I Have Played With This Now For Several Hours. It Is A 79 F-350 Supercab. The Dome Light Does Not Come On When You Open The Door Or Turn The Headlight Switch. If I Turn On The Cargo Lamp Switch It Does Come On. I Have Power To Both The Blk/blu And Grn/yel Wires At The Headlight Switch. Although When I Check The Blk/blu Wire For Power It Turns The Dome Light On. If I Use A Volt Meter On The Blk/blu Wire It Shows 12 Volts But No Light. Now If I Am Not Mistaken The Jamb Switches Ground The Grn/yel And Blk/blu Wires? There Is One Of Each At Both Jamb Switches. This Is Really Driving Me Nuts! I Would Love Some Help!!!!! Thanks In Advance

 

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It sounds like the dome light is geting it's ground through the cargo lite.

Unless I do not understand your post.

Make sure your door switches are grounded to a clean ground. (if they do go to ground in the first place)

 

It looks like the ground in the dome lamp may be corroded.

when you check the Black/Blue wire at the haeadlamp switch with a lighted tester, it lets the dome light come on right? Is that what you are saying? This could be cause when you grounded the wire through the tester it is alowing power to flow. Dome light finding ground through the lighted tester.


Same thing could happen when you turn the cargo light on. Dome light finds ground through the cargo lamp.

Try cleaning the ground connection at the dome lamp housing. Make sure it has a good ground.

Another possibility is this...

The Black/blue wire is corroded and is not making contact at connector #702. This connector is located behind the seat on the driver side. Check this connection as well see if you are getting power here at the connector or if it's corroded. See if you get power at this connector with the headlamp dome lamp switch or either door jamb switches are on. Dont have the cargo lamp on when you test connector #702, as this will energise the black/blue wire from the cargo lamp switch. We want to see if the black/blue wire can get power from the headlamp switch or door switches first if you understand.

If you have power to both sides of connector #702 located behind the seat, move up to the cargo lamp switch connector. Test the black/blue wires at the connector with the cargo lamp off, but the headlamp dome light switch on etc.. should have power to both black/blue wires etc...

Then check the black/blue wire at the dome lamp itself with the headlamp switch on.

This should find either a ground problem or a corosion problem or broken wire in circut #53 the Black/blue wire.

As for what the color wires do.


1: Circut #54 Green/Yellow wire: supplies 12 volt power from the fuse panel F-7 to the following...

A: Lighted Vanity Mirror if equipped
B: both right and left hand door switches
C: glove box switch and light
D: Cigar Lighter
E: Cargo Lamp switch
F: Headlamp switch, for dome light portion of switch
G: AM Digital Radio
E: Engine compartment lamp

2: Circut #53 Black/Blue wire: gets it's 12 volt power supply from Circut #54 Green/yellow wire when the following switches are activated (in the on position).

B: Both Right and left door switches
E: Cargo Lamp Switch
F: Headlamp switch, for dome light portion of switch.

3: The following lights are supplied 12 volt power from Circut #53 Black/Blue wire when the switches from section two are on.

1A: Dome Lamp
2B: Right and left hand underdash courtesy

4: Circut #55 Black/Red wire gets it's power from Circut #54 Green/yellow wire and powers the Cargo Lamp when the cargo lamp switch is active.


hope this helps.

 

That's what I think is happening too.

They don't ground at the door switches. They just transfer 12 volt power from one circut to another.

IE: 12 volt from Green/yellow wire, to Black/Blue wire when switch is on. When the switch is off it interupts power flow between the two wires.

The dome lamp grounds in the dome lamp housing.

 

thank you very much for the info. it has been an extremely long day. i actually fixed it before i got this message. i had to untape the entire harness behind the instument panel to find it. it now has a different system. i installed a fader. so now the door swtiches are grounding switches. now it is all so damn confusing but everything works the way it is supposed to. this truck has been a nightmare. someone else has cut and spliced every single connector on the truck. now most all of the external connectors are gm weatherpack style. once again thanks for the info!

 

Ford has not ever used a positive door switch. The switches are a ground switch. There is no power running through these. The dome lamps have current running to them all the time and the ground is switched. In fact almost all car makes and models work this way. It's safer than causing a vehicle fire when a switch dies or shorts out.

 

Sorry, but you are very wrong, the door switches are not grounded, and are indeed positive switches. They pass 12 volts when on, block 12 volts when off.

And the dome lamp does not have power running to them all the time. They are direct grounded to the cab metal.


It's been this way since at least 1973-1996.

If you do not believe me check the wiring diagrams.


The system is exactly how I explained earlier.