in first and second, but not third) then the transmission planetary may be at fault.  Ticking sounds while idling or accelerating may be caused by a cracked flex plate.  Buzzing sound may be caused by cooler line routing where the line is contacting the frame or engine / transmission.  Clicking and squeaking that follows road speed changes may be caused by a universal joint.  Do not disassemble the vehicle until the origin of the sound is clearly pinpointed.  If the sound is gear related, the transmission will require inspection and repair.



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