As part of my R-134a conversion I want to install an auxilliary fan.Most post say to just wire it in to the compressor wiring. Thing is that last fall I installed an IPD radiator that already has a temprature sensor that at this time is redundent.Would using sensor not be better as fan will not be running at high speed which would impede air flow ?

Wait a minute. If you want to improve the A/C performance, you want the aux fan to come on when the A/C is turned on. I suggest that you take a look at the 93 240s (and maybe 91-93) because it has an aux pusher fan that does just that. It comes on when the high pressure switch determines the high side pressure is too high and some advocate jumpering the high pressure switch harness to override this and have the fan come on whenever the A/C is on. For this setup, you don't worry about what the coolant temp. is doing.

forget the sensor...it may burn out at the wrong time (ask me how I Know) .Wire the signal to the relay for the aux fan to a manual switch on the dashboard. That way you turn on the pusher fan when you want it to blow. Like in heavy traffic or waiting to cross the bridge back from Mexico in 115 degree heat. You can turn it off at highway speeds so the fans blades don't block air to the radiator.
buena suerte,
el Raidman

Suppose I connect the compressor hot wire to the Aux. fan but have the sensor wired in series between the two ? Would this not give the best of both ?

Without diodes anytime the fan is on, so will the compressor.

The only technically correct method is: Power a relay with a fused +12v wire to the #30 terminal, and ground the #85 terminal; then to activate it: +12v->coolant switch->diode->terminal #85a on the relay AND (added)A/C high pressure/temp switch feed->diode->term #85a.

This will turn the fan ON if EITHER the A/C temp/pressures get high OR the coolant temps get high. Since you ARE NOT relaying on the electric fan for cooling by itself, you do not need a "failsafe" system.
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JohnG 1989 245 MT @216,500

Personally I have a fetish about keeping the radiator cool and will not tolerate overheating problems so I would go with the radiator sensor, how ever it really depends on what you are trying to do.


If you want to improve the A/C performance wire the fan to the compressor,
If you are trying to compensate for the heat to the radiator caused by the extra load then use the radiator sensor, It just depends on what you want the auxilliry fan to do.
As it stands right now my 245 with a new 3 row and an elec. "puller" fan and no A/C works fine...the wifes with new 3 row A/C and stock fan ...also works fine
and it gets plenty hot here, just yesterday I saw a dog chasing a cat...they were both walking.
Bret
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rust free in west Texas

The absolute BEST scenario is to slave a fan relay with TWO sensors, one for A/C (pressure OR temp), and the other for radiator temp (cold side). The trick is to use two diodes to prevent the coolant temp sensor from activating the A/C compressor (due to electrical feed-back).

That's the best method.
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JohnG 1989 245 MT @216,500