Instrument Lights - blow fuses - Vintage Mustang Forums
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post #1 of 10 (permalink) Old 08-18-2019, 10:54 PM Thread Starter
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Instrument Lights - blow fuses

Well my dash lights aren't working. I proactively replaced the headlight switch assuming that was my problem but when I tested them the fuse blew. I tried another and it blew.

So I guess I have a short to ground after the fuse block somewhere.

Can anyone suggest where to look first?

Last edited by Uncle Buck; 08-18-2019 at 10:59 PM.
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post #2 of 10 (permalink) Old 08-18-2019, 11:05 PM
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Sure. First thing I would check is every last instrument cluster bulb. If the filament broke on one and dropped down to short out the bulb that could do it.... it doesn't take much to blow a 2.5A fuse.

Next on the list would be to find, under the dash to the right of the steering column, the blue & red wire with a "triple tap" female bullet connector.... one or two "taps" may be used if you have a console/clock/other accessory needing instrument lighting.... and using an ohmmeter check the resistance from the fuse block to the end of the wire. You shouldn't have more than .01-.02 ohms resistance. If so, start tracing it back. When you're looking at potential shorts, don't forget the radio, clock, RallyPac, etc..
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post #3 of 10 (permalink) Old 08-19-2019, 07:53 AM
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My bet's on somebody cutting the radio light wire and just leaving it hanging against metal somewhere.

If it's not the radio light wire, and Bart's resistance check doesn't show anything obvious I'd start by pulling all the light bulbs out and see if it still blows. If it doesn't you can add them back on at a time 'till you find the bad one.

Every blue/red wire under the dash is part of that light circuit so take a look at all of them.
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post #4 of 10 (permalink) Old 08-19-2019, 08:13 AM Thread Starter
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Thanks for the suggestions.
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post #5 of 10 (permalink) Old 08-19-2019, 11:30 AM
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You could try turning the rheostat all the way to off on the instrument panel and see if it does or doesn't blow. It could also be any of the hot wires running to the front or rear running lights. I don't know if turning the instrument panel off will help isolate that or not but it might be worth trying.


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post #6 of 10 (permalink) Old 08-19-2019, 02:58 PM
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Uncle Buck, do you have an aftermarket radio in your Mustang?. If you do see how the radio is hooked up. Make sure the power & ground wires are not hooked to the plug from the harness that would have went to the factory radio because one is power for the radio & the other is power for the radio light. I saw a lot of people in the late 70's early 80's when I was working for a Ford new car dealer hook up their aftermarket radio with the radio power wire to the harness radio power wire & the radio ground wire to the harness radio light power wire & as soon as you turn on the lights it would ground out through the ground wire on the aftermarket radio & blow the fuse. I hope that makes sense.

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post #7 of 10 (permalink) Old 08-19-2019, 04:45 PM Thread Starter
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^^ I do. It's one of those vintage looking aftermarket radios. That makes good sense. I'll check that.
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post #8 of 10 (permalink) Old 09-16-2019, 10:14 PM Thread Starter
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Quote:
Originally Posted by MikeG69 View Post
Uncle Buck, do you have an aftermarket radio in your Mustang?. If you do see how the radio is hooked up. Make sure the power & ground wires are not hooked to the plug from the harness that would have went to the factory radio because one is power for the radio & the other is power for the radio light. I saw a lot of people in the late 70's early 80's when I was working for a Ford new car dealer hook up their aftermarket radio with the radio power wire to the harness radio power wire & the radio ground wire to the harness radio light power wire & as soon as you turn on the lights it would ground out through the ground wire on the aftermarket radio & blow the fuse. I hope that makes sense.
Bingo!

Today I decided to have a look at my instrument lights issue. With the wiring diagram in hand I looked up under the dash and saw the #19 circuit accessory triple plug. Normally it would run the lights for a rally pack and the stock radio. Well I don't have a rally pack or stock radio so I expected the accessory plug to be empty but saw a very suspect black wire plugged into it. Once unplugged the dash lights worked fine. Oddly, everything works fine including the radio which I half expected not to work.

I'll need to trace this mystery black wire to figure out where it goes but for now I'm guessing it's a radio ground wire that isn't necessary as the radio is grounding through other means.

And as a bonus feature all my dash light bulbs are good.

Gotta love simple fixes that could have been a pain.
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post #9 of 10 (permalink) Old 09-17-2019, 06:02 PM
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Good, I am glad that was the problem & it was a simple fix.

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post #10 of 10 (permalink) Old 09-17-2019, 06:32 PM
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And a cheap fix too, they are the best. Glad you found the problem.

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