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General TR Overdrive problem

Trevor Triumph

Jedi Knight
Country flag
My '64 Spitfire has a single rail transmission and associated overdrive. It has been misbehaving lately. It stopped working once and I replaced the fuse. Problem solved, I thought.

Recently, it stopped working and the fuse appears to be good. Driving home from an event, I turned it on expecting that it might have somehow fixed itself, at least for a while - this has happened previously - no such luck. Later I noticed while shifting into third gear, the car sounded as if the overdrive was engaged, (drove like it, too.) but as far as I could tell there should have been no current to the solenoid - the switch was off. So I drove home in top gear, seeming to be just a bit faster than idle.

Can the overdrive engage without power? I wonder too if transmission oil level could have an effect. Though as I recall, when oil gets too low it stops working.

I need to solve this so I can put this car of the road, so I can repair my wife's Spitfire shift lever.


Great Pumpkin - R.I.P
Don't back up! Or if you have to, do so very slowly and cautiously. If the car doesn't want to back up (kind of like the rear wheels are against a curb), don't force it. Either wait for the OD to relax (which may or may not happen), or call a tow truck to lift the rear wheels.

There are several ways for the overdrive to be engaged with the switch off. I would start by disconnecting the wire at the output from the relay, to be sure it's not the relay itself shorting out. Next check the drain hole in the operating plunger. It's not very big, and if it gets clogged, the OD might not disengage. (the hole has been exaggerated in the photo below)

Also check that the valve ball is in perfect condition, the spring & plunger are in place, and the solenoid is adjusted properly.
It's also possible that the OD clutch is jammed into the brake ring. A few raps with a brass faced hammer might free it up.

I'd probably do a pressure check next. If it's building pressure, there has to be something wrong with the control mechanism. If it's engaging without pressure, though, it's a mechanical problem inside the unit.

If all that fails, you may need to disassemble the unit.

Trevor Triumph

Jedi Knight
Country flag
This overdrive does not have a relay. The manual does not show it, as I recall. It could be that I am using the early schematic MkI page rather than the later 1500 wiring diagram. In any case, eliminate power to the solenoid.

Looks like i have some special time with my car in the next few days.



Great Pumpkin - R.I.P
Oh? You have a D-type, right?

If it's a J-type from a later Spit, then almost everything I posted above is wrong. Sorry about that.

This is from a Mk II owner's manual, but I'm pretty sure the Mk I was the same.

Trevor Triumph

Jedi Knight
Country flag
The single rail transmission and J overdrive are from a later Spitfire. Tomorrow I'll get to work checking wire connections and such. I don't think I have had to move the car in reverse so I'm not sure abut that... I guess I'll find out tomorrow.
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