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MG MGB Technical - OD engaging off throttle

Here's the background:

'69 MGB GT, just put a used OD unit in. Replaced both seals and the big screen on the OD unit itself. I took it on a vintage car tour this weekend where it got about 800 miles of hard use. It performed well with one exception. When the weather was really hot 90s-100s and the car temp was up the overdrive would engage by itself in 3rd and fourth gear when I was off throttle. If I got back on the throttle it would instantly go back into normal non-OD gear. When this happened the RPMs dropped about 500 rpm making engine braking problematic. Once the outside temperature cooled down to around 85 or lower the OD never engaged without me flipping the column switch. Also even on the very hot days it never did it first thing in the morning, only after the car was well heated up. Also the OD always performed flawlessly when engaged.
I checked and topped off the fluid with appropriate 20/50 Castrol, even adding a bit of ATF to see if it would help clean the system.
It got in the habit of unplugging the OD when it was getting hot out if I was on a tight twisty section, and then plugging it back in if there was any wide open roads coming up.

Any ideas of whats happening and how to rectify it?

-Bill
Bill Mertz

Here is a pic from the event. Good group of cars. This Abarth just blew a transmission.

Bill Mertz

This is a stretch but you have a very strange problem. If I read your post right, unpluging the overdrive solinoid wire stops it from happening. The only thing I can think of is that the wire harness to the transmission is getting hot and you have some wires melting. If the green power wire going to the reverse lamp switch is shorting to the yellow wire to the overdrive solinoid it can energize the OD. It would be a coincidence that when you back off the throttle the movment of the engine and trans is forcing the wires to connect. Disconnect the OD wire under the dash from the OD switch. Connect a test light to the wire from the OD and to ground. Go for a hot ride, if the light comes on you know you have a short in the wire harness somewhere.
John H

I think the 69 is the LH o/d so no vacuum interlock but still has the switch on the dash? The engine and gearbox will twist realative to the body as you apply power when the car is hot the mounts are more flexible so it twists further. This can cause the interlock on the gearbox to switch according to throttle stae, but this would also need a phantom power feed as well. It may be that you are getting a bypass of the O/D switch as well due the twisting. If its the switch on top of the gear lever then i would suspect torque induced short around there.
Stan Best

I hadn't realised disconnecting the OD wire stopped it, which makes it a simple electrical problem. Something is obviously connecting power to the OD wire when the engine moves under decelleration, i.e. two wires shorting together as John H says. Where do you disconnect it? The problem is obviously upstream of that, i.e. closer to the ignition switch than the gearbox.
Paul Hunt

I disconnect it under the hood at the bullet connector above the right front fender well. I do have the switch on the column stalk. How would I know the short is up stream of where I disconnect it?
Bill Mertz

You know the fault is upstream of where you disconnect it because disconnecting it stops it engaging when it shouldn't. But if you mean 'where is the short?' then it can only be between where you disconnect it and the manual switch, which is only a couple of feet of wire, and no connectors other than at the multi-plug for the column switch. On a day you are not going to need the wipers yopu could leave the bullet connector conencted and unplug the multi-plug, and that would tell you if it were between the two connections i.e. in the main harness, or in the stalk switch and its sub-harness.
Paul Hunt

This thread was discussed between 12/09/2008 and 14/09/2008

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