I installed a twin electric flexalite fan into my truck. The supplied thermostatic switch is crap so I went ahead and purchased a cooling jacket thermostat with relays. The fan flows 5,500 cfm with both running and is probably more than adequate for my needs.

The fan comes wired to run simultaneously and I seperated both to run on individual relays. They both draw a large amount of amperage and one should be adequate when A/C is selected especially for defogging on colder rainy days. The relays will be setup as to override the A/C relay should engine temperature exceed the thermostatic sending units range.

I drew up a primitive "schematic" below. I added a Diode into the A/C circuit so it wouldn't back feed into Fan1.


Would this work? If so, would Radio Shack or similar sell the proper Diodes for this application?

I know it looks terrible as I slapped this drawing together in about 5 minutes.

Anthony

 

What is wrong with the supplied thermostat? I have mine wired up and ready to install (in a Radio Shack project box):

 

There's nothing wrong with my flex-a-lite switch..just mount it behind the right headlight instead of on the fan shroud...that'll keep it from having heat related problems....just my opinion..JR

 

I read your previous experiences. That's where I got the idea to mount it in a project box. I am mounting it in the spot where the battery is--which is getting relocated under the bed.

 

I appreciate the response but if you looked at the diagram carefully you will see the fans are on seperate circuits. The Flex-a-lite supplied control box is not the proper way to operate the fans as it uses a radiator mounted feeler bulb stuck in the radiator. To get a true coolant temp measurement a tapped sending unit must be installed into the coolant jacket as close as possible to the head before the thermostat.

BTW, those mosfets should have air cicrculation around them and not sealed in a box.....

Anthony

 

here is one i drew up for mine...
this was for the ic fans and didnt draw as much juice, so just put in another relay to the other fan and your good..



jim

 

i dont know if a 12v diode will work, they are rated at 12v but low amps, this app will draw much more..,the relays should stop any current from back feeding...you should need three relays two fan and one ac, the fans are just like the diagram above but just add another for higher draw fans, the ac relay will be positive from ac when on in pin 85, and ground on 86, then also ground on 87, and to the sending wire side, or pin 85, in the above diagram on pin 30 of the ac relay, no doides should be needed..


jim

 

L In detroit, the setup above will require a diode as there is no way for power off of the a/c relay to not backfeed. I would have used the positive side but the sending unit switches the relays on via ground so I had really no other choice.

The diode would be installed on the low amperage side of the relay. I figure about 2 amps max.

 

the way i did mine was fan wires go to relay load output+12 chasis ground for -12, thenn the relay batt +12 in goes through a XX amp breaker not a fuse. The relay coil wires are +12 from ign on acc power so it goes off with the key, and - inputs from manual on switch/ thermostat grounds on/ and AC call from PCM with a diode in the a/c line so your ac doesn't run when your fans come online. Hope that is simple enough. I would use two relays but run them at the same time. You may run high head pressures onn the a/c system with only one fan. testing needed to confirm, or maybe remove one relay etc during the cold months if you live up north.

 

What diodes did you use? Preferably a model number or equivalent. I was just going to order the stock plug in type used in the distribution panel...

 

current cannot flow back through a relay....it will only trip from a open state to a closed state with opposing current on either side....
pins 87 and 30 will only be a short when the relay is tripped and it is only tripped when the ac is supplying positive voltage to it, or when it is on, otherwise it is like an open switch and voltage cant get back to the ac circuit..

jim

 

 

Went to Radioshack and bought some 2-3 amp diodes for a few bucks. Standard looking diode, the relay coil only draws a few mA.