Overdrive
The overdrive operates electrically, there is a solenoid on the side of the unit which controls oil pressure which is used to operate the internal clutches. It should engage smoothly under power. It may be quite difficult to do a smooth change on a trailing throttle. When it engages it should be quite quick… ~quarter second for the revs to settle. If it takes longer than this…and even longer under full throttle, then the overdrive clutches or brake bands are worn or mal-adjusted, which requires a specialist rebuild. Try to reverse the car, in the event there are some funny noises from the gearbox, or in a worst case the car won’t move, then the one-way clutch at the back of the overdrive has broken up (again, time to rebuild). A TR4A overdrive should operate on 2nd, 3rd & 4th gears, if it only operates on some of these then the interlock wiring on the gearbox needs examination (quite simple), if it operates on 1st gear as well….. then someone has bypassed the interlocks!!