Triumph Repair: J - Overdrive, electrical fault, d mode

Question
Hi Jim, I have a 77 Spitfire, the overdrive works up to 50 mph; once I go over 50 mph, it slips in and out of overdrive consistently; running between 40 and 50 mph, the overdrive will stay engaged.  What is causing this?  Many thanks!

Answer
Howard,

The overdrive works by using gear oil pumped into the engagement pistons, with an electrically operatated control valve.

What it sounds like is that at higher speeds something is causing the o/d unit to disengage, either hydraulically or electrically.

Does the problem happen in both 3rd and 4th gears?  Or only 4th?

My first guess would be that the oil level is a bit low, and at higher speeds you're starving the pump for fluid.

Otherwise there may be an electrical fault that's de-energizing the control solenoid, allowing the o/d unit to drop back into non-o/d mode.

While you're checking the fluid level it's worth hooking up a test lead at the overdrive solenoid and running with a voltmeter hooked up.  If you see the voltage dropping as you're moving around then it's a sign there is some sort of electrical fault.


Cheers,

Jim