There have been many threads over the years about how the fans & fan controller operate, but I can't recall one that summarized it.

Has anyone ever done a diagram like this that captured the common conditions and what the fans are supposed to be doing in each?

 

Look at the WSM in the fan controller section. IIRC there are coolant temp and refrigeration maps that show what you describe.

Perhaps the most obvious non-predictor of fan operation is the dash gauge reading. That sensor is in the water bridge for coolant before it heads to the radiator, so is pretty much at the hottest part of the system. The S4+ fan controller is looking at coolant temp at the bottom of the return side of the radiator, pretty much the coldest part of the system. It also reads frigen high-side pressure after the condenser. In my car on days that deserve AC operation, the fans run at medium or high speed *** soon as the AC button is depressed (correct per the WSM map...) and go to high in stop-and-go on warmer days. For those of us with working nose flaps, the flaps work in close association with the fan speed controls.

Oh -- Be to include the ATF and intake manifold temp switches to your matrix of fan drivers.

 

Is it true that the fans have two speeds, medium and high?

 

Pretty hard to figure out from the WSM ... there are a series of diagrams on 19-16 thru 19-18 that show fan voltage against (1) cooling water temp, (2) AT temp, and (3) Frigen [refrigerant] pressure. Each chart has one solid and one dashed line, perhaps each represents one of the fans? For example, on AT temperature, voltage on the dotted line goes to 8V at 105 and on the solid line at 110. So maybe that says one fan comes on at each temp threshold?

For refrigerant pressure both lines hit 8V at 3-4 bar, and then both go to 12V at around 20 bar. That would be consistent with the logic that both fans come on together when the AC is turned on.

On 19-26, it says "If both fans refuse to operate when the AC button is pressed while the ignition is on, the output stage is defective"

 

The fan "speeds" are defined by the voltage applied. The WSM diagrams show that there are variable fan speeds available for refrigeration pressure, and variable speeds based on coolant temp. (see section 19, starting on page 19-15) The controller uses the higher of the two requirments.

Looking at the coolant temp chart, the fans move immediately to the lowest speed available at 8 Volts when coolant temp --at the radiator sensor-- increases past about 78ºC. Voltage rises progressively towards 10 Volts by 86ºC rising, and peaks at 12 Volts as temperature exceeds 95ºC. There is a small amount of deadband (approximately 2ºC) in the control as temperature drops.

Reiterating my previous reminder: The temperatures used for fan operation are sensed at the bottom of the cold side of the radiator, right where the coolant return nozzle is placed. The fan system considers how much heat is being shed by the radiator, regardless of how hot the coolant in the crossover and the outlet to the radiator might be. So fan operation is somewhat disconnected from what you see on the gauge, by heat-added from the engine and heat shed by the radiator. The fan system's only job is to deliver coolant back to the engine that's between 78ºC and 86ºC. From that, the engine thermostat controls how much coolant is recirculated and how much is passed through the radiator. Perhaps "confusing" the issue more is that the engine thermostat is in the return line from the radiator and in the bypass around the radiator.


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With those things in mind, installing a "colder" 75ºC thermostat will buy you a lower gauge reading only when fan operation isn't required. In the summer or even in low-speed driving on cooler weather, the fan controller is still trying to add cooling capacity when the return temperature is above 78ºC under the best conditions. It really works to keep return coolant temp around 85ºC. So with a 75ºC thermostat, all you'll "gain" is bigger swings in temperature as the thermostat and the fan controller fight each other as you drive. Will it drop your summer gauge readings? Not really - it only delays the proper warmup of the engine.

 

Do the fans get the same voltage under all conditions, or are there cases where one fan is off and the other one on, receive diff voltages, etc.? I thought that might be why the diagram has dashed and solid lines, to represent each of the fans ...

 

Both fans operate at the same speed. The dashed line with the little left-point arrows indicates the action of the control on reducing temperature, where the solid lines with right-facing arrows show the action on increasing temperature.

For the intake temp, the sensor is actually a wax-operated switch mounted top-center of the intake manifold. The difference in operating temps rising vs. falling is the deadband, in this case caused by the action of the little mechanical switch inside.

 

Both fans should always do the same thing. Both off, both at slow speed <-> both at high speed. if you ever see them doing different things - you have a problem.

Alan