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Dimmer/Rheostat help

  1. Sep 23, 2010 #1
    I'm hoping someone on this forum can help me out as I'm not very good with wiring or hardware components (more of a software kinda guy).

    I'm wiring in some new gauges into my car and need a little help dimming the lights on the gauges. The gauge has 1 wire (which requires a +12v source) to adjust the brightness of the lights. However the car's factory dimmer/rheostat has 3 wires (+12V into the rheostat, ground into the rheostat, and ground out of the rheostat). The rheostat dims the lights by increasing the resistance to current ground coming out of it.

    I'd like to be able to dim the new gauges using the factory rheostat (which also dims other lights in the car). But as you read above, the factory rheostat effects only the ground, not a positive wire that the new gauges use for dimming. So my question is, is there a way with some sort of electrical component, to take the ground that is coming out of the rheostat as an input, an output a + source that is in proportion to the resistance on the ground wire?
  2. jcsd
  3. Sep 24, 2010 #2
    The third wire out of the rheostat does adjust the resistance to ground just as it adjusts the resistance to +12 V. It is unlikely that that wire is used as a ground by gauges. It is much more likely it is used as a variable positive power supply. Have you tried connecting that wire from the rheostat to the wire for the lights in the gauges?
  4. Sep 24, 2010 #3
    The new gauges have (among other wires) a +12v for power, a ground, and another + wire with max input of 12v for the lighting. Attaching the gauge's ground to the rheostat would effect it's ability to preform it's functions because it would cause resistance on the main +12v power wire. I believe internally, the gauges use + wire for powering the gauges core functions, and another + input for lighting but both share a ground. Basically the only way to adjust the light would be to vary the voltage supplied to the + lighting wire. But the car's factory setup only varies resistance to a ground wire.
  5. Sep 24, 2010 #4
    If the rheostat has three wires attached to it +12V, gnd and a third wire, isn't the third wire also varying the resistance to +12V?
  6. Sep 24, 2010 #5


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    Check the rheostat with a multimeter, so you can figure out which lead accepts the incoming 12V and which one you should use as a feed to your light. I suspect the third lead is actually a ground, put there for your convenience. If you mount the rheostat in a hole in a non-conductive panel (plastic...), you won't get a chassis-ground, but you still have a way to ground the rheostat's case via that lead.
  7. Sep 24, 2010 #6
    The rheostat is a factory part on the car (it is used to dim the car's interior lights and gauges). It has 2 inputs (+ and grnd) and 1 output (grnd that has a varying resistance). I'm trying to use it's output to control the aftermarket gauges lights. They use a + wire with a max of 12v for full brightness. I cannot use the aftermarket gauges ground wire as it will effect the gauges core functions.
  8. Sep 24, 2010 #7


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    Can you test the voltage on other gauges/panel lights on the vehicle to see what the voltage is doing when you adjust the dimmer?
  9. Sep 24, 2010 #8


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    Then put the "+12v for power" on the rheostats +12 volt point (or some other 12volt source), the ground on the rheostats ground (or some other ground source), and the "other + wire" on the rheostats wire going to the interior lights.
  10. Sep 30, 2010 #9
    connect the gauge +12 to the output of the rheostat at same point as dash lights are hooked, then the black wire that is the power out would connect with the power out/ground wire of original dash lights, not the rheostat.

    basic circuit. that black wire is not a ground wire until you ground it. until then, it can be used as the +12 (- the resistance of the new lights) lead for another item.
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