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635, bottom radiator coolant outlet, large brass hex sensor with four pins, plugs to wires.

Two flats are broken off to prevent plugs from connecting. Is this about a/c only ? Not functioning yet anyway.

If it's big for relaying coolant temp to gauge, it's shopping time.

Early E24's used a double contact radiator switch. It's the same design as 528i E12s, but used one switch instead of 2 and it costs a hundred bucks instead of 15. It did have different temp settings though, I think it was 91/106C instead of 91/99 for the low/high speed operation. While the aux fan DEFINITELY needs to be on for A/C, I think the fan is a very good idea for just normal running as BMW designed it. If the car gets hot, the fan comes on. BMW didn't spec a high draw engine fan for these cars, so an aux fan is definitly a good idea. I think it's later E30's used a double switch with 91/99 set points which will fit in, but it takes a different plug. Ok if you have DIY junkyards nearby and a cheap fix. Otherwise, I'd probably plunk a 91 switch in there and wire it so it turns on high speed. Better a little extra noise than overheating.

So the four prong lower radiator sensor is basically for activating the electric fan in front of the smaller radiator for the air conditioning.

In front of the main radiator, rather blocking it some. As long as the a/c isn't used, car isn't driven much or on days hot enough to push the cooling limits of the main system, no big deal.

Yes, no.

Not exactly, the A/C system turns on the aux fan for A/C use, that double switch in the radiator only works with higher temps. Particularly on high, it does make a real difference in cooling. As to whether you really need it or not depends on the condition of your cooling system, including the fan clutch, which on 80+ cars is prone to failure, and how hot the ambient is.

I almost got in trouble last June coming back from 5er Fest when I got stuck in traffic with a non op fan clutch and no aux fan reinstalled yet. So, while if everything is right it shouldn't be necessary, I think it's a very good idea for it to be there and working regardless if you have A/C or not. Think of it as insurance...

In my car, when idling in traffic the engine will get hot enough to turn on the aux fan even when the A/C is off. Aux fan comes on for a while, cools the engine down, then shuts off for a while until the temp goes up again. The engine driven fan needs help.