Mopar Muscle Association UK

I have a non-functioning interior dome light on the 63 Dodge.
The console light works as it should when either door is opened. It even has a 3 way switch on the console that works (always on / on when door is open / always off).
But the dome light has never worked since we got the car. Plus it’s hanging down from the headliner so I thought I’d do 2 fixes in one.

The bulb holder is a metal housing that slots into a metal tray that hangs from the headliner. There’s no metal to metal contact between the housing or tray and the car body. So I guess one of the 2 wires is an earth? Bulb has 2 parallel pins, ie not offset.
There was continuity between the 2 contacts in the bulb holder so I figured the bulb was duff. I changed the bulb, but it still didn’t work. I thought it odd that the housing was not earthed so I touched the housing against the inside of the roof and it worked. Great, I thought, I just need to add an earth wire. Which I did.
But now it only works when the door is closed. Bulb goes off when I open the door.

Is it likely bad contacts in the back of the bulb holder?

(sorry meant to attach a photo but left the camera behind this morning)

find which wire has 12 volts

2 ways to wire a lamp

power to it all the time and a switch in the earth wire which completes the circuit to switch it on

or
earthed all the time with a switch in the power feed to switch it on

need to work out which wire is which how it is wired in and what is being switched by the door jam switches.

one of the wires should be earthed and one should have power. but this is complicated by having to open and close the door to check for each wire

both wires should be checked with voltmeter on 20 volts first. you should measure 12 volts or so, ( i say or so because it may be subjected to a dimmer switch which lowers the voltage) between end of wire and a metal bit of car with no paint.

if one of them has 12 volts with the door open and closed your lamp is wired as per the first example above and that wire goes to the contact on the bulb holder that is totally insulated from everything other than the nipple on the bottom of the bulb.

(if it only has 12 volts when the door is open then the other wire should not have 12 volts on it and we'll worry about this later).

carry on assuming 12 volts to light all the time and a switched earth.

the other wire should then be tested with a continuity tester. you should get a beep when the meter is connected to it and a metal bit of car when the door is open but not when closed

this one connects to the other connector on the lamp which should not be earthed to the case this connector should only be connected to the bayonet connector for the bulb i.e the outer part of its stub with pins

there was a piece of fibre board stuck to the roof in my car to stop the lamp housing and any connector in it shorting to the roof. these come off and end up floating about on the headlining and a clumsy sod then presses the lamp into the roof when getting out of the rear seat and melts the wires.

if you have had a short
then one or both of the wires that come up to the roof may have melted and shorted to themselves or the inside of the windscreen pillar
you will have to use the old wires to pull a new set through from the connector down in the sill or at the end of the dash box section.. in a 70s car this wire connects to the headlight switch and will have melted the connector and switch and burnt out the dimmer coil.

i want to say you have the wires backwards........!!! but i'm not saying that because you don't want to be pulling a new set of wires up the screen pillar because i got it wrong....

we also need to establish why you have continuity between both connectors on your lamp? was this with a bulb in, If so the bulb had failed in a stupendous way that you normally only see with halogen bulbs. if it was with the bulb removed then the blub holder may be broken

OR maybe not

if fact you could have a switched 12 volt feed from both doors to the lamp. both connectors in the lamp connect only to the nipple on the bulb
and the earthing is indeed via the case of the lamp connecting in some way to the body

suggest you try both doors open closed and both wires with the continuity tester and set to voltage. do voltage first.


summary

most likely 1 wire is 12 ish volts all the time and the other goes via both door jam switches and connects earth when door is open.
next likley 1 wire is 12 volts only when a door is open and the other goes to a constant earth potentially via the dimmer
least likely but still very possible both wires have 12 volt when door is open and the whole light fitting should be earthed as it uses its case connecting to the body for negative/earth


1960s car could well have overly complex set up just look at a 66 charger dash...


peculiar behaviour would ensue if the wires in the screen pillar have melted but i would expect smoke and fuse issues if your car has enough to bother with one for the interior lights, and the correct fuses.


Dave

Ooh plenty to digest there Dave, thanks. I'll print that off and sit in the back of the car with it.
I didn't think to check if the dimmer is turned down, though I'm sure i have in the past.
But with a new bulb it might just work...

The door switch usually completes the earth circuit hence 2 wires to the bulb holder, there should be a permanent live feed.

and continuity on your bulb means it is a GOOD one

i thought the bulb was blown !! filament supports melted and shorted... reading back i don't know where that came from

yes i agree continuity on the bulb good if its a good bulb

continuity on the bulb holder between both connections maybe not so..... depends