"Does anybody know if the one single wire on the back of the instrument panel, that is on the lower right if facing the instrument panel, does that one single wire control the overdrive arrow light?"
• Yes it does. The Arrow light always has +12V hard-wired to one side. The other side leaves the cluster on that single wire and connects to the relay output 87a.

Relay terminal 87a should be either at ground when the relay (and thus the OD/solenoid) is OFF – turning the light ON. Or it should be at +12V with the relay/OD/Solenoid ON – turning the light OFF (+12V on both sides, thus no bulb current flow).

"I read on a previous post that you said if the arrow light stays on constant that it could be the fuse. So that is why I was wondering about the rear demister switch not lighting up since it is on fuse #11 also with the overdrive."
• The light is ON when the relay is OFF—and a blown fuse means the relay would have to be OFF. But it also means:
A) There would be no voltage available from the fuse for either the OD or the demister
and
B) With no voltage from relay to OD solenoid there could be no OD shifting, which you say is working—so +12V from fuse is getting to OD relay/solenoid. Meaning the demister must be a separate problem.

If the relay is good, I'm afraid I'm stumped for now. That light wire connects to relay terminal (87a) and the OD solenoid connects to relay 87. Those two terminals are common to each other, and should be at ground potential when the relay is OFF (lighting the arrow and deenergizing the Solenoid).

With the relay ON, the +12V at 87 and 87a energizes the OD Solenoid (as it apparently does) and should turn the Arrow OFF, as explained above.


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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.