I'm new to the site and did some scouring/searching and turned up nothing. I have an '89 F250 and am having some serious trouble with the dome light/headlight circuit. I purchased and installed a new dome light from BroncoGraveyard a few weeks back and everything was working fine (the truck didn't have one in it when I purchased it). A few days later, I lost my dash lights while driving home so I bought a new headlight switch (mine was missing the dimmer coil, completely rusted and just sitting inside the dash). With the new switch installed, I discovered that the dome light was shorting to ground and causing the headlight circuit to trip the resettable fuse. I shielded the hot wire to the dome light and now the strangest thing is happening: the dome light comes on with the headlight switch pulled out and turns OFF when you open the doors. When the headlights are off, the light is operating fine and turning on when the doors open, off when they shut. What am I missing here? Is there a connector somewhere that may have been reversed by a previous owner? The wiring diagrams in the Haynes manual are useless, none of the '89 diagrams show the dome light circuit on them. This would also be a heck of a lot easier if I wasn't colorblind!

-Vin

 

Sounds to me like it's grounding back thru the light switch.

 

i had a weird dome light problem that went on for some time. i finally started snooping around under the drivers door sill and found that i had a bad joint inside a factory shrink-wrap type of connection. might be worth a look there.

 

Thanks guys, I looked into both.

Update: I checked the pin out for the headlight switch and confirmed that all of the leads are in the right place on the connector and none of them are shorting/grounded. I had a load in the bed the other night and discovered that the issue extends to the cargo lamp as well. With the headlights off, both dome and cargo lights work correctly, off with door closed, on with door open. When you pull the headlight switch, it works opposite, lights on with door closed, lights off with door open. If you turn the headlight switch all the way to the left, it turns off the dome light when the door is closed, but the cargo lamp stays lit (again, opposite of how it should work). Does anyone have a wiring diagram for that circuit? The Haynes manual does not show anything for the ‘89s. I’m thinking a wire has been swapped somewhere in the circuit between the switch and the lights themselves.

 

The dome light grounds thru the light housing to the cab. Could you be missing one of the screws inside the housing causing it to find another ground source?

 

Update: So I looked into the cables in the driver's side sill and nothing seemed to be missing/wrong. I ended up pulling the headliner and then the entire lighting harness from the connector in the driver's side b-pillar to both the cargo lamp and the dome light. It turns out there were two major issues here:

1) The truck had an option for map lights, and a previous owner had tied that wire (permanently hot) into the Blk/Bl hot feed to the dome light
2) There is a bridge wire with a diode that connects the two hot wires for the dome light and the cargo light which had gone bad and was allowing power to flow both ways. When you pull the headlight switch, the power is only supposed to flow through the one feed and through the diode to turn on both lights. It was allowing the power current to continually flow, keeping the cargo light on with the headlight switch pulled.

I replaced the diode and separated/terminated the map light wire and we've got fully functional dome and cargo lights!

Thanks for your help!

-Vin