Overdrive won't Disengage

[SteveJP]

Just been out for a 100 mile run along the various motorways around here at the legal limit to give the Spitfire a good blow through. However when coming down through 4th and 3rd the overdrive will not disengage using the gearlever switch, it only disengages when dropping into 2nd when the inibitor switch comes into play.

Once back on the 'B' roads it works perfectly - anyone else had this problem?

[Paudman]

Must be a short circuit through the gearlever wires, making contact and keeping it switched on at the switch. - check them for wear.

[Spitty2Dream4]

Yes, I've had this. I've put it down to needing the oil replaced or topped-up. It only seems to happen after a motorway run and takes about 100 meters before beginning to disengage.