Triumph Repair: Overdrive, overdrive units, laycock

Question
My 1973 Spitfire with the 78 engine has an overdrive that it controlled by a switch on the dash...It is currently not working. When I had the clutch replaced a couple of years ago they were supposed to check and replace it needed when they had the tranny out; however, they conveniently forgot. What's the easiest way to diagnose if I have a bad solenoid or switch and what is the easiest way to go about replacing it without dropping the tranny again.

Answer
Hi Linda,
There were two different overdrive units used on Spitfires. Either one can be removed with the transmission from inside the car by removing the center tunnel cover although you would not want to do that until you diagnose what is wrong. First look under the car at the OD and see if the electrical solenoid is mounted on the right side pointing rearward and is mounted with two screws. If so, it is a Laycock DE Normanville OD. (Early Spitfire)

If the solenoid is mounted on the left side pointing rearward and is itself screwed into the case with a large hex then it is a "J Type" OD. (Later Spitfire)

Both are operated by that solenoid. The earlier "DE" and the later "J Type" are electrically operated when you are driving in 3rd and 4th gear only and use a lock out switch on the gearshift system. So your first test is to see if the solenoid is activated. Turn on the Ignition on, put the car in 4th gear and check for power at the wires of the "DE" OD and check for power
on the yellow w/purple tracer and that the black is a good ground.

On the "J Type" the color codes of the wires should be the same.

If you don't get power to the solenoid then you need to remove the tunnel cover and start testing the lock out switch and power supply.

You should purchase a service manual for the car to make it much easier to work on. If you run into problems that the manual does not cover I will be glad to help.
Howard