overdrives....

If it was working fine before the swap then it's most likely the electical side of things rather than a problem with the overdrive itself. The live feed from the on/off switch needs to go via the 3rd/4th switch and a reverse inhibitor switch (sometimes in one combined switch depending on the car).

You can check where the fault is step-by-step...


Check the solenoid:
One wire to earth, then run a live wire to the other connection. You should hear a definate 'click' as the solenoid engages. If not, the solenoid itself is faulty.

Check the inhibitor switches:
With the ignition turned 'on', the overdrive turned 'on', and the car in 4th gear - trace the original live wire back from the solenoid to the next switch in the circuit (either 3rd/4th or reverse). Touch those two wires together and see if the solenoid clicks - if it does then that inhibitor switch is faulty. If not, then continue to the next switch and test that in the same way.

Work your way along the live feed until you get to the gearlever switch. If connecting the wires at the gearlever switch does nothing, then it's a fault in the feed from the fusebox to here (it's common for the wires to wear through because of the constant movement of the gearlever).


Hope that helps!
If you get stuck, post again and tell us how far you've got.