1971 Trans Am Gauges go Crazy when Headlights are Turned On
 

I just installed my rebuilt 80 AMP alternator and now when the car is running and I turn on the headlight switch or rotate it to turn on the dome lamp:

1. The Oil Pres. gauge goes into the red (to the left).
2. The Water Temp goes most of the way to the right.
3. The Fuel and Voltage gauges both go to the right.
4. The Tach drops to 250rpm.
5. The left Turn Signal light (on the dash) illuminates.
6. The High Beam light illuminates.

(See video in link below)

https://www.dropbox.com/scl/fi/680d7...a78snd6ik&dl=0

I also noted that when I turn the ignition on, the tach jumps to the 250RPM spot before the engine is started.

During the course of about a 15 minute drive, and turning the Headlight Switch on and off many times, there was one time where the gauges operated normally when the lights were on. But when I turned them off and then back on it was back to being crazy.

Any ideas on this?

Thanks!

simple question do you have the ground that connects the gauge cluster to the headlamp switch?

I'm asking this because what you're describing sounds like a ground issue.

The ground strap is a shaped piece that is supposed to be mounted BEHIND the upper left corner of the gauges, and BEHIND the upper right mounting screw of for the headlight switch.

What is the easiest way to verify both?

there is no "easy" way;
You have to remove the dash insert and visually verify if it's there.

I've never removed the insert before. But how does that give me access to behind the upper left corner of the gauges, and behind the upper right mounting screw of for the headlight switch?

Thanks!

the dash insert uses three phillips head screws along the top;
then there are two more screws (they may have been swapped for later hex head screws) that are accessed via holes in the dash structure visable after removing the filler panel under the column;
after all five screws are removed, the dash insert is (from my experience) best removed by pulling he top forward and down.

Once that is out the connector will be right there... or it won't.

https://i.ytimg.com/vi/L1NHfBfNa58/sddefault.jpg
(this image has the headlight switch an wiper switch screws removed.)

Thanks!

Are you referring to the blue wire on the left side of the tach?

No, that “blue wire” camera anomaly from the clear lens and blue internals of the speedo head.

What’s being referred to is that sideways U shaped piece of metal between the upper left speedo head/lens bolt, and the upper right hole in the HL switch bracket.

As a potential FYI…there was some escutcheon on my bird’s dash I needed to unscrew before the dash panel would come out. I’ll be darned if I can recall what it was, either lighter or HL, I think. It was pot metal too, not plastic.