Electric fan relay wiring [Archive] - Chevelle Tech

: Electric fan relay wiring

Mar 29th, 05, 12:59 AM
I need a little help here with how to wire in electric fans using relays.
Emphasis on Tric. Basic relay wiring is no problem for me, but I'm puzzeled how to do this.
Scenario; 3 electric fans(2 radiator 1 condensor), 1 temp switch, 1 A/C pressure switch. What I'd like to have happen.
A.)Temp switch closes=both Rad fans come on.
B.) A/C is swiched on= Condensor and 1 Rad fan come on.
C.) A/C switched on and temp switch closed= all fans on
Fan feed will be fused from Battery. Fans will ground at fan frame.
Control circuit, low volt, comes from Ign switch(+) and grounds for control will be from a temp switch and a pressure switch.
I'm trying to do this with as little amount of wire possible so it looks clean.
Assuming the control wire leading to the pressure and temp switch splice together
it looks to me that both rad fans will get a ground when only one is commanded. This is using a 4 post relay in the circuits(is there another type?)

Elree Colby
Mar 29th, 05, 9:22 AM
Hi Phil

As you know 85 and 86 are the pick up coil connections for a typical automotive relay. Connect one, 85?, to ground on each relay. Connect the temp switch to 86 on one radiator fan relay, use a diode to connect 86 on the first radiator fan relay to 86 on the second radiator fan relay. For the condenser fan relay you can connect to the clutch power, whenever the clutch is on the fans will turn on. Connect from the pressure switch or compressor clutch to 86 on the condenser fan relay and use a diode to connect 86 from the condenser fan relay to 86 on the second radiator fan relay.

Also I would use a auto fuse/circuit breaker in the motor power feed as opposed to a fuse. The auto fuse will reset when it cools. You won't be setting on the side of the road with a blown fuse and no replacement. :mad:

Mar 29th, 05, 11:54 AM
Thank You Elree. Does Radio Shack carry the Auto fuses or is there a local source to get them and the Diodes?
Stop by sometime. I'm still in the same house, 2519 S Pennington.
Redoing the wiring today as I have to pull the Rad for a leak
Calling is OK too 480 213 7616

Elree Colby
Mar 29th, 05, 12:59 PM
Napa or other auto parts will have the auto fuse. Fry's electronics will have the diode. If one's closer Radio shack will have them also, just a better selection at Frys.

Mar 29th, 05, 11:02 PM
I'm trying to follow the logic of how each fan is being activated.

From the initial post it sounds like you are using a one wire temp sensor that goes to ground at a certain temperture? And I thought that the AC pressure switch was a +12 volt feed, with the compressor grounding thru its bracket mount.

Is that correct, or am I confused?

Mar 31st, 05, 1:01 AM
Hoptup you are correct on the temp switch. The pressure switch is in the line and it closes at 180 psi It is also wired to close the circuit to ground when A/C is selected on. It is independent of the A/C Compressor control circuit.
Keep in mind that this is on the computer controlled TPI Elky

Elree Colby
Mar 31st, 05, 11:46 AM
The diodes will work equally well on the ground side of the relays. I would be concerned about using the refer pressure switch. If I understand it correctly it is the switch used by the engine management system. I didnít do any research however Iím thinking the switch is interrogated with lower voltage, around 5-6 volts, if so you will not want to connect 12+ volts to it. With the refer switch open there will be 12+ volts present on the ground side of the relay coil. In addition when the refer switch opens and the fan relay(s) drop out the collapsing magnetic field of the coil will induce a voltage spike that may damage components on the engine management processor board. You can install a diode in the refer switch factory connection that would prevent the possible higher voltage from feeding back into the engine management. To help with voltage spike you can install a diode across the relay coil.

The diode has a color band on one side. When used on the ground side the band will be toward the shared relay. The shared relay will have two diodes with the color band toward terminal 85, the diodes will then connect to terminal 85 on the other two relays. A wire from one radiator fan relay terminal 85 to the temp switch and a wire from the condender fan relay terminal 85 to the pressure switch.

To avoid any possible problems with the engine management system, you can use a fourth relay the picks up with the compressor clutch. The relay will switch the ground of the condenser/shared fan relays. This would be in place of the refer pressure switch.

If you don't have the relays yet I have new relays with or without mounting tabs, relay bases with or without mounting and the wire terminals that are captured in the relay base.

Mar 31st, 05, 4:30 PM
Got it all dialed in Elree
Initial problem that brought this on was a bad thermoswitch