Electric Fan control Diagram

[Curious Hound]

Hi! I have this posted in the general discussion forum, but thought this might be a better place to share it in case anyone else wants the info.

Following is an electric fan control wiring diagram with explanatory schematic to more clearly show how the circuit works. This setup is based on the Volvo 2-speed fan install in the tech articles and has the following features.

• Two speed fan operation
• Master cutoff switch on dash
• Provisions to ensure fan runs when Air Conditioning is running
• Interlock to prevent running both fan speeds simultaneously

This circuit is currently installed and operating in my '93 Ranger 3.0L. But it should work on just about any vehicle. In the photos, you will not see the master cutoff relay (R1) because I installed it in an extra space in my underhood fuse/relay box. I used the power door lock fuse to supply power. If you have any questions, feel free to ask here or PM me. NOTE; IT HAS BEEN BROUGHT TO MY ATTENTION THAT THE LABELS "SLOW" AND "FAST" ON TERMINALS 2 AND 3 ON THE TEMPERATURE SWITCH ARE REVERSED. The wiring connections, however, are correct.

The schematic;

This shows the BMW 2-speed temperature switch installed in the upper radiator hose. I later moved this switch to the lower radiator hose and it performed much better. (please ignore the intake plenum. That's a work in progress.)

Here are the speed relays and A/C enable relay mounted at the top of my fan support.

 

[RonD]

Nice, very nice

 

[solraven]

looking over that diagram, If I'm reading that right when the slow sensor trips the fan kicks on high speed, and then if the fast/high temp switch kicks on or the AC kicks on it switches the fan to low speed. Also if it helps, most 3pin temperature switches don't connect the casing to the ground wire, meaning you can reduce use of relays by running 12v to it instead of ground and connecting output to the fan as long as the amperage draw is within rating(don't use the Taurus fans). Looking at the wire diagram from the vehicle the switch is from, it looks like they where made to operate this way. You could also save a relay by having your cutoff switch cut ground to the sensor and a/c relay. I'm also curious if anyone has tried hooking pin 45 of PCM to a relay and seeing if it just works, or could be enabled in forscan? Possibly even adjust on off temperature. I was always curious why the thermostat housing had 2 temperature sensors...

 

[Curious Hound]

I'll look over the diagrams this evening when I have time and verify their correctness. But that setp served me well for 6 or 7 years until I removed the air conditioning from the truck. Then I simplified. The fans often draw 30 amps or more, at least in high speed. So, I prefer not to run that current through the temperature switch.

This was designed primarily for the older trucks with OBD1 (EECIV?) Ecm architecture. So there was no fan output from the ecm. Therefore, using g Forscan to enable or disable it was not an option. On newer OBDII trucks, I would expect that to already be enabled and in use.

The reason there are 2 temperature sensors on the thermostat housing is that one (the 2 wire device) is a sensor for the ecm and the other is a sender serving the gauge cluster in the dashboard. Again newer trucks may just have 1 sensor and allow the ecm to send data to the gauge cluster. I have wiring diagrams for a 97 and 02 superduty F250s that I can look at later to confirm that. OBDII architecture opened up some wonderful abilities in the area of data sensing and system controls.

 

[Curious Hound]

looking over that diagram, If I'm reading that right when the slow sensor trips the fan kicks on high speed, and then if the fast/high temp switch kicks on or the AC kicks on it switches the fan to low speed. Also if it helps, most 3pin temperature switches don't connect the casing to the ground wire, meaning you can reduce use of relays by running 12v to it instead of ground and connecting output to the fan as long as the amperage draw is within rating(don't use the Taurus fans). Looking at the wire diagram from the vehicle the switch is from, it looks like they where made to operate this way. You could also save a relay by having your cutoff switch cut ground to the sensor and a/c relay. I'm also curious if anyone has tried hooking pin 45 of PCM to a relay and seeing if it just works, or could be enabled in forscan? Possibly even adjust on off temperature. I was always curious why the thermostat housing had 2 temperature sensors...

It appears that I have the labels "slow" and "fast" reversed on the temperature switch in the first diagram. The wiring works correctly. But the labels are backwards. The second diagram, in my opinion, is easier to follow to see how it all works. The first diagram is more helpful to show people how to wire it. It depends a lot on your electrical knowledge and experience. Thanks for letting me know about the labeling error. I have made some edits to that posting.

One other note, the use of relays to control the fan speeds, instead of wiring the temperature switch directly from power to the fan, allows me to electrically interlock the fan windings to prevent both fast and slow windings to be energized simultaneously. Just something I have learned to practice over many years as an electrician.