Original submission by T:Back to Trimatic Index
A Trimatic is a 3 speed automatic gearbox driven by a Torque-Converter.
It operates hydromatically, meaning that it's internal operation is driven
by oil pressure.
The simplest description of the operation of a Trimatic is that it compares the vehicle's
road speed with the engine manifold vacuum. When certain roadspeed/vacuum conditions exist the gearbox shifts up or down.
Shifts can be forced by flooring the accelerator which activates the "kick-down"
or alternatively, moving the selector lever to the appropriate gear.
You can also induce changes by small movements of the accelerator pedal. Keeping the manifold pressure higher than the road speed (by progressively opening the throttle) will make the gearbox stay in gear longer. Similarly lifting the accelerator pedal slightly will make the the gearbox change up sooner. The gear box will change up or down naturally without the need of the Kick Down switch.
Though early hydramatic cars, like the Buick in "Rainman", could be push started
a Trimatic cannot. A Trimatic has only an engine driven oil pump which means the internals
will only operate when the engine is running. The Buick had an additional tailshaft driven
pump which meant the car could be pushed, dropped into Drive and the engine started that way.
The overall ratios of a Trimatic are dynamic and change with the road conditions and the engine's power because of the operation of the Torque-Converter and the gearbox's ability to monitor the Engine's vacuum.
Stronger vacuum (like when the engine is running on a cold night) means that the Engine is producing more power so the gearbox will change up sooner.
The Torque-Converter will load the Engine harder than usual when it detects the extra power being produced.
Propellers rotating in fluids display a coupling plot that is desirable for Internal Combustion engines.
Since the power output of an I.C. engine increases with speed and the coupling of a Torque-Converter also increases with speed, there are ideal environments for Trimatics.
Since red/blue/black engines do not need to rev high to produce power the Torque-Converter
in a Trimatic can deliver a higher road speed than would otherwise have been possible from lower engine rpm.
The speed sensitive dynamic coupling ability of a Torque-Converter is what gives a Trimatic dynamic gearing.
It is possible to achieve the same overall ratio in every gear of a Trimatic taking the pressure off the engine during upshifts.
The Ravigneaux Gearset
Trimatics use what is called the Ravigneaux Gearset. The Ravigneaux Gearset is like two ball bearing units built into the one housing.
Modulator:Attached to the rear of the gearbox on the driver's side is a vacuum sensitive device
called the Modulator.
The Modulator monitors the Inlet Manifold vacuum and generates an oil pressure inside the gearbox which is proportional to that vacuum. This oil pressure is used to determine the
shift-up/shift-down points as well as the rate that each shift will occur. Its very important that the vacuum is working properly, the vacuum comes from the engine, there is a small rubber hose connecting to a hollow metal pipe, this pipe goes to the rear of the gearbox and connects with a small rubber hose into the rear of the modulator. Be vary carefull when handling the engine side of the metal tube, if you moove it to much it may pop out from the rear of the modulator. Note if the car drives and is stuck in low gear and not even selecting 1 or 2 or 3 manually will not make it upshift them its highly probable that the vacumm is no longer connected to the gearbox. The metal hose near the engine is curved then about 4 inches of tubing, however from the end its about 1/2 long then a sort of ridge/washer is on the tube - it is very important that that ridge is in the rubber, if not it is not connected properly and you wont get vacuum to the modulator, althought it looks ok and corect it will not work. Also when the modulator is worn, oil will enter the vacum tube, this will get sucked up by the inlet manifold and will be blown out of the exhaust as smelly white cloud of smoke - this can also be caused by a leak from the brake master cylinder into the booster then sucked into the engine. A special thin tool like a spanner is needed to fit in the gap to unbolt the modulator
Inlet Manifold Vacuum Source:The Inlet Manifold Source is located at the rear of the Engine on the Passenger's side.
The connection includes a connection for providing vacuum to the brake booster.
There is a pin hole inside the unit that can slowly become blocked up by Exhaust Gas Recirculation ash causing slow shifts and eventually no shift into top gear.
The unit screws out and has a normal right handed thread.
Put thread tape on the thread before replacing it.
A Neutral/Safety Switch ensures that the Starter will only engage in Park or Neutral.
This switch also activates the reversing lights when the car is in Reverse.
The Neutral/Safety Switch is mounted on the Steering Column where the Column intersects the floor.
On T-Bar models the Neutral/Safety Switch is mounted under the Centre Console.