DRL resistor voltage?

[FFRWRX]

Obviously my electrical knowledge is letting me down here.

I'm wiring up a small fan motor to blow more cool air into the engine compartment. I needed a 12V source to tap off. The Daytime Running Light resistor is right there, so I thought that would be a good thing to use.

I've tapped off both wires and with respect to ground I get 0V on one and about 3V on the other. The daytime running lights do work and are supposed to get about 9V from what I've read. That resistor drops the voltage. I thought that if I tap into the correct wire I should get the full 12V. Apparently not.

What am I missing? I was wondering about going across both wires but don't want to mess up whatever system the DRL's use by introducing an unknown resistance (the small 12V cooling motor).

Screenshot 2023-08-04 at 5.56.48 PM.jpg

And when they say "caution high temp" on that, they aren't kidding....it gets HOT.

Rick

[RPGs818SNA]

Hey Rick,

The DRL resistor comes after the 12 volts goes through the headlight bulbs, which are about 3 times as resistive as the DLR resistor. So the bulbs drop 9 volts from the 12, leaving only 3 volts across the DLR resistor. You'll want to find another 12 volt source for your fan, preferably fused appropriately.

Cheers,
RPG

[FFRWRX]

Thanks very much RPG. I thought I was being clever by finding a power source right by the fan.

The switch on the top of the steering column turns the rear lights on/off. Not sure why I would ever need that; in fact, I was going to disconnect it since when working on the dash area I would accidentally turn those lights on which would drain the battery if not noticed. But if I tap the fan into that circuit I can use it to control the fan. Don't think I would forget to turn it off since I can hear it running.

Rick

[RPGs818SNA]

That's a clever use of the parking light switch, Rick, especially if you never need to turn on the parking lights, front side marker lights, tail lights, and license plate lights at the same time whether or not the ignition switch is on. It should support a small fan easily.

RPG

[RPGs818SNA]

Now back to reality with some details. The parking light switch provides power to the parking lights either from the parking light relay controlled by the ignition switch and light switch stem or directly from the battery when the switch is on. To repurpose that switch, you'll have to permanently connect the parking lights to the parking light relay and run a wire from the switch back to your fan.

I think you were looking for 12v power already in the back near the engine. Depending on the thoroughness of your harness dieting, you may be able to use the trunk light, rear window defroster, or rear accessory power. My guess is that all those are long gone.

I still like the parking light switch solution. It lets you run the fan for a while to cool the engine down after you shut it off.

RPG

[FFRWRX]

My wire diet was fairly extensive....no extra power leads in the engine area that were not required. But using the parking light switch was very easy.

The key is that I don't mind the rear parking lights coming on when I'm running my cooling fan. So I just tapped into a lead on one of the tail lights (one that has 12V when that column parking light switch is on) and ran that to the fan. When that switch is on, the fan is on (along with the rear lights, but that's fine). If I'm driving at night and turn the lights on, then the fan comes on too. I don't think there would ever be a situation where I would want the rear parking lights on but the fan off, so this works perfect for me. And as you said, when I shut the engine off I can still run the fan for a while to cool things down.

And if I decide I want/need a larger fan with higher current draw, then I'll use that lead from the lights to trigger a relay, and get power from the alternator which is pretty much right there....with a fuse of course.

I'll update my build page so you can see the fan that I'm using.

Rick

[RPGs818SNA]

Sounds like the perfect solution Rick. Your photo looks like an Attwood 1731 which only draws 2.6 amps. The 4 OEM tail and license lamps are 5W each, so they draw less than 2 amps. The wiring also supports a trailer connection which you don't have and is fused at 10 amps. So you should be good even with 2 of those fans. Or a relay as you described for even more power. Well done.

RPG