Starting to troubleshoot my inop dash gauge lighting

[767Jockey]

My dash gauges are Smiths, using a rotary rheostat type lighting switch. Given that electrical is definitely my least knowledgeable area, I could use a bit of help troubleshooting this.

On the back of the rotary switch/dimmer that controls the gauge lighting, there are only two wires. Am I correct in thinking that if I jump across these two wires, the lights should come on if the switch is bad, effectively closing the circuit and taking the switch out of the circuit. Then, if the lights don't come on I would have to investigate further as it's likely an issue in the circuitry?

[twobjshelbys]

Or to be safe disconnect the hot lead of the switch (pull the fuse) and use an ohm-meter across the leads of the switch while turning and closing the switch.

[ERA 626]

Originally Posted by twobjshelbys

Or to be safe disconnect the hot lead of the switch (pull the fuse) and use an ohm-meter across the leads of the switch while turning and closing the switch.

what would that do? you say " ohm meter" are you implying checking for resistance across the 2 wires? Im confused...

[Gaz64]

Originally Posted by ERA 626

what would that do? you say " ohm meter" are you implying checking for resistance across the 2 wires? Im confused...

With the hot lead (12v positive) disconnected from the rheostat input, we can then read the rheostat resistance with an ohmmeter while moving the rheostat dial.

I would check for 12v positive at the input lead with a voltmeter, DVM or DMM, digital voltmeter/digital multimeter.

Gary

[twobjshelbys]

Originally Posted by Gaz64

With the hot lead (12v positive) disconnected from the rheostat input, we can then read the rheostat resistance with an ohmmeter while moving the rheostat dial.

I would check for 12v positive at the input lead with a voltmeter, DVM or DMM, digital voltmeter/digital multimeter.

Gary

What he said.

Except I still have my Radio Shack VOM with a dial meter. Still in the original box. It's got to be close to 50 years old now. Works fine, has lasted a long time. I had a Heathkit VTVM but it was too large and clumsy for most things. I used it when building the Heathkit TV but I think I gave it to Goodwill decades ago.

[ERA 626]

he should be able to simply bypass the rheostat and touch the 2 wires together and the lights should come on.

[Gaz64]

Originally Posted by ERA 626

he should be able to simply bypass the rheostat and touch the 2 wires together and the lights should come on.

Yes, that would be correct, if the rheostat is the fault.

[767Jockey]

Originally Posted by ERA 626

he should be able to simply bypass the rheostat and touch the 2 wires together and the lights should come on.

Thanks what I was thinking, quick and easy. I don't even have to remove the rheostat from the dash to do it. In fact I'm thinking I don't even have to remove the wires from the rheostat to do it, just connect a jumper between the two wires on the back, that should bypass the switch and the lights should come on if the circuit is good. No?

[Gaz64]

Originally Posted by 767Jockey

Thanks what I was thinking, quick and easy. I don't even have to remove the rheostat from the dash to do it. In fact I'm thinking I don't even have to remove the wires from the rheostat to do it, just connect a jumper between the two wires on the back, that should bypass the switch and the lights should come on if the circuit is good. No?

Yes, just a jumper in parallel will bypass the rheostat, 0 ohms (jumper) in parallel with example 50 ohms (rheostat) equals 0 ohms. Lights should be full bright if rheostat is faulty.

[Blas]

If your dash lights are dim, I suggest removing the dimmer completely. The dash bulbs never get a full 12v even with the dimmer full on. Feel free to PM me if you have questions.
Blas