I don't want a manual toggle, I don't want it to run continuously with the compressor and I don't want to open the refrigerant system to add a high temp switch. Sounds like an external temp switch is the way to go. Questions is, at what refrigerant temp should the switch close? And what does that translate to in condenser fin temp? I guess I can take a thermometer to cars with aux fans and see at what temp the aux fan comes on. I was hoping for an established rule-of-thumb or range.

I guess I can use a temp switch like compu has but I don't know that 190*F at the condenser is an appropriate setpoint.

Sixto
87 300D

The second link from earlier post appears to be more/less the same part as compu's for about the same price but with greater range: 32 - 240.

Your setpoint will need to be based on your installation location and desired performance. Manufacturers select theirs based on the factory locations. Your variables for the probe temperature may be: in front or behind the condensor, in the fins, near the fan or near the edge, a healthy, surplus-generating alternator (on early and longer) or a near-maxed out alternator (on later/off-earlier), healthy cooling system or marginal?

If you're using a currently installed fan and want it to come on at lower temp than currently, I'd use a changeover relay with the adjustable probe set-up from one of the above options. The fan would be on pin 30, the current clutch switch power on 87a. The adjustable relay would close and connect pin 87 to pin 30 (the relay pins are standard terminology for relays).

If the new thermoswitch NEVER tripped or tripped later than the clutch, you'd have stock performance via pins 87a and 30. With the changeover-adjustable combo, you should theoretically be able to dial down the temps until you get the desired performance via pins 87 and 30, triggered by the thermoswitch across 85 and 86; but never lose the safety margin of the stock setup.