Fun with Lights

Hi Everyone,
I have a 1972 ford that has been acting a bit weird since I pulled it out of storage at my grandfathers. I’ll be going over a lot of the wiring shortly, but before I do I wanted to know if anyone had specific suggestions for areas I should check. Here’s what it’s been doing: With the lights on, the horn doesn’t work, and the taillights/dash lights are super dim. The headlights are ok, and the brake lights work as they should, it’s just the taillights and the dash lights. With the lights off, the horn works as it should overall. Does anyone have any ideas on where I should start?

First thing to check is the battery

So I actually just put a new battery in in November, she hasn’t had any issues with starting or anything, just the lights. Is there something specific with the battery I should check?

Have you charged it up?

I did, right after I installed it and again in the spring.

It’s an almost 50 year old car. Try changing the bulbs. Do one first and see if it makes a difference.

The dash lights could be the same or perhaps an outgoing potentiometer. That thingy knob you use to adjust the brightness of the dash lights.

Find and remove the corrosion from every ground connection you can find. Search hard! A car that old will have corroded grounds all over it.


What @Mustangman said, starting with the battery connections, and make sure the battery cables are in good shape. Check all fuses and the fuse box for corrosion, too. Do you have a VOM to check for voltage drops and resistances?

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I also concur what @Mustangman said about the ground connections. When there is a faulty battery ground to chassis connection there is then an added resistance in series with the ground connection that shouldn’t be there. The series resistance causes a voltage drop to occur across it when current flows through that resistance. So the load you are trying to apply power to doesn’t get all the power it needs to work. The headlights draw a significant amount of current draw so more voltage will drop across the faulty ground connection. Things like parking lights don’t need much current to operate, but if the headlights are on, the high voltage drop that is caused by the headlights will make the parking lights appear real dim because they can’t get the current they need due to the high voltage drop across the resistance. The parking lights may only have about 4 volts across them instead of 12 volts. That would mean the extra resistance would have 8 volts across it, to have a total of 12 volts across the total circuit.

This concludes our lecture on electrical theory.

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Yeah I hate electrical issues but when there are weird issues, checking/adding grounds would be the first course of action.