GM Hydra-Matic notes

As a side-note, the gearbox is specified in a strange way compared to automatic gearboxes I am used to. Most modern automatics have the engine drive the torque converter housing, which turns the pump, and the inner vanes of the torque converter drive the mainshaft. The output of that is then passed through the reduction gears.

Pontiac specified that the vehicle should not creep at idle, in Dr (drive) position. Even with the engine idle specced lower than the manual gearbox version (375 versus 550 RPM) the fluid coupling imparts enough force at 375 RPM to move the car forwards. At a guess the rationale for this was to not alienate their customers who had never experienced an automatic gearbox before.

Get in, sit down, start up and put it into gear, look there, the car does not move.. gently squeeze on the gas pedal sir.. there, the car begins to move- press a little harder, and just take your foot off the gas and onto the brake to stop. Yes, just like that sir, isn’t that a fantastic marvel of modern technology?

To stop the car from creeping yet retain an acceptable idle (the engine gets a bit unhappy as it approaches 300 RPM or lower) the engine drives the outer coupling housing directly (which is not connected to either inner torus) and that is taken inside the gearbox and through a 1.45:1 epicyclic reduction gear. The output of that is then sent back to the driving torus at 7/10 the speed of the input. As a result the driving torus is turning at about 260RPM at idle. At this speed it does not impart enough torque into the driven torus to move the car.

The output of the driven torus goes back deep inside the gearbox and drives a second epicyclic reduction gear of 2.63:1 ratio. The two gears driving each other in reduction results in an overall gearbox ratio of 3.82:1 in first gear.

The epicyclic reduction gears can be locked by the internal clutches to either reduce the output speed or be locked solid at 1:1 ratio. Combining the 2 clutches in a binary style fashion (00 01 10 11) provides 4 reduction options- 3.82:1, 2.63:1, 1.45:1 and 1.00:1 (direct).

The final drive passes through a third epicyclic gear, which, by holding the planet set still with a clutch will reverse the motion of the output shaft and provide a reverse gear. It is also locked out internally to first gear full reduction only- without that there would be 4 speeds in reverse with a top speed of about 60mph. Not ideal!

The gearbox also has a few interesting design features- two oil pumps, one on the engine driven side, the other on the output shaft. This is to allow oil pressure to be built up as the vehicle is towed, for two reasons. First, it provides lubrication without the engine running, second it provides oil pressure so that if the transmission is dropped from N to Dr it will engage a gear and be capable of bump starting the engine.

The oil pumps are designed to provide high pressure at low speed. This pressure is kept constant by a spring relief valve. The gears are selected by valves with varying strength springs. A governor valve (centrifugal weight that spins with road speed) increases pressure on the valves as the road speed changes, initiating gear changes. The throttle pedal is attached to the gearbox and serves two functions. First, it opens a valve that applies main pressure to the opposite side of the gear shift valves, meaning greater governor pressure needs to be built up before the valves will move and gear changes occur. Second, as the gears are changed it adds boost pressure to the servos so the bands clamp more firmly and quickly. This makes low speed, light throttle gear changes smooth to engage at low speed, and hard acceleration changes to snap tight quickly at higher road speeds.

It all sounds good but there are a lot of moving parts that all need to move in synchrony. If something jams, it’ll either disengage drive fully or attempt to engage two gears at once which causes undesirable operation, neck-snapping jerks in the drive or a redline-flare of the engine. In this case, it would change gear at a much higher RPM/roadspeed than it should have, and only when the throttle was snapped shut. Hopefully that can be remedied.

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