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1989 300SE 232k miles
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Discussion Starter #61
OK. So the jump that I did at the red plug is the pressure sensor, and the aux fans are to be at low speed.

I read where the engine temperature sensor is near cylinder 4 on my engine. And that I can turn my key to position 2, disconnect that sensor, and my aux fan should run high.

I’ll do those consecutively. That will determine if I have high and low speed.

I’m curious, because when I said my aux fan comes on, but not immediately, I mean substantially after the AC starts. To me, it’s well into normal pressures and more like during high operating temperatures.
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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Is that adequate for how you suggested direct wiring it?
No. That wiring turns on the low speed relay. Without thinking it through, I believe if you do the same thing to the engine coolant temperature sensor you will power the full speed relay. (If not, then simply unplugging the sensor will.) If the sensor is single terminal, then ground it.

Might be easier to simply jump the fan power connections at the relay, or, as I suggested before, run 12 volts directly to the fan motor. Or, once you get the potentiometer, you can turn it on by twisting the knob.
 

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I’m curious, because when I said my aux fan comes on, but not immediately, I mean substantially after the AC starts. To me, it’s well into normal pressures and more like during high operating temperatures.
The purpose if the low speed fan is to increase airflow across the condenser. When the condenser outlet pressure is reasonable the fan is not necessary. Higher pressures indicate the condenser isn't shedding enough heat, so the fan comes on.
 

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Discussion Starter #64
Okay. So the AC pressure switch (red cap) is supposed to activate when there’s low pressure. It turns on the aux fan at low speed when there’s low pressure. So I’m understanding that because my car instantly has high enough pressure in my AC, my aux fan not activating at low speed is normal and ok.

And the engine temperature sensor activates the aux fan at high speed. And that’s what I’m witnessing when my aux fan turns on. The aux fan is high speed and the engine is hot.

So they’re both working properly. But the question is: If my Pressure Switch (red cap) were to activate, would it be at low speed? If so, we’re golden. If not, there’s a problem that I only have one speed for the aux fan.

To determine this, I need to activate them both, myself.

Because they seem to both be working properly, I’m going to wait until my potentiometer arrives. I’ll need to access the engine temperature harness at that time. And I can simply jump the pressure switch then, too.

Also, I know I’m now into engine temperature concerns. Particularly with my 79 C thermostat. And my belief that me fan clutch isn’t working right...

I’ll let you guys know how my aux fan testing goes!

Thanks,
Bill
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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Now I'm confused. And, I'm too busy (lazy) to look anything up right now. There is a switch that is open when the pressure is low and prevents the compressor from running if the system is undercharged.

There is an engine coolant sensor that connects to some electronics that convert temperature to an on/off signal that triggers the fan full speed relay. This happens at about 105 degrees C and has nothing to do with the air conditioner. This is the one people put the resistor on to reduce the cut-in temperature.

There's another sensor, which I was thinking is pressure, that turns the fan on low (via the low speed relay), but maybe it's refrigerant temperature instead. This is to ensure there is adequate airflow over the condenser.
 

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Discussion Starter #66
ahhhh. Gotcha. Suffice to say, I’ll tap into the AC switch that runs the aux fan on low and the engine switch that runs the fan on high (when I add a potentiometer as an adjustable resistor). And compare their speeds. And that will let me know if I have both speeds working.

And not touch the other one.
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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Apologies to you Bill & John350..

Above I was looking at a different vehicle that's wired differently.
My description/assessment was incorrect.

For clarity, here is the proper diagram for the 300SE Aux fan.
 

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Thanks MBL! So, the aux fan comes on low when the high side pressure reaches 20 Bar (290 psi) and goes off at 15 Bar (218 psi). The other switch on the dryer is the combined compressor low pressure (2 Bar) and over pressure (30 Bar) cutoff.

BTW, there's no need to use a fuse when you jump the switch. That circuit is already fused and only powers the relay coil.
 

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Discussion Starter #69
Thanks MBL! So, the aux fan comes on low when the high side pressure reaches 20 Bar (290 psi) and goes off at 15 Bar (218 psi). The other switch on the dryer is the combined compressor low pressure (2 Bar) and over pressure (30 Bar) cutoff.

BTW, there's no need to use a fuse when you jump the switch. That circuit is already fused and only powers the relay coil.
Well that clears a lot! To my knowledge, I’ve never hit 290 psi. So I feel good again about the car running my aux fan at low speed.

I use a blade fuse because I read someone used one. And I have them. And they fit perfectly. And, most important, I know where they are at any five time. 😂
 

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Discussion Starter #70
I wanted to follow up. I jumped the switch at the AC switch to activate my aux fan at low speed. I immediately disconnected that, and unplugged the switch at my engine to activate my aux fan at high speed.

They are, in fact, 2 different speeds. And those switches (and the aux fan) seem to work properly.

Additionally, I “rebuilt” my viscous fan clutch by refilling its reservoir. My engine temperature is holding better at 80 C at idle, and my AC blowing temperature is staying colder at idle.

I have my potentiometer and may tap into the aux fan high speed switch at the engine, but I’m going to give the car some time.
 
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