Help with cab light and door switch
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Lead Driver
Joined: Mar 2008
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From: Near Los Angeles
Help with cab light and door switch
I have a 75 f250. The cab light above rear window has not worked for many years. I got a new bulb. No help. There is only one wire to the light, and that is grounded. I pulled the switch in door jamb. One wire has power. Nothing changes when switch is depressed. Can someone tell me how to test, what to test, and how to get the light working. Thanks.
Moderator & parts slinger
Joined: Feb 2007
Posts: 50,189
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From: S/W Missouri, Polk county
Straight from the FTE electric legend "fmc400".
For the dome lightwire, hot-at-all-times power comes goes to the headlight switch as a GREEN with YELLOW stripe wire. Dome light power comes back out as a BLACK with BLUE stripe wire. The same color scheme is used at the door jamb switches. BLACK with BLUE stripe runs out to the actual dome light. BLUE with a RED stripe is instrument panel illumination. It comes out of the headlight switch to the fuse box, leaves the fuse box again as the same color and is distributed throughout the dash.
There is no ground connection to the dome lightswitches or the headlight switches. These switches switch power, not ground.
1) Make sure you have 12 volts on both sides of the courtesy lamp fuse. This circuit is hot at all times, so it doesn't make a difference what position the key is in. If you have power on only one side, replace the fuse. If you don't have power on either side, then there is other work to do first.
2) Pull the dome lightswitches out and remove them. Make sure the terminal for the GREEN with YELLOW stripe wire in the connector has power.
3) Check the dome light switches for continuity. With the plunger all the way out (switch CLOSED), the switch should be shorted. With the plunger pushed all the way in (switch OPEN), the switch should be open. If not, replace the switch.
4) With power to the switch proven to be good, and the switches proven to be good, the dome light should lightup. If the dome lightdoes not lightup, remove the cover and check for power at the dome light(not the side that's permanently grounded to sheetmetal) with the door OPEN. If you have power here, yet the bulb does not lightup, replace the bulb.
This will prove out the door-side; if you get to the point that the doors turn the lighton but the headlight switch will not, it is time to replace the headligh switch.
For the dome lightwire, hot-at-all-times power comes goes to the headlight switch as a GREEN with YELLOW stripe wire. Dome light power comes back out as a BLACK with BLUE stripe wire. The same color scheme is used at the door jamb switches. BLACK with BLUE stripe runs out to the actual dome light. BLUE with a RED stripe is instrument panel illumination. It comes out of the headlight switch to the fuse box, leaves the fuse box again as the same color and is distributed throughout the dash.
There is no ground connection to the dome lightswitches or the headlight switches. These switches switch power, not ground.
1) Make sure you have 12 volts on both sides of the courtesy lamp fuse. This circuit is hot at all times, so it doesn't make a difference what position the key is in. If you have power on only one side, replace the fuse. If you don't have power on either side, then there is other work to do first.
2) Pull the dome lightswitches out and remove them. Make sure the terminal for the GREEN with YELLOW stripe wire in the connector has power.
3) Check the dome light switches for continuity. With the plunger all the way out (switch CLOSED), the switch should be shorted. With the plunger pushed all the way in (switch OPEN), the switch should be open. If not, replace the switch.
4) With power to the switch proven to be good, and the switches proven to be good, the dome light should lightup. If the dome lightdoes not lightup, remove the cover and check for power at the dome light(not the side that's permanently grounded to sheetmetal) with the door OPEN. If you have power here, yet the bulb does not lightup, replace the bulb.
This will prove out the door-side; if you get to the point that the doors turn the lighton but the headlight switch will not, it is time to replace the headligh switch.
Fleet Mechanic
Joined: Jul 2017
Posts: 1,448
Likes: 163
From: East Texas
Check and clean the hole where the grounding screw in the housing attaches to the sheet metal.
Do you happen to have an LED bulb?
The prongs on ours kept pulling out and breaking the circuit. We took it apart and very carefully re-soldered them in and added a drop of hot glue to hold the 2 halves together.
Do you happen to have an LED bulb?
The prongs on ours kept pulling out and breaking the circuit. We took it apart and very carefully re-soldered them in and added a drop of hot glue to hold the 2 halves together.
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Lead Driver
Joined: Mar 2008
Posts: 7,182
Likes: 380
From: Near Los Angeles
Thanks, guys. I have been cleaning the grounds. I have only one wire to the lamp, as in illustration #137 above. I pulled the door switch and have constant power to one wire, but the switch must have failed. No matter if switch is pressed in or it is in the 'out' position, power does not pass the switch to the other wire. The metal screw in switch has a 2" plastic tube attached to the back of it that pulls apart in the center when the switch is pressed. So, in this LMC replacement, does #2 insert into the end of #1? https://www.lmctruck.com/catalog/par...y_section/332/ And why are there 3 wires. Mine has 2 wires. I guess the extra wire is for a different model, and unneeded for my application.
Thanks for the diagrams.
Thanks for the diagrams.
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