I spent several hours yesterday pondering over how to power on the auxiliary fan whenever the compressor is running, as well as when the coolant reaches 100C/212F, and I think I've figured out a way to make it work using the original auxiliary fan relay, which is:
In early manual W116 climate control systems, the compressor clutch is powered on directly by a blue and green 2,5mm² wire coming from the temperature dial switch (no switches of any kind were used between the temperature dial switch and compressor; pressure switches were not yet in use). A blue 1mm² wire is connected to this wire at the temperature dial switch and connected to the RPM stabilization relay which would raise the idle in gasoline-powered models so that whenever the compressor clutch was engaged, the engine's idle speed would be raised to counteract the extra load.
To power the auxiliary fan whenever the compressor clutch is powered:
1. The existing red 2,5mm² wire at fuse #2 (which has only one wire, as its sole purpose on a W116 is to power the auxiliary fan) connects to terminal 30 of the auxiliary fan relay, providing it with a power source.
2. The extra blue 1mm² wire coming from the temperature dial switch of the manual climate control which would have gone to the RPM stabilization relay on a gasoline-powered model will now connect to terminal 86 of the auxiliary fan relay, which will switch on the relay whenever the temperature dial switch is turned on AND the ETR switch does not disable it.
3. A brown 0,75mm² ground wire at terminal 85 of the auxiliary fan relay will complete the circuit which will make switching on of this relay possible by a signal from the blue 1mm² wire coming from the temperature dial switch.
4. When the auxiliary fan relay is switched on by power from the temperature dial switch, it will allow power from fuse #2 to travel through the auxiliary fan relay and out of terminal 87 where it will travel through the existing black 2,5mm² wire toward the auxiliary fan.
5. The wires which went from the auxiliary fan relay to the refrigerant temperature switch on the receiver drier will be removed along with the temperature switch itself. The wires which went from the auxiliary fan relay to the coolant temperature switch on the thermostat housing will also be removed.
Doing the above should power on the auxiliary fan whenever the compressor clutch is engaged, however, I will be using the pressure switch on the drier, which will disengage the compressor clutch whenever refrigerant pressures are too high. However, the auxiliary fan's power wire will be connected before the pressure switch, therefore the auxiliary fan will remain running and reducing refrigerant pressures even when the pressure switch causes the compressor clutch to disengage.
To power the auxiliary fan whenever the engine's coolant temperature reaches 100C/212F:
1. An added-on 2,5mm² or 4mm² wire (possibly black in color) will be added to fuse terminal #2 (of which sole purpose was to provide power to the auxiliary fan through the auxiliary fan relay). This wire will then go to the coolant temperature switch on the thermostat housing which switches on when coolant temperatures reach 100C/212F and then switches back off once the coolant lowers down to 95C/203F.
However, the original coolant temperature switch (part number 006 545 14 24, red color, M4X1.5X1 threads, on at 100C/212F, off at 95C/203F) is a single-pin unit which grounds to the engine through the thermostat housing and is not suited for what I need.

I found that there is a coolant temperature switch (part number 006 545 15 24, red color, M4X1.5X1 threads, on at 100C/212F, off at 95C/203F) which has two pins and grounds from the second pin after going through the wires which complete the circuit.

2. Another 2,5mm² or 4mm² wire will go from the second pin of this 2-pin coolant temperature switch replacement and will run back through the firewall and will connect to the black 2,5mm² wire which powers the auxiliary fan (inside the firewall before it reaches the loom after the firewall so the part of the auxiliary fan wire harness in the engine compartment will be left untouched).
One thing I wonder about: Can the coolant temperature switch handle the full load of the auxiliary fan running through it? The original design is for it to switch on a relay, which then draws power from the fuse box to power the auxiliary fan, so there is very little current running through the coolant temperature switch and even small 0,75mm² wires are used. However, what I want to do is run the full load of the auxiliary fan through it (and of course using much larger wires). I just hope the switch can handle that much current.
3. If the coolant temperature reaches 100C/212F, the coolant temperature switch on the thermostat housing will switch on and allow power from fuse #2 to flow through it and toward the auxiliary fan, thus powering it. When coolant temperatures drop to 95C/203F, the coolant temperature switch will shut off and thereby shut off the auxiliary fan.
By doing the above, the auxiliary fan will always be powered by fuse #2, which is solely used for powering on of the auxiliary fan. A 16 amp fuse is stock, however, I have the freedom to install a larger fuse if necessary because of the higher amperage of the upgraded 16" second generation W126 auxiliary fan and since there is nothing else being powered by fuse #2.
Increasing the size of the wires in the auxiliary fan wire harness is not necessary because even the second generation W126 uses the same size 2,5mm² black and brown wires. However, because the added wires which will run between fuse #2, the coolant temperature switch, and the auxiliary fan power wire will be rather long as the coolant temperature switch is located on the opposite side of the engine compartment than the auxiliary fan wire harness, I may use larger 4mm² wires to and from the coolant temperature switch in order to keep down the wire resistance.
If the auxiliary fan is being powered by the coolant temperature switch, it cannot flow back and inadvertently power the compressor when the air conditioning is not on because the auxiliary fan relay which is switched on only by the power wire coming from the temperature dial switch will be switched off.
When the air conditioning is switched on and the auxiliary fan is running with it, if then the coolant temperature switch engages, though there will be two powered circuits colliding, because both of their power sources is from fuse #2, there should be no consequences except possible decreased wire resistance as the fan would be powered by two wires instead of one. If the coolant temperature switch shuts off, the auxiliary fan will stay running until the either the air conditioning is shut off or the ETR valve shuts it off. Or, if the air conditioning is switched off, the auxiliary fan will stay running until the coolant temperature switch shuts off. The power cannot backflow into anything and cause inadvertent powering on or damage.
I still think ROLLGUY's wire harness and relay to power the auxiliary fan whenever the compressor is engaged is brilliant, due to it being plug-and-play, easy to install, and easily removed if desired. As long as a person doesn't wish for the auxiliary fan to come on with increased coolant temperatures, that's the way to go. And so far I have not ever observed increased coolant temperatures in my car--even in stop-and-go long commutes in 120F Phoenix traffic--with the exception of when I was going up the steep grade at The Grapevine in California at 70 MPH while returning from Washington with my car fully loaded inside and out.


So, most people probably don't need the function of the auxiliary fan coming on with increased coolant temperatures anyway, and what I'm planning to do requires drastic measures which is beyond the scope of many people--even most mechanics aren't going to spend hours/days figuring this stuff out. But, I'm already so deep into it with my manual climate control conversion, so I might as well do as much as I can.