Many times, a shift quality concern may be corrected by the owner, without special tools. Sometimes the owner may think that the transmission requires over haul to correct shift quality
concerns. In many cases the transmission simply requires a good look for the root cause.
How Is Shifting Controlled?
Every transmission, sans the modern electronically controlled versions, have two hydraulic pressures working inside the valve body to control shifting at each shift valve. These pressures
are throttle pressure and governor pressure. These two pressures oppose each other while driving. Throttle pressure represents how hard your foot is into the throttle (more throttle
equals more throttle pressure) while governor pressure represents vehicle road speed (the faster the road speed the more governor pressure). Both pressures serve to act on the shift
Example, you drive normally with moderate accelerator pedal pressure. The throttle pressure is minimal, so the opposing governor pressure is able to act on the 1-2 shift valve at lower road
speed so the shift may occur at 15 MPH. Another example, with hard acceleration, the throttle pressure is maximized, so the opposing governor pressure must build to a higher value (more road
speed) until it is able to act on the 1-2 shift valve which may result with a 1-2 shift at 30 MPH. This does not take into account any downshift mechanism that may be used, such as a downshift
cable, electric downshift solenoid, etc. Also, throttle pressure may be managed by a lever, or vacuum modulator. But governor pressure is always managed by a fly weight device driven off
the output shaft.
Why Would Shifting Be Abnormal?
Abnormal shifting can take the following forms. Early shifting, late shifting, or no shifting.
- Throttle linkage or lever disconnected, binding
- Note: It is possible to be binding full open, or full closed, or somewhere in the middle. If vehicle uses a throttle lever or throttle cable, check that it is connected, moving with the
throttle linkage and not binding. It should be connected to the transmission external throttle lever or internal linkage (cable system) where cable enters transmission. In most cases the
issue is near the carburetor where linkages have been removed, bent, rusted, or modified.
- With early Hydramatic, this lever is critical to shift quality. Make sure it is intact, not binding and follows the throttle movement. You will see info about special tools to bend
the throttle arm and so on. If it shifted correctly at one time, make sure it is all connected and working correctly. Then try lengthening or shorting the rod travel at the transmission
side adjustment block and see what happens. It may just need to be tweaked. However, if the transmission is internally varnished, has leaking internal seals, or contaminated, it needs to
- Throttle lever incorrectly adjusted
- With adjustment, either too short, or too long, this will affect shift timing. When too long, throttle pressure is higher than gas pedal pressure and will cause a late shift. When too
short, throttle pressure is too low in relation to pedal pressure and the shift occurs too early.
- Vacuum modulator vacuum line pinched, plugged or disconnected
- The vacuum modulator (if used) can delay shifts, or cause shifts to be early or not at the correct time. This can sometimes be intermittent. There may be collapsed rubber vacuum
lines, air intake leaks, incorrect engine timing resulting in manifold vacuum issues, plugged vacuum line or vacuum port at intake manifold, pinched or rusted metal vacuum line (check entire line),
transmission fluid leaking into vacuum line obstructing vacuum signal, and in some cases a stuck or binding throttle valve or vacuum modulator diagram. When in doubt about the vacuum signal,
install a vacuum gauge in line at the modulator side of line and check for good and responsive engine vacuum. Responsive means that as the throttle is changed, the vacuum signal immediately
follows throttle changes. Any delays would indicate a plugged line, leaking line, or restricted port at manifold.
- Vacuum modulator leaking oil
- Be sure to pull the vacuum line off the modulator and check for transmission oil. If oil is present, replace the modulator. When there are shift issues that do not appear to be
related to vacuum line plumbing, then replace the modular. While the modulator is off, pull the throttle valve from bore and make sure it is not stuck.
- Throttle valve stuck (requires internal transmission repair)
- This is tricky. If the throttle valve is stuck, or sticky, there is contamination in the valve body. In most cases, the transmission should be removed, disassembled, inspected, and
- Detent (down shift device) Stuck
- Transmissions that succeed the original Hydramatic design also use a detent or kick-down device. The original Hydramatic did not use a detent. The detent may be an electrical
connection and electric kick down solenoid (THM-400) or a detent cable (THM-350). The THM-200 uses a throttle valve cable and also serves as the detent (no modulator). If the detent is
stuck “wide-open-throttle” position it will delay the shift. If disconnected, the result will be sluggish downshift on acceleration demands.
This can be tricky to test for. In all cases of no-shift, be sure to check fluid level and condition. No metal or contamination or heavy varnishing. You will have to test drive
the vehicle during a wide open throttle test to see if the unit shifts from first (generally 35 MPH or more). Yes, the engine will be screaming. One of two results will follow the
wide-open-throttle shift test:
- Transmission shifts
- This indicates that the throttle pressure is at maximum and governor pressure could not initiate a shift until sufficient road speed occurred. Find out why throttle pressure is too
high. Likely the kick down cable or lever is stuck open or the electrical solenoid (THM 400) is getting 12 volts all the time or the solenoid is stuck).
- Transmission does not shift
- This would indicate that the governor is not producing sufficient pressure. May be caused by stuck governor valve, governor circuit leak, governor bore damage (enlarged), or contaminated
valve body. This will require careful inspection and repair needs determined. Likely overhaul is required.
Starts Off In Higher Gear
- Here is another example of either stuck valves or a hard part failure in the transmission or a band is stuck on or off or severely misadjusted. This will require careful inspection and
repair needs determined. This can be also caused by a stuck open governor valve. In any case, there is likely contamination and the unit needs overhaul.
Age Related Issues
- With age comes hard or cracked seals, both internal and external, varnishing that causes parts to stick, internal and external leakage, and a host of other problems. The only way to correct
age related issues such slipping, shift concerns, or other function issues is a complete teardown, inspection, reseal and rebuild. No amount of transmission fluid changes or additives will
result in a functioning transmission.
Early Hydramatic (Dual Clutch, Dual Band) Shift Characteristics
LINK TO HYDRA-MATIC DRIVE CHARACTERISTICS - Scroll To "Design"
We hope that this helps trouble shoot some common causes of shift issues. This is not an exhaustive diagnostic process. The service manual will help with detailed pin point diagnostic
steps. But in most case you may determine the root cause through inspection and easily identify and correct the problem.
Enjoy Your Classic Car or Truck!