73 dash lights

[Gary S.]

Hi to all. I am helping a friend diagnose a situation with regard to his 73 coupe. When he pulls the headlight switch out, he does not get any dash lights. If he fully rotates the switch he gets the courtesy lights. Partial rotation of the switch doesn't do anything for lights. I now that these are two separate functions controlled by the same knob, so we are working under the premise that the headlight switch is bad. I just took it out and there aren't any obvious or visible defects. Is there a common ground for dash lights as well as a separate ground for courtesy lights? The archives seem to be pretty empty on this topic. We have full access to the headlight switch and the plug.

From my 72 wiring diagram I see that the console lamps get power from "20 gray" but where is there ground? The Lectric Limited diagram shows the console lamp symbol as having a little "leg" on it. Is that a ground against the instrument cluster? I know that the courtesy lights get their ground through the bird cage so that doesn't help the instruments lights with grounding.

Any ideas here? Is it possible for the headlight switch rheostat to fail without any visible sign? His switch looks good, the little "heater coil" wire loop is intact, and there aren't any other visible problems. This is such a simple switch that we are wondering if we have a switch problem, a ground problem, or a wiring problem. We have already tested the light bulbs and fuse so they are ruled out.

TIA
Gary

[Joe L.]

Re: 73 dash lights

Gary-----

The dash lights do, indeed, ground through their connection to the instrument panel. These sockets are "single wire" type so they have to ground as-described. There is a more-or-less full round metal ring on the dash light socket that "snaps" into the dash light hole and this is how the grounding is effected.

My strong suspicion is that he needs a new headlight switch. Failure of the dash lights only is exactly what happened to me years ago when I had to replace mine. There was absolutely NO indication from appearances that the switch was bad.

The original switch used for 1973 Corvettes was GM #1995166. This switch was later replaced by GM #1995191. The later switch is functionally improved and more durable than the original. It's configuration and connections are a bit different, but no one will ever see it after installation. Unfortunately, it's GM-discontinued but it is available in reproduction.

[Gary S.]

Re: 73 dash lights

I suspected that others might have had a similar problem. Thanks Joe.

I will pass this along. We have already found the switch in his catalog.
Gary

[Jim T.]

Re: 73 dash lights

Gary did you test the resister wire in the switch by testing it from one end to the other to see if is in fact capable of conducting current? The break in my 70's switch was obvious without testing and the switch in my 68 was inoperative most of the time due to the contact piece not making sufficient contact with the resistor wire to conduct electrical current.

[Gary S.]

Re: 73 dash lights

Jim, that was the one test we did not do. I don't know why but I do know better! Regardless, not only does the switch not dim the lights, the plastic is brittle and is starting to break up. So, he is ordering a switch today. I will probably test the old switch just to see.
Gary

[Dave S.]

Re: 73 dash lights

Gary,

I had a failed rheostat(?)in the headlight switch on my 71. It is a well cared for original car but it still failed. There was no visible signs of failure at all.

[Jack H.]

Re: 73 dash lights

The rheostat windings are circular and, from the front side, you're only seeing half of their surface area. Fatigue fractures or electrical overstress breaks in the winding can occur on the backside of individual winds as well as on the side where the windings abutt the insulating ceramic. So, the end-to-end continuity test IS your best way to confirm operability....