The relay (drivers side, inside the lip, engine compartment, above the wheel well) the red wire, pin 30 on the relay, is the hot, the 87 pin red/white (on most models) is the power to the fan. The 86 pin is power, FROM the thermo switch (lower left of radiator) to the relay actuator (only has power when radiator exit temp. is above 190-210F.). the 85 pin goes to ground. The 86 and 85 pins for the relay, actuate the coil (switch) in the relay. Jump the connector, for the thermo switch and see what happens, fan should turn on (I keep a ready made jumper, tied to the wire by the connector). If it doesn´t, it´s most likely power. Check the fusible link, for the red wire (to the 30 pin), with the key on. Check the yellow wire at the thermo switch for power (with the key on, comes from the igniton switch).
Have seen a very hot motor 220-240 (by the temp. sensor at the back of the block), with air in the system, fail to turn on the aux fan. The thermo switch for the aux fan, lower left of the radiator, was still relatively cool, due to lack of circulation. The book says, the thermo switch for the aux fan, turns on (switches at 190, all I have tested switched at 207-209 deg. F.). What I´m saying is, it´s possible for the motor to overheat due to lack of circulation, from trapped air, a faulty thermostate, a partially plugged radiator or what not, without turning on the aux fan.
I ran a switch, jumping the thermo switch, to the dash, so I could turn on the aux fan myself. If you plan on installing a manaul switch, near the connector for the thermo switch is a good spot. I just cut off a little insulation and soldered two wires onto the existing wires.
Usually when the aux fan fails to come on and the motor is hot, it´s due to air trpped in the system. Sometimes the thermo switch, sometimes the relay is faulty or corroded.
