Friday, August 5, 2011


Fan control is accomplished in two ways. The fan always runs when the air conditioning compressor clutch is engaged. In addition to this control, the fan is turned on by the temperature of the coolant which is sensed by the coolant temperature sensor which sends the message to the on-board computer. The computer turns on the fan through the fan relay.

Switching through the on-board computer provides fan control for the following conditions.

1. The fan will not run during cranking until the engine starts no matter what the coolant temperature is.

2. Fan will always run when the air conditioning clutch is engaged.

3. On non-air conditioned vehicles or with air conditioning off the fan will run at vehicle speeds above about 40 mph only if coolant temperature reaches 230°F and will turn off when the temperature drops to 220°F. At speeds below 40 mph the fan switches on at 210°F and off at 200°F.

4. There also is included a method to help prevent "steaming" (water vapor evaporated by hot water circulating through the radiator evaporating moisture on the outside of the radiator and when there is no ram air to blow it under the vehicle) the fan will run only below 60°F ambient, from 100°F 195°F coolant temperature only at idle, and zero vehicle speed and then only for three minutes.

You can check the coolant temperature with a thermometer inserted into the radiator.

The first thing you'll need to do is gain access to the relay connector. check for battery voltage on the gray wire. If there isn't any check fuse 24 in the power distribution center. If the fuse is ok but there is no power at the relay there is a broken wire.

If the power is ok check for ground on the solid black wire. If you are missing ground you have a poor ground or broken wire to find. You could find the problem or lay in a new ground.

If these are ok plug the relay back in, start the engine and turn on the a/c. Wait a few minutes and then check for power on the dark green wire. If there is power then the wire is broken between the relay and the fans somewhere.

If there is no power take a jumper wire and ground the light green/dark blue wire at the relay with it plugged in. The fans should run at full speed. If not and everything else has checked out ok, the relay is bad.

If grounding that wire does turn the fan on then we'll go to the engine controller mounted next to the power distribution center. Unplug the gray connector and find the same light green/dark blue wire in pin 73. Ground the wire there and see if the fans come on. If not then you have a broken wire between the controller and the fan relay.


Take out the air box, including the filter housing below it, and you will see the relay mounted to the frame rail below it. why is it there you ask? it needs to be there for a constant air flow to keep it cool, as it is a solid state relay with no moving parts.

ps. the radiator fan will not come on as soon as the a/c is turned on, the high side pressure needs to be built up to a certain pressure for the fan to come on, if the ambient air is too cold, or the a/c freon is low, the high side pressure will never build up high enough to trip the fan.


the gray should be 12volts positive from the fuse
the dark green goes right to the fan
the black is a ground for the relay
the light green(with a dark blue tracer) is the computer command of the relay (provides a pulsed ground)

i would jumper the gray with the DARK green wire with a paper clip, the fan should work.

the new relay has to be bolted up to work correctly, as it is also case grounded. it's mount to the frame is very important, it must be tight and use the new screws supplied, as the vehicle's frame is also the 'heat sink' it keeps the relay cool.