... in my post below, where at the end I added the following:

Seems like most any simple relay would work, with a push-button switch (ON) and grounded thru the 4th gear switch (the "self"canceling" part).
Solenoid current would be a consideration.

That's dead wrong. A special "latching type" relay is required. The simpler relays would do just what you don't want — they would re-energize as soon as 4th gear was re-established.

Once the ground is lost (by going out of 4th gear), the latching relay I described will not automatically "switch on" again when 4th is re-engaged. The energizing pulse from the OD push-button switch is also required.

The simple On/Off switch on pre-'81 OD's that Randy suggests would also allow the unwanted "automatic" OD re-engagement you want to avoid. You need the combination of a PB switch AND the later (electronic) relay.



P.S. Apologies for the duplicates below. The BB system was stalled for a short time I guess.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.