Triumph Repair: TR 6 Overdrive, cone clutch, cruise speed

Question
I have a '71 TR6 that is a daily driver.  Recently, the overdrive has begun engaging and disengaging erratically.  When the engine is newly warm in the morning, the overdrive engages properly, but shortly after the car will disengage from overdrive.  Once the transmission is thoroughly warmed up, the overdrive won't engage at all.  I replaced the solenoid a while back and the actuator arm moves as it should; I've also changed the oil and cleaned the screen in the bottom.  One of my friends told me that there is a ball bearing valve on a spring that might be the culprit, do you have any ideas?

Answer
Hi Rick,
The adjustment of the solenoid arm is very important. The arm moves a small rod that lifts the ball off of it's seat allowing oil to be pumped to the cylinders that shift the cone clutch to operate the overdrive.
The car Commission number should start with a "CP" or "CC" for you to have the "A type" overdrive. The "A type" is easy to ID. It has the solenoid mounted straight up on the left side. When the solenoid is activated the arm on the right side should be able to insert a 3/16" rod through the hole in the arm on into the OD case. If the rod will not slide in all the way, you need to adjust the arm on the solenoid side.
When the arm is in the correct position a rod inside on the right side lifts a ball valve up high to allow the oil to go to the cylinders that engage the OD.
The solenoid has a piston that is raised when activated and that piston his a small pin inside the solenoid which changes the "Pull" magnet to a lighter "Holding" magnet.

I found that you can divide the problem into the OD or the solenoid by lifting the lever on the right side and inserting a 3/16" rod (exactly 3/16")all the way into the OD case and with the car on stands or a lift and start the car and operate the car in 4th gear. Then watch the speedo when you get up to cruise speed to see if the OD is working. At very slow speed, the OD will not operate because the pump will not have built up enough pressure. As you speed up the pump pressure will build and the OD will start operating.
YOU MUST !!! NOT!!!!! PUT THE CAR IN REVERSE WHEN THE PIN IS IN PLACE OR IT WILL DESTROY THE OVERDRIVE.
If the OD works with the pin in place than the adjustment is wrong or the solenoid is failing when hot.
Another way to test the system is to drive the car with the tunnel cover off and have someone drive the car while you sit in the passenger seat and when the OD fails check with a 3/16" rod to see if the solenoid has put the arm on the right side in the correct position. This will tell you if the failure is inside the OD or just the solenoid not holding the correct position.
Let me know,
Howard