Adding an auxilary electric fan

[Joe M]

to my friend's 390 68 Cougar.  He is leery of dumping his mechanical fan and relying on just an electric.  Car heats up a bit when stopped so he asked me to add a pusher fan that he could activate if he gets caught in traffic.  He purchased a Summit Racing fan that draws 8 amps.  My question is, at what point should a relay be used to activate an electrical component?  Thank you.

[jayb]

It depends on how you are turning on the fan.  Will it be on a switch, or is it going to be controlled by a temperature sensor plumbed into the cooling system?

If its just on a switch, there are a lot of switches that can handle that fan, but beware that when you switch on you will have an inrush current of around double the rated current of the fan.  So, make sure you get a switch that can handle 15-20 amps.

If you are using a temperature sensor, they can't sink the current required for a fan, so you will have to use a relay.

Hope that helps - Jay

[Joe M]

Thank you for the comeback, Jay.  He just wants me to run it from a toggle switch so he can supplement his engine-driven fan.  I'll certainly make sure the switch is rated high enough.  He is also running a Mallory Uni Lite.  Could the momentary increase cause a spike that could damage the module?  I know he has a  Mallory voltage protector wired in.

[jayb]

I doubt that there is any chance of damage to the Unilite when he flips the fan on.  He might see a temporary voltage dip or spike on the battery voltage when the fan is turned on or off, but I doubt this would damage the Unilite, especially with the protection module in place.

[Joe M]

Thank you Jay!

[FE_Rex]

FYI - usually the late model Ford cube relays have a diode suppression system built-in.