Headlight switch backlight dimmer blows horn fuse...???

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Caco2120

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So here is a new one. I can only assume the 2 are related as i didnt notice it before.

Drove my car for the first time in a while at night and noticed no dash backlights. I turned the knob on the headlight switch to turn them on, and low abd behold, it blows my horn fuse. (#8)

Only thing i have done recently is convert to a 1 wire alternator.

Independent wire to battery. Unplugged old voltage reg wires at firewall from the factory plug.

Horn works, and fuse wont blow until you mess with the dimmer knob.

Headlights, turn signals, indicators, all work.

Lowered column and pulled instrument cluster looking for pinched or fried wires....notta.
 
My guess is it's time to disconnect the battery, drop the fusebox and get into the firewall plug and start tracing things out with an ohmmeter, there's probably a melted mess somewhere inside of the harness in the under dash wiring. IIRC, the dome light operates off of a switched ground with constant 12 volt positive to the light.
 
Lord I get tired of asking this. Year make and model. And yes, it is important. And no I cannot guess from your avatar. Dash lights get their power from the tail /park circuit, which feeds the dimmer control, and the dimmer control feeds the inst fuse in the panel, and from there onto the dash lamps

"Twist" of the light switch also grounds the low end of the courtesy lamps
 
Lord I get tired of asking this. Year make and model. And yes, it is important. And no I cannot guess from your avatar. Dash lights get their power from the tail /park circuit, which feeds the dimmer control, and the dimmer control feeds the inst fuse in the panel, and from there onto the dash lamps

"Twist" of the light switch also grounds the low end of the courtesy lamps

1974 scamp.
High and low beams work.
Blue indicator light works.
Horn works.
Turn signals and indicators work
No backlights at any time (they are newer bulbs)


Turn the dimmer knob
Horn fuse blows
 
1974 scamp.
High and low beams work.
Blue indicator light works.
Horn works.
Turn signals and indicators work
No backlights at any time (they are newer bulbs)


Turn the dimmer knob
Horn fuse blows
Dome light works also on the door open switches. Does nothing with the knob twist .
 
Lord I get tired of asking this. Year make and model. And yes, it is important. And no I cannot guess from your avatar. Dash lights get their power from the tail /park circuit, which feeds the dimmer control, and the dimmer control feeds the inst fuse in the panel, and from there onto the dash lamps

"Twist" of the light switch also grounds the low end of the courtesy lamps

Thanks for the help btw. What are courtesy lamps? The underdash lights? Like the map light?

So part of the switch puts out power for the back lights...but it also grounds a circuit?
 
Every time there is a weird electrical problem it is always a 74-76. I pulled my hair out on a couple of these cars. Even installed a used harness from another car . Then the problem switched to something else.

These cars have bulk head and harness plug problems. They must have went to a different vender for the ends. Once you have an issue with one thing . You will be chasing open circuits . These cars have a corrosion problem on the wire ends. Fix one and you cause another by wiggling something you shouldn't have touched. Good luck it could be something simple like slamming the hood real hard.

The only thing that turns me off on a 74-76 A-bodies is the park bench bumpers and the "electrical problems".

Find a complete 73 harness and components front to back. You will never regret it.
 
'75 and especially '76 are pretty radically different.
 
Illumination is the orange wires. There are more than the inst' panel illumination. If you have column shift you have orange wire to a lamp at PRNDL. There's a horn wire in the steering column also. I don't know the problem is in the column, just thinking where the 2 circuits get close together. Good hunting
 
Every time there is a weird electrical problem it is always a 74-76. I pulled my hair out on a couple of these cars. Even installed a used harness from another car . Then the problem switched to something else.

These cars have bulk head and harness plug problems. They must have went to a different vender for the ends. Once you have an issue with one thing . You will be chasing open circuits . These cars have a corrosion problem on the wire ends. Fix one and you cause another by wiggling something you shouldn't have touched. Good luck it could be something simple like slamming the hood real hard.

The only thing that turns me off on a 74-76 A-bodies is the park bench bumpers and the "electrical problems".

Find a complete 73 harness and components front to back. You will never regret it.

Yeah its been a struggle.

Bench tested the cluster. All good there.

In a weird development i noticed a little indicator light that i wired in for a line lock indicator will illuminate with the ammeter unplu
Illumination is the orange wires. There are more than the inst' panel illumination. If you have column shift you have orange wire to a lamp at PRNDL. There's a horn wire in the steering column also. I don't know the problem is in the column, just thinking where the 2 circuits get close together. Good hunting
My column light is no longer in use, but you are correct. That fuse blows parking, tail lights and horn.

Orange wire into the round gauge cluster socket also appears to be the path to the floor lamp under the dash. Im contemplating snipping the orange wire. And putting both ends of the pigtail on different sources. The gauge backlight on the parking lamps, and the floor lighting on my (already toggle switched) map light

Hate to fix it with more redneck splicing, but i think a clean job of the above is bettet than tracing whatever grounding issue this actually is.

Unless someone has a better idea
 
So...i think i got it fixed the "right" way. The orange column wire i think was my culprit. I had messed with my column, and i think it was pinched. So it was grounding out on the column. It was inop. And painted over. So i just snipped it at the plug. Whalla...fixed...backlights back. Dimmers work. No blown fuses.....

Fixed until i look at it...breath in its vacinity...or slam the trunk
 
OK all the oranges are "dimmer controlled" feeds to the dash lamps. The functional path is tail fuse.........headlight switch.........through the switch park/ tail section...........to the dash dimmer............out on a tan wire........to the fuse box........through the inst fuse......out that fuse on all the orange wiring

Courtesy / dome, same thing pretty much. Some cars have lamps in the console to light the floor, some cars have a lamp up under the dash to light the floor
 
Best way to "shoot" this stuff usually is take one problem at a time and try to find it. Sometimes, you end up finding "the other." Once that if fixed move on.

You have a factory diagram? I have a scan you can download "I think." It is low quality but readable. The 73 one is very close over at MyMopar
 
Best way to "shoot" this stuff usually is take one problem at a time and try to find it. Sometimes, you end up finding "the other." Once that if fixed move on.

You have a factory diagram? I have a scan you can download "I think." It is low quality but readable. The 73 one is very close over at MyMopar
Thanks all. I do have the mymopar diagram....its close. Lol

Just got lucky this time. I remembered i snugged the column up and grabbed my non-used old gear selector shaft when i did it. Musta pinched that light wire....hoping anyways. Still working today
 
The MyMopar aftermarket diagrams are somewhat easier to follow, BUT LEAVE THINGS OUT. Park that into the back of your mind. There are times a factory diagram is what you need.
 
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