Dome/Map Light Circuit. Can anyone explain how it works? Diagram?

[OneEyed]

I'm trying to understand just what exactly is going on with my 1967 Newport. I am in the process of installing a car alarm. I want to use the dome light to trigger the horns(instead of siren).

I can use a switch to ground or a switch to 12v as the trigger, however the factory dome light circuit seems to have no ground at all. There are two wires leading to the dome light, Pink and Yellow.

I have a constant voltage at the Pink wire and the Yellow is nothing, no ground nor voltage when the door is open and voltage when the door is closed. So, therefore, both wires have voltage to the dome light when the door is closed. Where is the ground? and how can I get the signal I need to trigger the alarm?

I think I can put in a relay, but then it would have a constant voltage on it til the door is open, which then it loses voltage rather than gaining a ground or 12v. Will that relay with a constant voltage act as a load and put a tiny drain on the battery? Have I confused anyone yet?

 

[OneEyed]

I tried to follow along in the FSM but i cannot see where it ultimately gets a ground. Plus it looks very confusing because it works into all four doors plus the map light w/switch.

I really just need a trigger for the alarm I guess and I don't want to install four more separate pin switches in each door. No motion or shock sensors either.

 

[Big_John]

There's 12 volts positive to the dome light all the time. IIRC that is the pink wire. The switch returns ground to the light. That is the yellow wire.

You can do it a couple ways, but I would use a Bosch/Tyco relay and connect power to the relay. Then I would use the wire coming from the jamb switch to provide ground to the relay. Siren or horn on the other side of the relay and you are all set.

 

[Big_John]

BTW, the jamb switch gets ground from the outside of the switch where it screws into the jamb.

 

[F Preston]

There's 12 volts positive to the dome light all the time. IIRC that is the pink wire. The switch returns ground to the light. That is the yellow wire.

You can do it a couple ways, but I would use a Bosch/Tyco relay and connect power to the relay. Then I would use the wire coming from the jamb switch to provide ground to the relay. Siren or horn on the other side of the relay and you are all set.

100% correct.

 

[OneEyed]

I went ahead and hooked my alarm trigger, ground signal, to the yellow wire on the dome and it seems to work, regardless of the fact that I get voltage on both sides of the dome light. Maybe it has a diode built into the circuit? When the door is closed I get voltage across both wires to the dome light. Still doesn't make sense to me, although everything works...?

 

[Big_John]

There's no diode. It's a very simple circuit.

You will get voltage across the light bulb. The filament in the bulb is a conductor. Take ALL the bulbs out of ALL the dome lights and you won't get voltage anywhere except for the +12vdc feed.

If the alarm needs a ground trigger, then you are correct to connect it to the yellow wire.

 

[1960dart]

1968 Newport
I too am very confused by this map light circuit as I can’t turn it off. I have been working to correct many years of bad wiring mods and have now arrived at the point where the map light bulb will not turn off unless I turn on the headlights. I can then dim it and togle the map light on and off. If the headlights are “off” I cannot turn off the map light bulb. Also the map light does not react when I open the doors but the rear pillar lights do turn on and off when the doors are opened.

Pink wire is constant 12v, yellow only has power (12v) when headlights are turned on.

Anybody have any ideas why or how to fix this problem.

 

[MrMoparCHP]

Each door has a switch to ground, all those wires (2 or 4) connect to the dome light switch (only as a splice not a pass through), that switch connects to ground. a wire (yellow) from that splice goes to the dome light.

If your dome light will not shut off then either you have a stuck switch or there is a short in the yellow wires.

It is because of the multiple switches they chose to switch the ground and not the power as typically done.


Alan

 

[mr. fix it]

1968 Newport
I too am very confused by this map light circuit as I can’t turn it off. I have been working to correct many years of bad wiring mods and have now arrived at the point where the map light bulb will not turn off unless I turn on the headlights. I can then dim it and togle the map light on and off. If the headlights are “off” I cannot turn off the map light bulb. Also the map light does not react when I open the doors but the rear pillar lights do turn on and off when the doors are opened.

Pink wire is constant 12v, yellow only has power (12v) when headlights are turned on.

Anybody have any ideas why or how to fix this problem.

By way of the map light comment... where is this map light located? I'm not a Newport owner so please educate us thanks.
The first thing that comes to mind is the 12v wire is pinched somewhere after the bulb causing the light to come on 24/7 regardless of the door being opened or the dimmer switch position.
This may be a tricky one to sort out...

 

[MrMoparCHP]

By way of the map light comment... where is this map light located? I'm not a Newport owner so please educate us thanks.
The first thing that comes to mind is the 12v wire is pinched somewhere after the bulb causing the light to come on 24/7 regardless of the door being opened or the dimmer switch position.
This may be a tricky one to sort out...

