Dome lights in a crewcab

[68crew]

Currently being stumped by the dome light circuit in my '68 crewcab which has been in the family since new. While in my brother's hands, the dome light stopped working and was never fixed. I did an off the frame resto and it is the last circuit I am struggling with. The front doors have the standard jamb switch(containing a hot wire and the lead to the dome), the rear doors have what looks like is a universal jamb switch. The rear doors were wired in at one time but I have no clue where to wire them to. From what I have been able to test, these rear switches just provide a ground when the doors are open since the two wires appear to be soldered together. I've looked at the schematics and it makes sense with the front two switches but not with a grounding type switch in the rear . I've blown 2 fuses with my attempts to wire in the rear switches. Hope someone can help.

[tbone6]

Is it the same dome lamp for both front and rear doors? Is so, the switches definitely should not go to ground. Did you replace the wire harness during your resto?

[68crew]

There is one dome lamp for the entire cab. I did not replace any of the wiring harness. The two rear switches had the wires cut under the carpet and I was attempting to wire them back in somehow. The rear switches appear to be original to the truck(brother doesn't remember) and there doesn't seem to be enough clearance for the style of switch that is on the front doors.

[tbone6]

By the way, welcome to the site
The switch style doesn't really matter as long as they actuate and function correctly. The switches should have one wire hot at all times (probably green wire with yellow stripe) and the other wire (black w/blue stripe) should connect to the wire that goes to the dome light, not directly to ground. The likely reason you are blowing fuses is that there are 12v going directly to ground through the switches. You should be reading some resistance through the bulb to ground, but not a direct short.

Start by disconnecting your battery and removing the splices you made to connect the switches. Find the 12v hot wires (reconnect battery as necessary, just be sure to disconnect again) and tape them and set aside. Next test the wires you think are going to the dome light for continuity. The easiest way to ensure you have the correct wire going to the dome is to unplug the bulb while reading continuity between the wire and ground. When you pull the bulb you should lose all continuity, plug it back in, continuity returns. The wire from the one side of the switch goes to the dome light. This wire (if original) should be black with blue stripe. Do not get hung up on wire colors at this point, the diagrams for these year trucks are sometimes flaky.

Once you find the correct wire to the dome light, connect one side of the switch to this wire and read continuity through the switch. Actuate the switch while reading continuity to make sure the switches are working correctly. You should read open circuit with the switch in (door closed), continuity with the switch released (door open). If it works the other way the switch is the wrong type. They should be NC (meaning normally closed, in the relaxed state, ie-door open).

Let us know what you find...

*edited for spelling...again

[68crew]

I am pretty sure if I connect just the front switches, the dome will work. I checked all continuity thru these wires, switches, and the dome light and all seems correct. The wires in the rear switches are soldered together and make ground when door is open, ungrounded when door is shut. In order for this type of switch to work wouldn't the lead to the dome need to be hot and then the switch would provide the ground? But that is inconsistent with the front door switches which provide the hot and the dome housing providing the ground. Could it be possible that these switches were added later and never wired in? Does anyone have solid knowledge that the rear doors even operated the dome light? Thanks for the feedback.

[guhfluh]

Those rear switches in your truck are not going to work with the factory dome light circuit without modification. You will either have to change the rear switches out to the same style as the front switches, or you will have to wire in a relay to the rear switches, that powers the dome lamp wire. The rear switches would ground the relay coil, and the relay would tap into the dome lamp power wire. Hard to explain without drawing it though.

My crew didn't come with any switches though, so I am going to have to wire them all in and figure out what fits. Can you post pictures?

[68crew]

I'll try to post some pictures later today. I am leaning toward omitting the rear switches unless I get some convincing evidence that these doors came with switches from the factory. Thanks for the help!

[68crew]

In a moment of clarity, my brother remembered that he attempted to wire in an alarm system when he first got this truck in 1986. He was unsuccessful and never touched them again. I put in a lot of time trying to figure these things out. I guess the crew cabs just came with the front doors operating the dome light. Thanks again for all the help. I will post some pix soon of this F350 crewcab long bed.

[tbone6]

[Calfdemon]

Just saw this post now, but my crew cab only has switches to operate the dome light on the front door. The rear doors do not turn on the light (as it seems you have already figured out).