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767Jockey 07-11-2020 05:31 PM
Starting to troubleshoot my inop dash gauge lighting
 
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 07-11-2020 05:43 PM
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 07-11-2020 07:42 PM
Quote:
Originally Posted by twobjshelbys (Post 1479586)
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...:confused:

Gaz64 07-11-2020 08:47 PM
Quote:
Originally Posted by ERA 626 (Post 1479591)
what would that do? you say " ohm meter" are you implying checking for resistance across the 2 wires? Im confused...: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 07-11-2020 09:17 PM
Quote:
Originally Posted by Gaz64 (Post 1479594)
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 07-11-2020 09:51 PM
he should be able to simply bypass the rheostat and touch the 2 wires together and the lights should come on.

Gaz64 07-11-2020 10:21 PM
Quote:
Originally Posted by ERA 626 (Post 1479598)
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 07-11-2020 10:21 PM
Quote:
Originally Posted by ERA 626 (Post 1479598)
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 07-11-2020 10:28 PM
Quote:
Originally Posted by 767Jockey (Post 1479601)
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 07-12-2020 02:42 PM
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


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