Gauges quit when headlights turned on

Canadian Poncho · A Poncho Legend!

Gauges quit when headlights turned on

4SPEED427 · A Poncho Legend!

I would hazard a guess that it's going to be a ground.

Do you have a ground wire from the cluster to the instrument panel? If not, just try grounding it with a test lead with clips on each end to see if that helps. It sure sounds like a back feed and that is so typical of a bad ground.

Canadian Poncho · A Poncho Legend!

I figured a bad ground as well. I'll give that a go.
Thanks Carl!

seventy2plus2 · Canadian Poncho Superstar!

good tip Carl. Todd, as you know our 69's & 70's have pretty much an identical dash, most of which is plastic. There 3 pieces of tin which ground the various parts of the instrument cluster, radio, cigar lighter, headlight & wiper switch together, and if I recall one of them ties to the metal part of the dash under the radio (it has a different screw than the rest).

I'm a bit suspicious of the tin piece that joins the headlight & wiper switches to the cluster, as everything is fine until you pull on the headlights.

dualquadpete · Canadian Poncho Superstar!

Todd, try wiggling the main plug into the back of cluster to printed circuit. mat be a bad conection to ground there???

4SPEED427 · A Poncho Legend!

Sometimes I think better after a nap!

I have had situations with a bad bulb or socket. Since it happens when you turn on the lights, I wonder if it's possible that a condition like that could be back feeding as well. If grounding makes no difference, I'd unplug park/tail lamp sockets and headlights to see if that makes a difference.

SD 396 · Poncho Master!

Check the main ground straps from the engine to the body and chassis. There should be one off the battery to the fender and two from the engine to the firewall. If the engine to firewall straps are original you may want to get new ones.
Have a look at the screws that hold the wiper motor on the firewall. One of the bolts though the grommets with the steel washers are where the ground goes to the wiper motor. If they are rusty or corroded the connection will be limited.
Also check the headlight grounds on the rad support. The screws that are used tend to get loose and you will get a bad ground connection. When this is the case the power will continue to look for a ground another way and rob ground from other areas.

Canadian Poncho · A Poncho Legend!

Thanks guys. I'll have a look.

Canadian Poncho · A Poncho Legend!

I decided to spend a couple of hours today finding the problem. Unfortunately the battery was completely dead! It showed .4 volts on my meter! I did a partial charge and put my meter between the negative battery cable and negative terminal of the battery. I had 4 volts. It then dropped to 1 volt. Now I'm not sure if this is related to my gauge issue or if it's something else. Right now the battery is on charge for a few hours. I'll dig into it later..

OURWAGON · Guru

SOME YEARS HAD A GROUND WIRE IN TRUNK HARNESS.
FASTENED IN NEAR THE TAIL LAMP AREA WITH A SCREW.
I HAVE SEEN THAT ONE CAUSE DASH PROBLEMS.

Canadian Poncho · A Poncho Legend!

Some more info. I was wrong about the 4v draw. Now that the battery is charged, it's actually a full 12 volts drawing at all times. I pulled all the fuses, one at a time. The only one that made a difference was a cig lighter-ctsy lamp fuse. The draw drops to 8.6 volts when that fuse is pulled. I disconnected the headlamp switch and for kicks the wiper switch. No difference.. This one has me stumped.

Canadian Poncho · A Poncho Legend!

Do you have a photo of this tin piece? I didn't see anything like this when I removed my headlight switch..

seventy2plus2 wrote:

good tip Carl. Todd, as you know our 69's & 70's have pretty much an identical dash, most of which is plastic. There 3 pieces of tin which ground the various parts of the instrument cluster, radio, cigar lighter, headlight & wiper switch together, and if I recall one of them ties to the metal part of the dash under the radio (it has a different screw than the rest).

I'm a bit suspicious of the tin piece that joins the headlight & wiper switches to the cluster, as everything is fine until you pull on the headlights.