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My two-relay fan solution.
Pardon me, but I think I may have misunderstood something and responded too hastily in my earlier post. I’ll try again to explain what I have in mind with what I hope will be a little more clarity. This is what I have in mind for my '82 245 w/M46.
Here’s my idea for how to have the benefit of the add-on condenser fan available at low speeds when it is needed most and unavailable at high speeds when it is not needed at all: I think of this as a two-relay solution.
The first relay would be wired up as you have described, signal from the compressor coil and through the radiator thermostatic switch, 12 volts from the fender wiring block, fan (load) assigned to terminal 87 on a “normally open” type relay. [I imagine a normally open relay would work like this: when there is no signal voltage (from the compressor), or no ground (through the radiator thermo switch), the circuit is “open” and no voltage is delivered to the load (your new fan).]
The second relay in my two-relay solution would be in addition to all of the above. This second relay would be the five-pin type with two “load” terminals: terminal 87, as above, “normally open,” and a terminal 87a “normally closed.” This second relay could be wired into the signal circuit of the first relay anywhere between your compressor, thermo switch, and ground. The signal circuit of the first relay becomes the load of the second relay (connected through the second relay at terminals 30 and 87a). As the second relay is “normally closed” (circuit “on”) to terminal 87a, the first relay will operate as normal as long as the second relay is not energized. It would be as if the second relay were not there.
But, when the second relay is energized, its terminal 87a would switch to “open” (circuit “off”), interrupting the signal circuit of the first relay, and the first relay (and your new fan) would be disabled. This second relay would get its signal voltage from the overdrive circuit, anywhere between the overdrive relay and the overdrive solenoid. You could tap into the circuit right near the relay (mine’s behind the dash next to the glove box). You could mount both relays together under the hood to make future fault tracing a little simpler. Terminal 87 on the second relay will be unused and should be carefully insulated.
Hope this helps. Let me know.
Steve.
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