headlight dimmer switch

[jeem]

Have 99 Monaco Diplomat when turning headlights on and adjusting dimmer switch to light up gauges, gauges drop about 20%. By turning dimmer switch off gauges go back to normal, Help.

[Robi.1014]

Rheostat. That's what is being used to dim (or brighten) the dash lights as you turn the headlight knob Clockwise or CCW.

In "old days", the rheostat was a resistance coil on which an electrical contact (a "wiper") would slide. Today's electronics allow this function to be performed by a variable resistor, and the resistor's output controls a chip (a power transistor) to vary the voltage that is available to the dash lights.

In both cases of technology, age and use can cause the wiper to not make good contact, and thus the dash lights act crazy or unpredictably.

Fix? Usually replace the whole headlight switch in total. Or the way I fix it: ignore the problem.

[sc3283]

OP....I do not know if your coach has smart electronics(computer boards that control function of certain items) but if it doe not...it sounds as if you have a bad ground to your gauges. If you have the wiring schematics, take a look and see if the gauges have a common ground. This ground will ground the gauge assembly for it's function and ground the gauge housing for light function. The headlight switch does control the brightness of your gauge lights but will not cause your gauges to change their readings upon turning the headlights on.

A rare but possible problem could be a dash light bulb that is defective and supplying a partial 12v positive supply to the gauge bodies. Yes if this were to occur most likely a fuse would blow....but we have all had bulbs create problems that do not blow fuses.

Do all the gauge lights illuminate? if not...replace that bulb and retry

Bill H
96 Dynasty

[Robi.1014]

Thanks to Bill ... which made me re-read your question. My comments (above) don't apply to your problem.

Now that I'm "focused", (and as Bill says) a poor ground (or poor wiring of the grounding of dash equipment) is a great place to start. If grounding proves not to be the problem, then looking at the "high side" (the 12VDC that feeds the gauges) is the next step.

Basically, you're looking for a change in voltage when you make a change at the headlight switch. A ground should be ground (e.g., you should not see 1/2 a volt between the grounded side of a gauge and a known-good chassis ground). A similar argument applies to the "high side" -- i.e., 12VDC measured on the high-side of a gauge (to ground) should not change as you manipulate the headlight switch. If it does, then a voltage drop is occurring ... and can be corrected when you find where the drop is occurring (usually: an under-sized wire being asked to carry too much current).