Electrical Interior lighting issue

Jack1424

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Sep 15, 2020
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I tried doing a search but could not pinpoint what I’ve got going on specifically. I’ve got a 1990 GT, all original, and recently when I attempt to turn my lights on, nothing inside is illuminating. Exterior is illuminating just fine with zero issues. Dome light works as it should. But I’m getting no instrument cluster lights, radio lights (factory radio that still functions as it should) and lighting for environmental controls. Any help would be greatly appreciated.
 
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Jack1424

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Sep 15, 2020
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I went ahead and tried rolling the dimmer switch and no flicker. However the dome light will still turn on via the dimmer switch. Could it be a bad fuse? Or should I just try swapping out the dimmer switch and/or the actual headlight switch as well?
 

7991LXnSHO

wanna catch the space herp
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Check to see if there’s power to the rheostat at the right time for your dash lights to be on. If there is, I’d replace that.
 

Mustang5L5

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I believe there is a 5 amp fuse. I am poolside at the moment so can’t grab my wiring diagram to pull up the page.
 

KRUISR

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The 5 amp fuse circled is between the rheostat and the rest of the interior/gauge light circuit (instrument cluster, radio, etc.).
 

Jack1424

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Just to update on this thread, I pulled the dimmer switch out and upon inspection, noticed that some of those finely coiled springs on the wheel had broken, thus leaving me to believe that was my problem. I found an OEM replacement dimmer switch on eBay that was in good condition so I bought it for 30$. I just got it today and tested the new switch out, and now all my interior lights are coming on but only when the dimmer switch is selected to the dome light. Do you guys think it’s a bad switch or a wiring issue with the car? At least I’m making some progress…
 

Mustang5L5

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Can you measure the output voltage on the blue/red stripe wire coming out of the dimmer? Should be progressive up to 12v as you spin the dimmer wheel
 
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7991LXnSHO

wanna catch the space herp
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First, did the summer fix your dash lights? OEM used is not the same as NOS. The rheostats are a common failure point, and have been forever.
Second, The interior lights should come on in the on position And when door is opened. If your. dash. Lights work now, and TBE dine light comes on when switched, I think you can move on from the dimmer dash switch.
 

Jack1424

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Mustang5L5, I did what you said and I think you may have helped me pin point it. Upon looking at the blue/red wire connection up on the light switch itself (not the dimmer switch side) it will not stay secured into connector for the light switch. So I’m thinking it’s the blue/red wire. Would you all agree? And if so, is it an easy job to swap that wire out? Thanks guys
 

jrichker

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The plastic shell often fails to hold the connector pins in place. If the connector shell isn't melted, a dab of high temp hot glue on the wire side of the plastic shell of the connector will hold the wire in place.
The small hot glue guns and high temp hot glue are available at Walmart or hobby/craft stores. for $10-$12
 
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