Triumph Repair: overdrive issue

Question
The overdrive in my 1980 spit. does not stay engaged. I have read that if the fluid level falls The OD will not engage. I toped of the tranny and the OD functioned for several miles but soon became erratic and then failed to engage. The transmission is leaking But too slowly to cause a low level so quickly.
   Your opinion will be very much appreciated,
         Craig

Answer
Hi Craig,

The overdrive is a two speed automatic transmission. When the OD is disengaged it is just a direct drive through the OD and no gears are in use in the OD. It has a cone shaped clutch inside that is spring loaded to a matching steel cone on the shaft and the whole thing is locked together and even when the trans is low on oil it is still locked as one unit.

When the electrical solenoid is activated it allows the oil pump to pressureize two pistons in two cylinders to push against the springs and forces the cone to lock on a case maunted cone and it stops and that forces planet gears to rotate. so then the rear shaft is spinning faster then the input shaft thus over drive. If the oil level is low or any other thing in the system cuts the oil pressure down then the cone clutch is released and the springs take over. It can however just get low pressure and then the cone clutch can slip.

Slowing down too slow can also make the pump not maintain pressure and that will cause it to either slip or disengage the OD.

You will need a manual on your car to test the oil pressure and a high pressure gauge. The electrics can also fail to operate the solenoid which opens the ball valve to allow the oil pressure to get to the cylinders.

It is easy to test if it totally fails but difficult to test if it only fails at times. It is not hard to test the electric supply by connecting a test light to the solenoid wire and drive the car and watch the light. If it stays on when the OD fails then you know the problem is from the solenoid on into the pump. Next would be to connect a high pressure gauge to the test port and watch the pressure at the time of failure. This would test the pump. If it maintains the correct pressure at the time of failure then the problem is inside.

Howard