Odd Electrical Gremlin

[MooseStang]

Lights off, all gauges and turn signal lights normal.
Lights on (even just parking), No speedometer of idiot cluster lights, turn signal indicators have faint glow, and red horsey hi beam indicator glows even though low beams are active.
Switch to hi beams and goes to faint glow.
Also, gas gauge pegs at past full.

Otherwise, turn and emergency flashes operate as expected.

I just replaced the light switch a few months back. Could it have gone bad already?

 

[Steve McMahan]

Just my less than 2 cents, but sounds like loose ground or missing ground

 

[Hemikiller]

Agree with Steve, sounds like a bad ground. Start at the battery negative cable.

 

[mrgmhale]

I am on the road. So at the moment I can’t give you much help. If you do not have a good electrical schematics now is a good time to get one. The Forel Publications 71-72 colorized wiring schematics are good to have. 71 is not as thorough as 72, and 73 is really decent. Even if you have 71 or 72 Mustang, getting the 73 is worth the added money. The pdf File schematics are inexpensive.

https://www.forelpublishing.com/estore2/Scripts/index.html

I also agree that you have a ground circuit problem. The 73 schematic shows where all grounds are located.

 

[lectricegg]

I would also do a very good visual examination of the printed circuit on the back of the instrument cluster. Some of those traces are grounds to the instruments and dash lights.

 

[MooseStang]

So, I tried my old switch (no dash lights) and the gremlins ran away. This leads me to believe that my 6 month old light switch has failed.
I'm now rebuilding my old switch (which I probably should have done in the first place).
Hopefully when I'm done the gremlins will still be gone and I'll also have dash lights.

 

[MooseStang]

Well crap. After cleaning my old switch, it now behaves like the newer.
Pulling the gauge cluster tomorrow.