Trimatic Introduction

From Holdenpaedia
Jump to: navigation, search

Page created by T Feb 15th 2010:

Back to Trimatic Index 

L R Trimatic from an EFI VK Commodore.  Click to Enlargen. Photo by 383HGBrougham.

Trimatic Introduction:

First Release:

Trimatics first appeared in HG Holdens.

Origin of the Name:

Trimatic Gearboxes derive their name from the fact that they have 3 forward Speeds.They also have a single Reverse Gear, Neutral and Park.

Trimatic Selector Positions:

The Trimatic Console is marked Low 2nd Drive N R and Park.Low provides 1st Gear with Engine Braking and is good for negotiating steep hills and rough terrain.2nd provides 1st Gear and will allow the Trimatic to change up to 2nd Gear but not 3rd.Drive allows the Trimatic to select 1st, 2nd or 3rd Gear for the operating conditions require.Note that 1st Gear in 2nd or Drive does not provide Engine Braking.Neutral allows the Engine to spin freely without providing power to the Rear Wheels.Reverse is engaged to make the Car back up.Park is used for securing the Vehicle so that it won't move.When engaged, Park prevents the Car from rolling, whether the Engine is running or stopped and is a vital Selector setting.When the Engine is stopped a Trimatic does not provide any forward or reverse Gears so the process of securing the Car by engaging a Gear doesn't exist except for the use of Park.Park engages a Pawl and Ratchet that prevents the Car from rolling whether the Engine is running or stopped.When parking a Trimatic, always make sure the Selector is in Park and double check that the Pawl has engaged before  leaving the Vehicle. The Pawl will cause the Car to freeze on the spot when it has fully Engaged. Be aware that the Car will roll a short distance as the Pawl engages so make allowance for this movement and steady the Car with the Footbrake at the same time.Once the Pawl has engaged, pull the Handbrake on to provide double assurance that the Vehicle can't roll.On a Hill an extra safety precaution is to point the Front Wheel towards the Kerb so that in the event of a Pawl and Handbrake failure the Vehicle can only roll a short distance or not at all.Never engage Park when the vehicle is moving or the Rear wheels will lock and a loss of control of the Vehicle will result. In extreme cases the Pawl will shear off and the Governor Hub will be destroyed causing an expensive rebuild or replacement.Note that in Drive, Low, 2nd or Reverse a Car fitted with a Trimatic will move by itself when the Engine is running so always stop the Vehicle, select Park and fully engage the Handbrake before exiting the Vehicle if you intend to leave the Engine running for any reason. This includes checking the Oil Level.Consult the Vehicle Owner's Manual for full details on any and all Trimatic operation and procedures.


The Starter is not supposed to activate in a Trimatic unless the Selector is in Park or Neutral. Be aware that in a used Car the Neutral Safety Switch may be faulty or out of adjustment meaning that the Starter might be operational in *any* Selector position.

Where Used:

Trimatics first appeared in late HG Holdens and were last fitted to VK Commodores and WB Vans, Utes and One Tonner Commercial vehicles.

Trimatics As An Hydromatic Gearbox:

Trimatics use Oil Pressure generated by an Engine driven Oil Pump to enage their Gears and provide lubrication. This qualifies them as being hydro-matic, hydro meaning their shifts are performed by Oil Pressure. They use Air Pressure generated in the Inlet Manifold to calculate the Engine's Power.They have an internal Governor that generates an Oil Pressure proportionate to the Roadspeed to calculate their shift points.In addition, comparison of the Governor Pressure and Inlet Manifold Pressure is used to calculate the rate that the Gears shift.A Trimatic will change up Gears rapidly if the Throttle is wide open and the Car is accelerating rapidly.It will also stay in each Gear longer.A Trimatic will change up Gears slowly if the Throttle is narrow and the Car accelerating gradually. It will also change up sooner. A Trimatic will change down Gears rapidly if the Throttle is wide open and a Kickdown is ocurring.A Trimatic will change down Gears slowly if the Throttle is closed and the Car is slowing gradually.


A Kickdown occurs when the Accelerator Pedal is floored and the appropriate Roadspeed and Inlet Manifold Air Pressures occur.e.g. If the Car has slowed on a Grade and the Roadspeed is low enough, flooring the Accelerator Pedal will cause the Trimatic to change down from 3rd to 2nd Gear.The Kickdown will be rapid because the Inlet Manifold Pressure will be high (atmospheric) meaning that the Engine will be producing maximum Power.

Kickdown Types:

Trimatic Kickdown mechanisms are Cable, from early HG through to early HQ, then became Electric Kickdown from late HQ through to the last Trimatics.

Trimatic Body:

All Trimatics use the same Body, but the internal components in the Body are changed to suit the Engine it is coupled to.A V8 Trimatic will have more plates in each Clutch Pack and Shift Valves that make it change up sooner to exploit the extra Torque that's available.A 4 Cylinder Trimatic will have fewer plates in each Clutch Pack and Shift Valves that make it change up later to compensate for the lack of Torque a 4 Cylinder Engine provides.

Torque Converter:

For each Capacity of Engine there is an unique Torque Converter.A V8 Trimatic uses a low stall Torque Converter to exploit the extra Torque a V8 provieds.A 4 Cylinder Trimatic uses a high stall Torque Converter because a 4 Cylinder Engine provides much less Torque than the V8.  A 173 and a 202 have differrent Torque Converters to provide the best coupling for each capacity even though the 2 Torque Converters are physically interchangeable.The difference is that the 202 Converter engages at a lower RPM and causes the Engine to work harder at lower RPM than does the 173 Converter.One way to tell the stall speed of a Converter is when engaging Neutral.A high stall Converter will be largely unnoticeable when the Selector is moved from Neutral to Drive and the Engine is Idling.A low stall Converter will provide a noticeable thud when the Selector is moved from Neutral to Drive and the Engine is Idling.

Extension Housing:

This holds the Output Shaft, Bearing, Seal, Speedo Drive and Governor. The Gearbox Mount bolts onto the Extension Housing.




Back to Trimatic Index