Howdy,

The early M46 OD circuit did use a two pin fourth gear switch, Volvo PN 381251-8 (1975-78) or 1235353-8 (1979-80). The switch on the shifter was a slide switch (ON/OFF), Volvo PN 1234508-8. One wire of the slide switch was connected to fuse 11, the other wire was connected to one side of the fourth gear switch (and the OD indicator). The other side of the fourth gear switch was connected to the solenoid. You can see this circuit in the Green Book TP30057-1 available at www.k-jet.org. Navigate to the 200 series Green Books. It is the first book of wiring diagrams listed. The OD circuit is on pg. 98 of 145.

If the shifter setup you have has a push button you can use that but you will need to configure the OD wiring to use a relay. But not just any relay will do. Because the push button is momentary, the relay is an electronically latching relay that also has an AND function. (Button pushed AND fourth gear TRUE will turn on the solenoid.) The relay is Volvo PN 1259750-6; it should be easily obtainable at a pick and pull. (In a 740 the relays are behind the fuse box. In a 240 the OD relay is up behind the Heat/AC controls.) Here is the circuit:




I have no idea if the parts for the early slide switch are available. You might be better off using what you have and getting a relay. Regardless, you should have enough information to gather the parts you need to make it work.
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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)