1988 GT headlight problem - Mustang Evolution

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Old 10-07-2015, 08:35 AM   #1
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1988 GT headlight problem

My headlights have always been a pain. they've started hardly ever working. my amber lights work, and I can run them on bright, but my regular lights aren't working. they worked fine until last week. I checked the fuse box, checked the actual headlight switch, which is brand new. I've checked all my connections. nothing is working. anyone know what is going on?


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Old 10-07-2015, 12:47 PM   #2
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Look at the diagram below and locate the alternator that has a black circle drawn around it.Now look above that circle and you'll see fuse #4.That fuse goes to your exterior lights.You stated you've checked that fuse already & its not blown so the next thing to check is the 16ga fusible link that's shown to the right of fuse #4 in the diagram.This link is inline with the black/orange wire between the firewall & starter relay.The diagram shows its location.If the fuse,switch & bulbs are OK,this fusible link is blown or this wire has an open in it somewhere between the starter relay & fuse #4.If you disconnect the wiring harness from the switch and test the harness with a voltmeter,you should get battery voltage.If not,there is an open between the harness & fuse,an open between the fuse & starter relay or the black/orange wire's fusible link is blown.If voltage is present at the switch harness,the switch is bad or the wiring between there & the lights are bad(highly unlikely though since all of the lights aren't working.The 2nd picture below shows the fusebox and fuse #4's fusible link(16ga)is called the "Lamp Feed".
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Old 10-09-2015, 04:14 PM   #3
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Are all of your exterior lights working (parking, turn signals,brake lights,license plate lamps)except for the headlights??Is the blue high beam indicater on the dash working correctly or does it stay on high beam when you flip the turn signal lever??Check the headlight wiring harness on both sides after viewing the diagram below.The left side of the page has letters A thru E & the top/bottom of the page has numbers 1 thru 3.
Locate sector B2 & E2.The headlight wiring harness grounds are located at those two positions.Once you find sector B2,locate Ground C,that's the ground for the passenger side headlight.Now find sector E2,locate Ground D,that's the ground for the driver side headlight.Make sure both of these wires are tight and corrosion free.The diagram will show that both of these ground wires are mounted on the top front section of the fender apron,close to where the fender meets the radiator support.
If the high beam indicater is always lit,even after you flip the lever to turn the lights on low beam,the multifunction switch (turn signal lever)might be defective (keeping the headlights on high beam constantly) The headlight bulbs could also be blown.They've got dual elements that allow them to operate on both high & low beams.Let's say the high beam element is blown on both bulbs & the multifunction switch is defective.If the switch has the lights on high beam & it won't let you switch them over to low beam,neither headlight will work.Its possible the switch could have shorted out & that could also cause the headlight bulbs to blow.If the switch itself is OK,but the indicater is still stuck in the high beam position, the wire that's part of the indicater lamp could have a short in it somewhere.I do know,if the charge indicater lamp(for the alternator)wire or bulb develops a short or open,it will cause the alternator to malfunction and not charge the battery so the same scenerio might be possible with the high beam indicater too.Since the headlights,parking lights,taillights,side marker lights,instrument cluster & license plate lamps are all on the same circuit (fuse #4 & 16ga lamp feed fusible link)all of these lights would be non functional, if fuse #4 or the 16ga link was blown.So if every light works(except the headlights) use the diagram below & the info above to trace any wiring or component specific to the headlight circuit.
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91 LX 342ci(10.5:1)
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TFS track heat
XE276HR .544" .544"
Flowtech 1 3/4"
Prochamber w FM
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Old 10-10-2015, 11:26 AM   #4
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When I turn my lights on, only the amber lights come on. headlights won't come on. I can turn them on bright and those work perfectly. everything else works fine. it's only my low beams that won't come on at all. it started out they wouldn't come on sometimes, then I'd flick them on bright and back to regular, and they would come on. now they won't come on no matter what I do


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Old 10-10-2015, 02:00 PM   #5
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OK get a test light out so you can check the headlight wiring harness.Flip the multifunction switch(turn signal lever)so the lights are set on low beam,disconnect the harness from the headlight,turn the headlight switch on,clip the test light ground clip to the neg battery terminal & touch the test light probe to each one of the 3 headlight harness terminals, one at a time.Confirm you have one of the terminals with voltage(test light should be lit)with the lights on low-beam.If not, the multifunction switch is bad,one or more of the wires could be loose where it plugs into the multifunction switch or the low beam element in both headlight bulbs is blown.
The multifunction switch has the High/Low beam selector switch built into it.So the headlight switch is routed through the multifunction switch as well... That section of the multifunction switch can fail,while the other sections(turn signals,windshield wipers,etc)of the switch can still operate correctly.So only part of the switch might be defective rather than the entire assembly.
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91 LX 342ci(10.5:1)
TFS 190cc(stage ll)
TFS track heat
XE276HR .544" .544"
Flowtech 1 3/4"
Prochamber w FM
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Old 10-10-2015, 06:27 PM   #6
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Back when these cars were new there was a tsb for this problem.it involved replacing the headlight switch and dimmer switch. There was also a wire,can't remember which one, that ran from headlight switch to dimmer switch that would melt the connector on the switchs. If it was burned you replaced wire and connectors on both ends.. Seems like this was on foglight cars may be your problem

replace dimmer switch, was the connector bad when you replaced headlight switch?
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Old 10-13-2015, 03:33 PM   #7
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connector wasn't bad no


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Old 10-13-2015, 04:50 PM   #8
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Yeah I do remember there being an issue because of the fog lights,now that you mention it.I did find another wiring diagram with a retrofit wiring harness that's supposed to prevent the headlight switch overheating issue.
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91 LX 342ci(10.5:1)
TFS 190cc(stage ll)
TFS track heat
XE276HR .544" .544"
Flowtech 1 3/4"
Prochamber w FM
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