Relay question…
 

A follow up to my thread on a non-operational radiator fan in the E46.

If I want to put a switch in the car to operate a relay for the fan, what size wire and relay would I use?

The fan currently has a 50 amp fuse, so a 50 amp relay? Would it be better to go with a higher amp relay? Is there such thing as too much?

Using and SPST switch, is it better to use the switch with the positive or ground wire?

There are 50amp relays specifically designed for radiator fans. The relay can be the same or stronger than the fuse, but not weaker.

The switch side of the wiring can be tiny, since there's basically no electricity running on that side. 18 gauge is fine. The switch can be on either side of the relay, it won't know or care.

For the fan wiring, 8 gauge should be plenty, assuming it's connecting the relay to a fuse panel,right in the engine bay. 6 or 7 feet total path between the fuse, to the relay, and back to the ground point.

Exact answer, and why we use relays.

Question, Isn't the switch going to power the relay coil? I think that's how I'd wire it.

The fuse protects the wire. So if you've got a 50a fuse, that's typically sized for the amperage required for the fan when the fan first starts (LRA if I recall correctly) If you put an ammeter on the wire when it's running, it's probably not pulling anywhere close to 50 amps. That's a lot of juice!

Thanks, Pazuzu.

I was going to use the factory wiring and fuse, as it’s heavy wire powering the fan, and hook it up to the relay. From there to the fan. Negative wire run to the switch.

If I go with all new wiring, there is a positive lug for jumpstarting the car in the engine compartment. Can I use that for both high power to the fan and low power to the relay, with the negative wire running from the relay to the switch? Both power wires would be fused.

Edit. Accessing the fuse box is behind the glove box and a major PITA to get to.


http://forums.pelicanparts.com/uploa...1658071902.jpg
http://forums.pelicanparts.com/uploa...1658071902.jpg

This picture is from when I was testing the fan with direct power. When I finalize everything, I will solder all the connections.
http://forums.pelicanparts.com/uploa...1658071902.jpg

The cut black wire with green tracer, is the wire from the DME to trigger the fan on and off. I’ve been trying to determine if there’s a break in the wire back to the DME, so it’s cut. Edit. Checked the black with green tracer and it’s good.
http://forums.pelicanparts.com/uploa...1658071902.jpg

I use this on my 89 Silverado along with an AC Delco aux fan. Been 3 summers so far in AZ, no problems, works like a champ. Hayden 3651. Lives underhood.

https://www.amazon.com/Hayden-Automotive-3651-Adjustable-Thermostatic/dp/B000C39C72

Just curious, why not switch to a manual fan?

The automatic E46 and I think all M3's had manual fans, only the manual cars had electric fans.

Good question. I was thinking the electric fan pulled more air and only when needed.

That'll work

That should work

But this is a better answer. If you put the fan on a switch, the day will come when you're distracted or whatever and don't trip the switch by hand. Then things go poorly. Let an aftermarket thermostatic do the work for you.

I saw that but it says it it controls two fans up to 30 A. Can it be used to trigger the 50 A relay?

Can I make this even harder? haha

Ya you could- use the output of 3651 to trigger the factory fan relay input: the wire from the ECU that triggers the relay.

Also, the 3651 is not only triggered by temp, but has an additional input for manual override.

In my case, it's set to go on when the AC Compressor runs OR it hits 212F, then it cools to 195 or thereabouts.

I simply put the probe in the fins right next to the upper hose, then adjusted the temp settings by letting vehicle get warm while sitting. AC compressor fires or vehicle hits 212 my aux fan comes on.

rjp

Enough for a welder. This is the first I have heard of a 50-amp automotive circuit.

Thanks, Randy. I have wire, a wired fan relay, and the 3651 coming for install this weekend.

If you read about a Charleston, SC man burning the house down, you’ll know it’s me.

NP

PS Note that the GREEN wire on the unit requires +12V input- it's supposed to be hooked to the AC Clutch which typically is +12V. Of course, if you wanted a manual override you can instead wire GREEN to a positive source (ignition) and interrupt it with a switch in cabin.

Good luck

rjp

Big motors (like cooling fan motors) have a current spike when they start up, called the inrush current. A fan might clip along all day long at 15 amps, but starting it might take 40 amps. It's the kick that overcomes the inertia.

So, your 15 amp fuse and relay might be fine when it's running, but you'll end up smoking one of them with a cooling fan that turns on and off several times a trip.