Map lights are generally in or under the dash, when I say in I mean near the bottom.


Alan

 

[mr. fix it]

Map lights are generally in or under the dash, when I say in I mean near the bottom.


Alan

I thought so that one should be "quote-unquote" easy to fix! LOL!
If the wiring is butchered up this might explain a lot or it could simply be pinched with the ashtray...
I would test with a jumper wire to the door switch if it is easy enough to get at.

 

[1960dart]

The map light is in the center of the dash connected to a “map light” switch so it can be turned on with out the headlight circuit turned on. It is the only light in the front seat area. The two rear dome lights work as expected. It is a very odd circuit as the yellow wire appears to be the ground sometimes, but not always. If I put a volt meter between the switch and ground I get -12v

 

[mr. fix it]

possible defective headlight switch?

 

[MrMoparCHP]

The map light is in the center of the dash connected to a “map light” switch so it can be turned on with out the headlight circuit turned on. It is the only light in the front seat area. The two rear dome lights work as expected. It is a very odd circuit as the yellow wire appears to be the ground sometimes, but not always. If I put a volt meter between the switch and ground I get -12v

The ground is only connected to ground when the door is open or the dome light switch turned on. To test you would remove the bulb, close all the doors and make sure the switch is off, check continuity from the light socket housing (not the button in the socket) to ground, beep you have a short, otherwise you are good.
Then remove the door switch and unplug the wire, light should go off (replace bulb first), all other doors closed. Check all other door switches (leave previous ones disconnected). If the light stays on, move on to the dome light switch, unplug it, still on then there is a short in the yellow wire. Unpluging the switch only works on cars with a separate light switch from the dome lights.


The only car I have with a map light is my Barracuda and it is tied to the dome light without its own switch. Presumably it could just get switched power and ground to the dash, would need to see a diagram for the C-Body map light.


Alan

 

[1960dart]

The ground is only connected to ground when the door is open or the dome light switch turned on. To test you would remove the bulb, close all the doors and make sure the switch is off, check continuity from the light socket housing (not the button in the socket) to ground, beep you have a short, otherwise you are good.
Then remove the door switch and unplug the wire, light should go off (replace bulb first), all other doors closed. Check all other door switches (leave previous ones disconnected). If the light stays on, move on to the dome light switch, unplug it, still on then there is a short in the yellow wire. Unpluging the switch only works on cars with a separate light switch from the dome lights.


The only car I have with a map light is my Barracuda and it is tied to the dome light without its own switch. Presumably it could just get switched power and ground to the dash, would need to see a diagram for the C-Body map light.


Alan

This circuit does not appear to use a conventional ground. It appears to be a circuit that sometimes grounds and sometimes feeds power to the circuit. It really has me stumped.

Part of my problem debugging it is the fact that I have a generic “1968 Chrysler” wiring diagram. It appears to be the same as my Newport but wire colours are not the same and I don’t know if it is actually the same. I am not able to find the specific wiring diagram so some of this is guess work.

 

[MrMoparCHP]

This circuit does not appear to use a conventional ground. It appears to be a circuit that sometimes grounds and sometimes feeds power to the circuit. It really has me stumped.

Part of my problem debugging it is the fact that I have a generic “1968 Chrysler” wiring diagram. It appears to be the same as my Newport but wire colours are not the same and I don’t know if it is actually the same. I am not able to find the specific wiring diagram so some of this is guess work.

It is very much a conventional ground, I can draw the loop if needed.


Alan

 

[1960dart]

It is very much a conventional ground, I can draw the loop if needed.


Alan

Sorry, but this is not a conventional circuit. Yes I can trace this circuit to ground in many locations but that doesn’t help. The following is how the original poster of this thread stated it.

“I have a constant voltage at the Pink wire and the Yellow is nothing, no ground nor voltage when the door is open and voltage when the door is closed. So, therefore, both wires have voltage to the dome light when the door is closed. Where is the ground?”

 

[1960dart]

Sorry, but this is not a conventional circuit. Yes I can trace this circuit to ground in many locations but that doesn’t help. The following is how the original poster of this thread stated it.

“I have a constant voltage at the Pink wire and the Yellow is nothing, no ground nor voltage when the door is open and voltage when the door is closed. So, therefore, both wires have voltage to the dome light when the door is closed. Where is the ground?”

Here is a link to the wiring diagram I am working from.
http://www.mymopar.com/downloads/1968/68ChryslerA.JPG

 

[MrMoparCHP]

Pure and simple
Pink (+)
Yellow (-)

Each door switch has yellow wires
The dash switch has a yellow wire
Those are all spliced together

The ground is open
The positive is hot (always)

Connect any one switch to ground, you close the circuit.

It is that simple.

If you are not getting 12v across the pink and yellow one of them is open.


Alan