1940 Dome Light switch wiring

[Bill Bickle 15098]

Questions lead to more questions.  My 40 LaSalle (5267) has the dome light switch on the back passenger side upholstery panel for people in the back seat to use when necessary.  Apparently , by turning the instrument panel switch you can also operate the dome light from there.  Was this possible originally and , if so, how do you wire the vehicle to do both? I am asking this because because of  my limited ability to interpret this operation on the wiring diagram.
As always, any help would be appreciated.

[Brad Ipsen CLC #737]

Don't think that is right.  The switch is hot all of the time (changed in 41 to a grounding switch) so dome light is not tied to instrument panel light switch.  With the convertible this is a wire that you will never see again once you do the top without an extreme amount of redo.  Check many times before the top is done.  Do you have any knowledge that your switch is correct.  You see this style a lot but I believe the correct ones are chrome and a square black/brown sliding switch.  Maybe 20 years ago I saw boxes of the all plastic oval style for sale at Hershey.  The chrome switches are extremely unreliable so there would have been a market for replacements when these cars were young.   

[CraigD]

I also am trying to install the dome light and pillar switch on my 1940 LaSalle 52 Special sedan. This was not installed when the restoration was done. All new wiring was installed during the restoration. At the light, there is a 6V line (always on) and a wire that goes to the pillar switch. The only way I can see that this is supposed to work is that power is always supplied to the light and the pillar switch is used to complete the connection to ground. The problem with this is that the ground of the socket for the bulb (outer ring) is mounted to the frame of the dome light housing, so when you use metal screws to attach the frame to the mounting bracket, it is grounded. I must be missing something, but I don't know what it is.

Craig Duerling
Cambridge, MD

[Steve Passmore]

Its usually a twin contact bulb and the grounding wire comes from the second contact.

[CraigD]

Thanks Steve. That would make a lot more sense, but unfortunately, the bulb and socket that came with the car is a single contact socket with the outer sleeve forming the ground. I guess it's possible that the restorer purchased the wrong fixture, but it fits the mounting bracket .

Craig

[Steve Passmore]

The only way that would work then is for the power to go to the switch then on to the light.

[bcroe]

If the bulb is grounded on one side, the only option is to switch the battery power to it.  If
the car is equipped with grounding door switches, you can work around that by having them
operate a relay, which then supplies power to the bulb(s).  Or use a transistor.  Bruce Roe

[CraigD]

I'm going to try to electrically isolate the fixture from the body by painting it and using grommets. Thanks for your help.

Craig Duerling