Auxiliary Fan
 

I'm posting here b/c it's not specific on the application. What I need to do is add an auxiliary condenser fan to a truck, my concern is how to wire it.

I'm working on a 06 Dodge 5.9 Cummins, which has an inherent problem with the electrical fan clutch. This truck has been hopped up, so who knows why it won't work properly. Restoring to factory is not an option.

What I'd like to do is wire in a pusher fan that is activated when the compressor is engaged. I need a schematic. The best I can gather is that a relay energized by the compressor coil wire can be wired in so I can run the fan directly off the batteries. I'm not an electrician when it comes to wiring in extras, but I can only guess that I need a 4 post relay?

How would you wire something of this nature? I'd rather not wire in a dash mounted switch as this requires competence to turn the fan on when not moving down the road.

The reason I want to do this is because the high side pressures spike when the factory clutch doesn't want to behave. The OE compressor locked up, and it's the fan clutch's fault, and it's a huge problem with these vehicles. Right now I'm testing the quality of my parts and my work, and have told the owner to turn the system off when under 30mph. That's not acceptable, but I can't find a good answer either. The folks on the Dodge boards repeatedly call the condenser the evaporator, so I can't feel very comfortable with any solution that might be offered there.

How would you wire it?

Many aux fans will come with a wiring harness that has a variety of options built in. Some kits come with thermo switches, relay switches, manual switches, the works.

Honestly, get a decent kit and it will probably have anything you could need, and instructions on how to wire it to do what you want. A $100 kit is big enough to replace your entire engine fan probably, so as an aux fan you could go half that or less.

This is a low budget solution, thankfully I have all the fans I need, but it's all aftermarket replacements for OE. I plan to hack one up and mount it to the condenser. I'll have to get a data sheet for the fan and I hope my vendor(s) will be able to supply one.

My brother suggested a wiring solution, and I'll admit being sub-par in wiring. I can read a diagram, but that's about it.

Anyway, low amp in-line fuse between compressor coil wire to relay, use relay as switch, high amp in-line fuse from battery to relay, wire from relay to fan, ground fan. Good enough? Sounds solid to me, the fan comes on when I want it to, which when the a/c compressor is engaged.

Seems like you're good to go.
Low side is battery-switch-fuse-compressor-relay-ground
High side is battery-fuse-relay-ground

The fan will turn on every time the compressor kicks on. Your basic 40 amp Bosch relay works perfect for this, unless the fan is too big. You can get standard-style 50, 60, 75amp relays as well.

You may want to put a zener diode and resistor from the relay output to ground. This will help bleed the energy from the DC motor and not create High Voltage sparks.