Join Date: Dec 2007
Location: Rio Grande, NJ
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Last night, I noticed the headlights on my 1990 F-250 wouldn't turn on. Everything else in the electrical system is fully functional (Blinkers, Parking Lights, Cabin Light, Gauges, Starter, Etc.) so I reasoned I had merely blown a fuse. Nothing is what it seems though.
I checked the fuse continuity, tested good.
Pulled out fuse #4 to check voltage - read 12 V across terminals
used Haynes manual to determine the TAN/WHT was the headlight output of the fuse box into the MAIN LIGHT SWITCH. The manual says that the RED/YEL wire is the output of the MAIN LIGHT SWITCH into the DIMMER SWITCH. This isn't the case in my truck, and it is my belief that this isn't the case in any 1987-91 ford F-Series or Bronco. The dimmer is probably just a rheostat or pot inside the switch itself. What the manual should show, at least in my case, is the RED/YEL wire going out of the cab via that large, white, circular feed-through plug-thing. After that, I can't really make out what is going on in that rat's nest of a wiring harness without doing some serious dismantling to find out where the RED/BLK and the LT GRN/ BLK wires of the headlamp assemblies actually are connected to the RED/YEL wire from the switch.
Anyways, I ended up checking voltages between the TAN/WHT wire and the RED/YEL wire to check the functionality of the MAIN LAMP SWITCH. These are the results:
OFF position: 12V
PARKING AND INSTRUMENT LIGHTS position: 12V
HEADLAMP position: 0V
This is telling me the switch is working the way it should be. This also tells me that the RED/YEL wire is the switch output to the headlamps
I also checked the filaments on the headlamps, the headlamp ground to neg battery terminal for continuity, and voltage across headlamps with the MAIN LAMP SWITCH in the HEADLAMP position, which read 0v. My reasoning is that a connector must've gotten wet when I was flushing the radiator yesterday, and is shorted out (although I am pretty sure a shorted connection would've blown the fuse). The only other connections I haven't checked (correct me if I am wrong) are where the RED/YEL wire meets with the RED/BLK and LT GRN/BLK wires for the headlamps, and where the two sets of RED/BLK and LT GRN/BLK wires split up to go to their respective headlamps. To check those, I would have to find them first, which would require tearing the wiring harness apart. Talk about a pain in the ***..
I didn't think of fusible links as the culprit, because all the other electrical systems on the truck are fully functional. Wouldn't a blown link take out other systems as well?
P.S. I also did some continuity checks with the headlamp sockets, and the ground had continuity with the hot battery lead. Does this mean it's shorted? (the lamp was pulled out at the time)
Last edited by EarthPig; 12-15-2007 at 01:47 PM.