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1984 Mustang lighting issues

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Old 06-10-2011, 05:00 PM   #1
Join Date: Oct 2010
Vehicle: 1969, Ford, Mustang
Location: South Dakota
Posts: 7
Default 1984 Mustang lighting issues

Hi Everyone,
I have a 1984 20th Anny Mustang GT. My fog lights have not worked for some time. Also when driving my headlamps will shut off completely. I have to pull back on the high beam lever and hold it to get any light at all. The car is not junk, I am the second owner and it only has 42000 miles on it. I have always stored it inside and it has no rust or wear. I cannot believe I am fighting this problem on this car. I put a test light on the switch, selinoid (under dash) and those are functioning properly. I get no voltage at the pigtail plug in the front nose. I put a tone on the wire and get the ring all over the body on ground points. Any suggestions on what to look for??? Thanks,
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Old 06-10-2011, 08:12 PM   #2
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Vehicle: 1986 LX, 87 GT Hatch, 89 GT Vert
Location: Spfld, MA
Posts: 9,240
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You could have an internal short in your instrument cluster that is capable of backfeeding a ground through your headlight switch. If you want that diagram, let me know.

Check your cigarette lighter socket for pennies. The illumination circuit ties into the headlight switch as well, which a penny inside it will create a backfeeding ground.

If you have the Light-Out Warning indicator on your dash, check the Resistance Wire shown in the diagram.

I wouldn't do much driving until you get to the bottom of this. Shorted headlight circuits can cause fires in any number of places throughout the car. An 84 stang with 42k on the clock has almost surely been a victim of critter nests, and critter teeth.

Click the image to open in full size.

Last edited by mattdel; 06-10-2011 at 08:31 PM.
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Old 06-11-2011, 11:21 AM   #3
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Vehicle: 1984 1/2, Ford Mustang, GT350 5.0
Location: NY
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I also had the same problem with my 20th Anni, replaced the headlight switch... nothing... but a possibility.
Took me a few months to find the problem, after the aftermarket radio the previous owner put it started to turn off and on several times.
Investigated and found out one of the wires on the radio for positive was hooked to where the ground should have been cause both of them to ground out.
1984 1/2 Ford GT350 DD in OR - (5.0, AOD, Carb, Flowmaster Exhaust)

1983 GT Convertible (Current project)
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1984, 84, 84 mustang fog light, circuit, dash, gt350, headlight, light, lighting, lights, mustang, problems, schematic, switch, working

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