This diagram may help explain how the logic works:
N39 is the resistor pack underneath the left side of the engine bay. V7 is the stock fan. F18 / F54 is the thermo-switch in the radiator.
Lets look at N39 first. This is how the three fan speeds are achieved. Put power to E1 and it has to go through two resistors before it gets to the fan, hence slow speed.
Power to E2 and it only goes through one resistor - middle speed.
Power to E3 and it goes directly to the fan - max speed.
Top left is relay J26. You can see this is switched from the F18 thermo-switch, so this comes on when the radiator outlet exceeds 87*, and switches on the fan at slow speed. Note also the connection to track 95 - this is the HVAC controller, allowing it to request the fan to come on, which afaik it does whenever the compressor is engaged.
Top middle is relay J101. This is switched from the F54 thermo-switch if the radiator outlet exceeds 93* and brings the fan on to medium speed. Note also the connection to track 45 - this is the AC condenser pressure switch. If the refrigerant pressure gets too high, ie not enough cooling air on the condenser, that will also switch the fan to medium speed.
Top right is relay J135. This is the last resort, and is switched from the instrument cluster in case water temperature cannot be controlled by the thermo-switch. This brings the fan on at maximum speed.
This is mine:
Vnew is my Spal fan which has replaced the viscous fan. The output from V26 (slow speed) is disconnected from the resistor pack and instead connected to my new fan. So, any time fan slow speed is requested (either by the thermo-switch or by the HVAC) the new fan comes on instead. Medium and maximum speed on the original fan still works as they should, the main difference being there are now two electric fans running instead of one plus the visco.
My fan is getting switched 12V from a relay, not via the resistor pack, so it always runs at full speed, however its power consumption is much less than that of the original fan which can pull ~40A on full speed.
