Triumph Repair: Relays, electro magnet, duty coil

Question
How does a relay work? For a TR3A Overdrive, can you bypass it and would it continue to work?

Answer
Hi Robert,
Almost all relays are installed to take the amperage load off of the operating switch and wiring. In this case the O/D switch operates a solenoid which in-turn moves a lever to allow oil pressure to engage the overdrive.

The solenoid draws too much amperage for the switch to handle so the switch only operates a light duty coil in the relay that is an electro magnet that closes a set of contacts in the relay that connect the current to the solenoid. Keep in mind that there is a interrupter switch on the transmission that prevents you form operating the O/D in 1st, 2nd or reverse. This should be kept in the circuit as "Reverse" can destroy the overdrive.

If you have just found that the relay is bad, you can replace it with most any relay used to operate a solenoid or such, like a starter relay or a fan relay from almost any brand of older car.

The markings on your relay (if original) should be "C-1" for heavy power in (Brown wire), "C-2" wire to the O/D solenoid. "W-1" = Ignition on power to the relay magnet. "W-2" to the trans sw and on to ground.

Most 4 pin relays today are marked with numbers. "30" is Heavy power in to the contacts and "87" is the load (solenoid), "85" & "86" are the two ends of the small electro magnet to activate the relay. (note that some relays have a diode in the magnet circuit so you may have to reverse the leads on "85" & "86" to make it work.) Many numbered relays have a small diagram on the relay showing the connections.
I hope this helps, let me know.
Howard