Trimatic Modulator

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Page created by T Oct 13th 2009:

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V8 Trimatic. Item 2 is the Modulator. Click to Enlargen. Photo by Hot308VK.

Trimatic Modulator:


Bolted to the offside rear of a Trimatic Gearbox is the Modulator. This device measures the Engine Vacuum underneath the Throttle and converts the Vacuum into the forward or backward movement of a Pullrod.The amount of Vacuum determines the position of the Pullrod. Since Trimatics work by comparing the Roadspeed with Engine Vacuum it's the job of the Modulator to provide the Engine Vacuum information to the Trimatic.
The Trimatic Modulator. Click to Enlargen. Photo by HQ_SS.
How to fit the Vacuum Hose to the Modulator. Image by T. Click to Enlargen.

Tools Needed:

7AT15 Spanner for bolting/unbolting the Modulator.
The 7AT15 Modulator Spanner. It is 1 1/4" AF and 3.0 mm thick. Click to Enlargen, Photo by T.

Removing The Vacuum Pipe:

Remove the Metal Hose from the rear of the Modulator by twisting and pulling the rubber.

Removing the Modulator:

A special thin Spanner 7AT15 is used to remove or replace the Modualtor because the space between the Modulator and the Trimatic Case is small. The Modulator Spanner is 1 1/4" AF and 3 mm thick. Unbolt the Modulator by turning it anti-clockwise with the Spanner.ATF will pour from the Trimatic when the Modulator is removed so use a Drain Tray of some sort to catch this Oil.Modulator.gif A Pushrod (Actuator Pin) fits into the end of the Modulator. The Pushrod will fall out when the Modulator is removed. Make sure you keep this clean and ready for re-installation. The Pushrod is a vital part of the Modulator's operation and transfers the Diaphragm's movement to a Spool Valve to generate a representative Oil Pressure.The Modulator Spool Valve is held in place by the Modulator and is easily removed for inspection and cleaning. It may also fall out of its own accord. After cleaning and inspection, refit the Modulator Spool Valve with its widest end facing the Modulator. A round Mark on the end of the Spool, if present, will indicate contact with the Pushrod. 

Correcting Overfilling:

Removing the Modulator:

Removing the Modulator is a way of draining out excessive ATF if the Trimatic has been overfilled. Take a reading from the Dipstick to work out how much ATF to drain (from the low to the full Mark is 0.5 Litres). Place a Drain Can with graduations on it under the Modulator then remove the Modulator from the Trimatic. When the excess amount appears in the drain Can, refit the Modulator .Recheck the Oil Level after correcting the overfilling.

Bleeding the Radiator Lines:

By loosening one of the Trimatic Lines at the Radiator a small controlled leak can be created. By matching the amount drained the Trimatic Dipstick level can be corrected.

Testing the Modulator:

The Modulator must not have any Airleaks. The Diaphragm can get an Hole in it and leak. Test the Modulator by blowing into the Vacuum Pipe. It must hold pressure. If it leaks it must be replaced or ATF will be drawn into the Inlet Manifold and the Trimatic will not shift Gears correctly. 

Replacing the Modulator:

Fit the paper Gasket to the Modulator and place the Pushrod into the end of the Modulator.Screw the Modulator into the Trimatic Casing finger tight. Tighten the Modulator using the 7AT15 Spanner. 

Replacing The Vacuum Pipe:

Refit the Vacuum Pipe as per the image on this page. Pull the rubber connector back along the Vacuum Pipe.Plug the metal Vacuum Pipe into the Modulator.Twist the Rubber along the Vacuum Pipe until it sits half on the Modulator Pipe and half on the Vacuum Pipe as in the Image. Note that rubber can become hard and brittle with age and need replacing. The rubber is simply a piece of standard rubber hose available from Repco. It is the same size as the Vacuum Advance Module Hose. 
The Inlet Manifold Vacuum Source located at the rear of the engine. Note there is a bump in the centre of the Rubber showing that the Vacuum Pipe has been fitted correctly.  Click to enlargen. The left most pipe provides a vacuum source to the Heater and Air Vent Controls. Photo by Vernonv

253 V8 Engine. The Photo shows the Vacuum Tap that supplies Vacuum to the Modulator. Note that the tap point is underneath the Throttle. Photo by Energy4anarchy. Click to enlargen.
L R Trimatic from an EFI VK Commodore. The Modulator is lower right. The Detent is to the right of the Modulator. Click to Enlargen. Photo by 383HGBrougham.
the Holden part number for that 5 way plastic T fitted at the rear of the manifold is 92009622. Info by myeh.  Click to Enlargen. Photo by mr self destruct.
Trimatic Vacuum Source from 308 VK Commodore.  Click to Enlargen. Photo by mr self destruct.
Trimatic Modulator. This is the non adjustable type.  Click to Enlargen. Photo by Snowdownunder.

Vacuum Source Part Numbers:

T, the Holden part number for that 5 way plastic T fitted at the rear of the manifold is 92009622. The other 3 way T fitted at the front of the manifold on 6 cylinders is 90229623. Not sure if it is worth adding the part numbers to the pedia..End of submission by myeh.   




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