Radio lights cause #2 CDI to flag

[crash7]

We've had an "intermittent" #2 VOR head for a few months, in our club's 172. Some days I'd fly & the needle would work. Other days, it was centered & flagged.

I finally discovered (sorta) what's causing it. Near the Master switch, there are two knobs for the interior lighting. One for the overhead red light & another to bring up the "back-lighting" in the CDIs & radios. When the radio "back-lighting" knob is brought up at all, the #2 CDI goes dead. Turn the light off, #2 CDI works. I also noticed that the #2 head does NOT light up when the "back-lighting" is turned up.

I haven't had time to crawl up under the panel yet. Is this due to a short somewhere? One of the lighting wires is bare & touching the back of the head? Or just some elec interference?

I'll update as I dig into it. I'm just curious for opinions at this point.

 

[SCCutler]

I'd look first for a grounding issue - something is finding a ground path the wrong way. I'd remove the CDI, clean all connectors and apply contact lube, verify all grounds.

Other possibility, induced current. That's less likely.

 

[weirdjim]

We've had an "intermittent" #2 VOR head for a few months, in our club's 172. Some days I'd fly & the needle would work. Other days, it was centered & flagged.

I finally discovered (sorta) what's causing it. Near the Master switch, there are two knobs for the interior lighting. One for the overhead red light & another to bring up the "back-lighting" in the CDIs & radios. When the radio "back-lighting" knob is brought up at all, the #2 CDI goes dead. Turn the light off, #2 CDI works. I also noticed that the #2 head does NOT light up when the "back-lighting" is turned up.

Can you guaran-damn-tee me that it worked OK once upon a time? The answer will vary tremendously if you don't know for sure that it once worked with the light up or down.



Jim