Dual Electric Fans on Carb'd Engine

[Neil B]

Hey guys, I've got twin Spal fans that pull some serious amps when they kick on. I currently have both fans on a single 185 degree sender. The problem is that the load from these fans causes idle speed to drop about 300 rpm. If I get my 'fans off' idle speed and mixture where I want it, the engine will stall when the fans start. This is a carbureted engine and I really don't want to add complexity with any type of IAC solenoid or PWM fan controller. I am considering either wiring 1 fan to key on ignition (always on) and then having the other fan come on at 185 degrees via the sender. Or, I could run 2 senders and stagger the fans to 185 and 195 degrees, but I think I would still have to run a higher 'all fans off' idle speed. Thoughts?

[rickpaw]

Holley sells an AC on solenoid that bumps the idle speed up when the AC is on. You can use the fan on signal to trigger the solenoid. Holley P/N 46-74.

Just curious, why would an increased load on the electrical (with the fans on) cause the idle speed to drop?

[carbuff]

My car does the same. It's the alternator kicking in to supply the increased load. My Holley HP controller bumps up the IAC position quite a bit to compensate. And then when the AC kicks on, it's even worse. It was a little tricky to get all that working correctly as well, so it's not just a carb problem.

[Neil B]

Originally Posted by rickpaw

Holley sells an AC on solenoid that bumps the idle speed up when the AC is on. You can use the fan on signal to trigger the solenoid. Holley P/N 46-74.

Just curious, why would an increased load on the electrical (with the fans on) cause the idle speed to drop?

From what I have read, that solenoid is not powerful enough to bump the idle on it's own. You have to blip the throttle and then the solenoid will hold it at the higher idle. So I'd have to blip the throttle every time the fans engage.

[gray86hatch]

Use a pwm to control the fan. On dads car with this you could not tell the fans were on. Almost never need to run at full speed.

On a normal drive on 80 degree day the fans dont come on unless stopped. I have the low at 195 and high at 205. Norm right on the tstat at 195

Tim

[ProServ]

Old thread but in case this helps anyone else.

You could stagger each fan as you mentioned or you could wire the fans like GM often wires the dual fan vehicles so the two fans come in staggered, both a low speed initially and then both at high speed.

First the two fans come on at low speed (series wiring) and then at a higher temperature they both go to full speed (parallel wiring). You would need two temperature switches and three relays or you can use a temperature based relay controller with a single temperature sensor (but you still need a harness with three relays).

Look at page 6 of this document for an example of how this would be wired.


BTW - the product in the above instructions could also be used to do the staggered (full speed fan 1, then full speed fan 2) control you also mentioned.

Originally Posted by Neil B

Hey guys, I've got twin Spal fans that pull some serious amps when they kick on. I currently have both fans on a single 185 degree sender. The problem is that the load from these fans causes idle speed to drop about 300 rpm. If I get my 'fans off' idle speed and mixture where I want it, the engine will stall when the fans start. This is a carbureted engine and I really don't want to add complexity with any type of IAC solenoid or PWM fan controller. I am considering either wiring 1 fan to key on ignition (always on) and then having the other fan come on at 185 degrees via the sender. Or, I could run 2 senders and stagger the fans to 185 and 195 degrees, but I think I would still have to run a higher 'all fans off' idle speed. Thoughts?