Instrument lights won't dim on '69

[jonstr]

Flight Judging wasn't stressful enough, so I'm trying to get my '69 ready for a Performance Verification. :eek I have a minor electrical problem and am stumped.

When you rotate the headlamp knob, the gauge lights are supposed to dim. They don’t on mine. I assumed this was a pretty easy fix, and I ordered a new headlamp switch. I got an OEM switch, and it hooks up correctly and everything operates correctly EXCEPT that the gauge lights still will not dim. It seems a mighty big coincidence that the new switch I got has exactly the same problem as my old (apparently original, by the look of it) switch.

Can anyone help me or point me in a direction with the following questions?

1) Is there another component that has anything to do with the gauge dimming function?

2) Is there a way to test the rheostat function on the switch itself?

On that last question, I’ve fumbled around with the wiring diagrams, the switch, and an ohm meter. I’ve located the wire (16 gauge dark green) that controls the on/off function of the instrument lights, but I cannot get it to read variable resistance with either the old or new switch. This would seem to point to a defective switch, except that I can’t get this test to work on the light switch from my ’63 either. That switch does dim the lights on my ’63, so I know it works correctly.

Thanks in advance for any suggestions.

 

[Tom Bryant]

jonstr

I moved this thread to the C3 Tech Forum. I think you will have a better chance of getting a reply to your question here.

As far as the dash lights dimming problen goes the reostat is integral with the light switch. It could be that somebody in the past installed a jumper wire to give direct current feed to the dash lights. Maybe in leu of buying a new switch. Have you checked the fuse box and harness for anything that doesn't look like it belongs?

Tom

 

[vigman]

Read this post!

http://www.corvetteactioncenter.com/forums/showthread.php?s=&threadid=7722&highlight=Dash+lights

Vig~

 

[jonstr]

jonstr

I moved this thread to the C# Tech Forum. I think you will have a better chance of getting a reply to your question here.

As far as the dash lights dimming problen goes the reostat is integral with the light switch. It could be that somebody in the past installed a jumper wire to give direct current feed to the dash lights. Maybe in leu of buying a new switch. Have you checked the fuse box and harness for anything that doesn't look like it belongs?

Tom

Thanks Tom. Yes, I've checked for creative wiring, but haven't found any yet. I'm going to follow through the thread that Vigman pointed me to and see if that produces anything.

Jonstr

 

[JohnZ]

Something is amiss, as this is a pretty simple circuit; power comes into the light switch, goes through the rheostat, and comes out in a 16-ga. dark green wire that goes to the fuse block, to the input side of the "inst. lps." fuse. On the output side of the fuse, it changes to a 20-ga. gray wire, which comes back to the instrument panel harness and feeds ALL the illumination lamps. You should be able to verify the rheostat operation with a multimeter and check for varying voltage at the 16-ga. dark green wire exiting the light switch as you turn the knob.

 

[jonstr]

Re: Read this post!

http://www.corvetteactioncenter.com/forums/showthread.php?s=&threadid=7722&highlight=Dash+lights

Vig~

OK Vigman. At the tail end of that post you said "...ready for the next mystery." Well here it is.

I've done most of the stuff in your thread already, but to be sure I just ran through it all again. Here are the results in a nutshell (trying to follow along in the order you lead Bob through them):

Fuse is good, contacts are clean.

Tail lamps and all parking lamps work fine.

The dimmer pot is corroded on my original switch, clean on the new one. Neither gets a flicker out of the dash lights as you work it, they stay on full bright all the time.

Red wire on the h/l switch is hot.

Jumper red to dk green and dash lights come on.

Continuity of dk green wire between h/l switch connector and right side of fuse block is good.

Continuity between left side of fuse block and any of the dash lights is good.

Dimmer shows no variance in resistance when measured between any of the tabs you mention (red, dk. green, n/c) on either switch (old or new). They show either no resistance or infinite resistance - no variance.

Using a test lead (actually I don't have one, so I used my meter and looked for voltage) between ground and the dk green I get 12 volts, and it does not vary as you rotate the knob.

The ground connection wires at the back of the speedo and tach appear to be clean and show a solid continuity to ground, as does the ground connector at the cigarette lighter.

The only things I did NOT do were actually disconnect, clean, and re-assemble all the ground points. Since the ground seems to measure out good, and my problem is not that the lights don't work, that didn't seem to be necessary. If you disagree, let me know.

Everything I can find points to a bad dimmer in the h/l switch, but it seems like a big coincidence that the new switch I got has the same problem as the old one. That said, I've ordered another switch to test. In the meantime, if you have any othe suggestions, I'm all ears.

Thanks,
Jon

 

[jonstr]

Problem solved. Despite the odds against it, the new switch I bought had the same problem as my 30 yr old switch. Another new switch solved the problem.

Jonstr

 

[Tom Bryant]

That's good news. Glad it was an easy fix.

Tom