My fans won't come on (neither one) and I'm trying to figure out exactly how it works electrically and hopefully the rest will fall into place Does the temp sensor have current to it all the time? Is it measurable current? Enough that you could check with a test light? Is there a temp switch as well as the temp sensor? What exactly turns on the fan relays? Is it a signal from the temp sensor or from the computer? Thanks in advance! Ryan

On a dual fan set-up, the temp sensor in the intake and the ecm control the driver side fan and will turn on around 230 degrees. The pass. side fan is controlled by the cooling fan switch in the pass. side head. It is located between the # 8 and 6 spark plugs. The switch will turn on the pass. side fan around 240 degrees. This will happen if the driver side fan fails to turn on. The pass. side fan is also activated by the AC pressure switch when the AC is turned on(if the AC works). Not forsure if you can measure sensor output with a test light. Best with a scanner that shows datastream so you can monitor engine temps.

THe AC doesn't work because all the refrigerant has leaked out but I jumped the switch which made the compressor come on but not the fan. Any other suggestions?

Also, what kicks the relay's on? Does the computer send a signal to the relay or does the temp sensor send on to the relay?

As far I know, the ecm sends a ground signal to the relay. If you jumped out the ac switch, make sure that it is the high pressure switch on the small ac line in front of the accumulator and not the connector on the front of the compressor that activates the ac clutch or the connector on the back of the compressor.

Thanks. I did jump it at high pressure switch. Since two of the four wires going into the relay's have current, I'm going to assume that it may be the ECM. I'm still at a loss though---even with a wiring diagram.

The signal that turns the fan relay on is a ground. For the driver side fan, the temp sensor on the manifold sends a signal to the ecm, and then the ecm sends a signal to the relay to close the ground and complete the circuit. On the secondary fan, its relay is contolled either by the fan switch b/w the 6/8 cylinders, or by the pressure switch on the a/c line. When the coolant fan switch reaches about 240 degrees it closes the ground for the relay and thus routes power to the fan. For the pressure switch, I don't know at what pressure the switch closes to complete the ground but that is how the relays work. I'm an avionics tech on F-16s and I deal with wires/relays all the time. Relays normally don't go bad. If you want to test your relays, at least for your secondary fan, unplug the connector to the fan itself (for hearing purposes only) and ground out the wires to either the cool fan switch or the pressure switch(next to the battery, just pull the plug off) and when its grounded you will here the relay click. That is how you will know if the relay is good. If you do not hear the click, then you have three options: you do not have power to the relay, the relay is bad, or a blown fuse.