Wiring in an illuminated switch on the ground side - possible?

McLovin · 12-10-2021, 12:33 PM

I’m trying to wire in an illuminated switch to manually trigger an aux cooling fan (in a car).

For ease, the switch needs to be put in line on the ground side.

I currently have an unlighted switch, that of course is a simple on off switch.

The illuminated switches seem designed to work on the power side, not the ground side, but it seemed to me that would be easy to get around. Not so, it seems!

This discussion seemed promising:

http://67-72chevytrucks.com/vboard/showthread.php?p=7589718

He’s doing (did?) the same thing. It seems like he succeeded. But the last guy who posted casts some doubt. And when I tried to do what the poster did, it didn’t work. The switch turns the fan on and off, but the LED stays lit at all times.

Is it possible, or does that switch only work when used to switch on the positive side?

McLovin · 12-10-2021, 12:34 PM

The switch.

908/930 · 908/930's Garage

I think so, wire a fused 12v to the + terminal and then a jumper from the NO to the - terminal and ground wire to the C, wire from NO to the ground on your fan.

Is there a relay for your fan? If not you should install one and your switch could be wired normally.

McLovin · 12-10-2021, 01:00 PM

I also thought that would work, but when I do it, the light stays on whether the switch is on or off.

McLovin · 12-10-2021, 01:17 PM

There is a relay.
But the way the add on switch wiring is set up now, it piggybacks the coolant temp sensor (which grounds out when triggered).
So I’d have to find the relay and completely redo the wiring.
Which I can do, but I was hoping for an easier way (ie, just replacing the non lighted switch with lighted).
I’m now thinking that’s not possible.

908/930 · 908/930's Garage

Possibly a small relay behind the switch to get the led light out of the circuit.

McLovin · 12-10-2021, 01:24 PM

Also, the expert in the Chevy thread I linked above seems to have a point, he says fan relays are also ground triggered.

His proposed solution is more than I want to do to achieve a light, though.

“ Also you can't just wire into the fan relay(s) for the same reason, they are switched on the ground side as well.
I would say that the easiest way out would be to wire in another relay to switch the fan on on the hot side. Easy to do just constant ground the relay coil at 85 and then switch the 86 side with the LED toggle. You'd have to jumper from the 30 terminal on the fan relay to 30 on the LED relay and jumper from the 87 on the fan relay to 87 on the LED relay. This would jump the fan relay and supply positive current to the fan through the LED relay.”

McLovin · 12-10-2021, 01:25 PM

Quote:

Originally Posted by 908/930

Possibly a small relay behind the switch to get the led light out of the circuit.

But the led light is integrated in the switch. How can a relay get it out of the circuit?

908/930 · 908/930's Garage

Pretty sure the + and - terminals are only for the LED.

McLovin · 12-10-2021, 01:33 PM

Yes, I agree.

What’s puzzling me is the OP in the Chevy thread seemed to make it work, using the same kind of 4 lug switch I have. I think he did what you first suggested (I think, but am not sure, his description isn’t clear to me):

“The newer switches I bought have separate lugs for the LED, so I switch ground, supply the output to the - side of the LED, and power the + side constantly. Works great”

908/930 · 908/930's Garage

It should work, check again, the NC and - go to your fan, C will go to ground. And 12v power to the + side.

McLovin · 12-10-2021, 02:39 PM

NC or NO?

McLovin · 12-10-2021, 03:08 PM

Tried it again, the light stays lit.

908/930 · 908/930's Garage

Hmmm, do you have a voltmeter? check the voltage difference of the wire that goes to your fan to the +12v at your switch. Should be 0.

RANDY P · RANDY P's Garage

I'd put put a bosh 5 prong relay on the output side of that button.

Wire the illuminated button as required but instead of + output from switch to accsy:

On the relay
85- Ground (-) you can loop it together to 87
86- directly to + output on that switch diagram.
87- Directly to good ground.
30-current ground to fan assy.

What will happen: When you flip that switch on, 86 will see (+) fron your new swtich, 85 will already be (-), causing pins 87 and 30 to connect- grounding your fans and turning em on. Just safer that way.

I would do it that way instead of trying to run a lot of current thru a small switch like that. See type B relay on the bottom, that's what I am referencing below

McLovin · 12-11-2021, 08:22 AM

Thanks Randy, that worked.
Used a 4 prong relay I had laying around.

McLovin · 12-11-2021, 08:30 AM

Thanks also for your help 908.
I’d still like to figure out a more elegant approach that doesn’t need the relay and extra wiring, but I think with the switch I have it’s not possible.

908/930 · 908/930's Garage

No prob, really better to have a relay in there anyways. Check how many amps the fan requires and the rating on that relay.

It is strange that the LED was still on, with the switch off there should have been 12v on both sides of the LED and should remain off.