Dimming dash lights cause speedometer to fluctuate

[Larry Dedrick]

To all:
     Any idea what causes my speedometer to fluctuate irratically when I adjust the dash lights. This condition is related to the headlight switch when you turn it clockwise or counterclockwise to dim or brighten the dash lights.

       Thanks in advance.        Larry D

[Edward Buker]

As the wiper passes around the resistive coil it probably produces a bit of minor arcing. That will produce some RF interference that may be latching onto the tachometer circuit which most likely uses pulses from the alternator....so extra pulses. It should stop acting erratic when you quit moving the dimmer control.

Later Ed

[Larry Dedrick]

Thanks Ed, I want to figure this out and see if I can cure the problem.

                       Larry D.

[Edward Buker]

Larry,

As long as it happens only when rotating the dim/bright knob it should not pose any problem. It is just an act of nature given RF noise causes this.

Later Ed

[Larry Dedrick]

Thanks again Ed.