MaXJohnson said:
The easiest way to add a fan switch to the early XJ is to tap into the wire coming from the temp sensor (in the radiator) to a switch with the other side of the switch to ground. Since the sensor provides a ground to the circuit when it reaches critical temperature, you can manually supply the ground with the switch.
My 87 and 88 are wired from the ignition switch (yellow wire) to the thermal switch at the radiator, through the thermal switch (when closed) (grey wire w/tracer) to the 85 pin on the relay (with A/C the wire color is different to the 85 pin) , out the 86 pin to ground. The models with A/C have splice (into the grey wire/w tracer) and diode pack, to keep the current from traveling backwords into the HVAC system. I usually wire in the switch near the thermal switch connector, upstream of the diode pack, just because this is the way it was designed to work. Power out of the thermal switch (or bypassed with a toggle switch) through the diode pack to the relay. This setup, should avoid, supplying power to anyplace it shouldn´t be.
Should know the answer to this, but the 90 is still Renix? got it in the old head the changeover was in 91?
Dan if yours is still the Renix, supplying voltage to the 85 pin on the aux. fan relay is probably OK, but if the thermal switch is closed, you could be supplying power back through the system to the computer and some other things. Or in other words, with your switch on (depending on where your getting your power from), the thermal switch closed and the iginition off, you could still be getting power to the off side, of parts of the ignition switch (under certain conditions). Who really knows how this loop will affect things? Safest place to get power would be from the yellow wire to the aux. fan thermal switch (or PROBABLY any yellow wire out of the igniton switch bundle?).
Or better yet just jump (two wires) the thermal switch at the radiator with your switch. A piggy back splice, will leave the thermal switch in the system, in case you forget to turn your switch on.
