Triumph GT6 Overdrive Wiring

It is common to find various different overdrive options on several gearboxes fitted to British classic cars throughout the fifties, sixties and seventies. It is also common to find issues with the wiring.

Whether it is an MGB, Triumph Herald or Land Rover, plenty of British cars were fitted with an overdrive option on their gearboxes, providing a higher final gear ratio, reducing RPMs and improving motorway cruising.

We were lucky to have this gorgeous magenta Triumph GT6 in for recommissioning. Its overdrive was not working, but as is mostly the case, it was only the wiring that needed to be replaced, as the solenoid and gearbox were in good condition.

The overdrive is controlled by a switch in the gear knob that activates a Lucas relay, usually mounted on the bulkhead. This in turns operates the solenoid at the gearbox which engages the overdrive gear, sometimes in both third and fourth gear.

A previous owner had bypassed the relay, wiring the solenoid directly through the inside of the gear stick to the gear knob switch, which had become faulty. This is not recommended as the solenoid requires a large current to operate and this wiring can be prone to short circuiting if damaged by the entry holes in the metal gear stick.

We reinstated the original Lucas relay and made a new separate wiring harness for the overdrive switch and solenoid circuits, keeping the two separate and protected with fuses.