Roy Deal

[APO77]

Over Drive

I have had an Auto Electrician investigating in the operation of my overdrive.

A relay has only 3 connections in stead of 4.

He has state that on using o/d in 2, 3 and 4  gears that if the revs drops  then the unit comes out of o/d.

I cannot see how this functlions.

Any comment please.

[stillp]

You need a better electrician.

Pete

[RobH]

+1 

Lucas 6RA relays come in a number of weird configurations.  One of which has an internal connection between two of the pins, meaning it is a "4 pin'' relay which only has three pins.  Perhaps yours is one of those, though it really isn't the right type for the overdrive - it is the type used for the horns on a '4A.  The common pin is used for power input and feeds both the contacts and the operating coil (below). 

He is just plain wrong about the OD dropping out if the revs lower - it isn't supposed to do that. 

The problem with using that relay with an A type overdrive is that the relay coil and the contact are powered from the same supply.  It is supposed to be wired with a 4-pin relay so the low-current relay coil operates from the ignition-switched supply while the contacts are powered from an un-switched supply. That way the ignition switch does not have to carry the high pull-in current of the overdrive solenoid. 

Edited November 10, 2023 by RobH

[Lebro]

Only thing I can think of is in 2nd gear, if revs are low then the pump in the OD may not pump enough oil to keep up the required pressure.

Bob

[stuart]

On a Healey 100 they had a speed switch on the box output shaft so below 30mph overdrive would drop out, perhaps the boy is confused

Stuart.