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Discussion Starter · #1 ·
My Dome Light Issue

1968 Mustang Coupe 289 V8 C4 is bone stock all original, made 8/25/67

The dome light does not work with both doors open and the headlight switch turned all the way CCW (to turn on the dome light). Here is what I have checked.

1. The light bulb is good. 2. The dome light fixture has a good ground from car body to fixture and or the bulb side. 3. After burnishing the center of the socket, there is no 12v to the socket (car battery was connected). 4. Having reviewed the 1968 Mustang shop manual page 19-48 (interior light schematics), it indicates there is a fuse in the electrical circuit (circuit 37A power to circuit 54) Am I correct about this and where is this fuse? 5. Just to be sure, I checked all the fuses in the fuse block which were all good.

Also, it appears there are no lights working in the gauge cluster, but the water temperature and oil pressure gauges work. The fuel and ampere gauges do not work.

So, could I have two door switches and a headlight switch all fail together? As I understand the dome light circuit, the switches are wired in parallel, so one bad switch would still allow the others to feed power to the bulb.

The headlights, turn signal lights, taillights all work when the light switch is set to the appropriate place. The hazard switch works correctly.

These are the possibilities as I see it; a broken wire somewhere in the dome light circuit downstream from the light switch, a bad light switch, a loose power feed to the light switch, a bad fuse or ……. some other gremlin.

Any thoughts would be welcome. Thanks.
 

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In case you have problems interpreting the schematic, above...

1. The Instrument lamps fuse is the small 4 amp one in the fuse block and the BLU/RED wires carry the current to the instrument cluster lamps.

2. The Courtesy lamps fuse is the 14 amp one at the lower right of the fuse block. If the clock and cigar lighter ALSO don't work the fuse could be blown or another issue causing no power in the circuit.

3. The power feed for the courtesy lamps runs from the fuse box to the headlamp switch knob then out to the lamps. The door switches are connected in parallel so that the circuit can be closed by EITHER the headlamp switch OR the door switches.

4. With no Courtesy lamp function by either the headlamp or door switches and using a simple test light and process of elimination you can probe the circuit as follows, looking for the break...
a. The 14A fuse in the fuse block for power on both sides of the fuse.
b. The GRN/YEL wire coming IN to the headlamp switch at the terminal labeled "D1".
c. The GRN/YEL wire going out to the Courtesy Lamps with the knob fully twisted at
the terminal labeled "D2".
d. The GRN/YEL wires coming in to the Door Switches.
e. The BLK/BLU wires going out from the Door Switches to the Courtesy Lamps,
when the switch is not depressed.
f. The BLK/BLU wires and Courtesy Lamp center socket contact in the Courtesy
Lamps.
g. The Courtesy Lamp housing - there should be no voltage here. Voltage at the
housing would indicate an ungrounded socket.

Somewhere in these steps you'll find a break in continuity that explains your lack of Courtesy Lamps.
 

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Discussion Starter · #4 ·
Well, I finally found the problem! There is a two wire connector on the right of and sort of behind the glove box. I have the glove box out for cowl repair and found the two wire connector was UNCONNECTED! I plugged it together and the dome light works and the door switches are good. Some one in the far distant past, not me, unplugged the dome light and did not reconnect.

Now I have to figure out why my fuel gauge does not read out. I installed a new fuel gauge sender with a new fuel tank. I know the fuel gauge itself is good because I have the gauge cluster out for install of a volt meter and I checked all gauges. Hmmm!
 
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