Just joined? Please introduce yourself.
Classic Edge Designs, LLC Prime Custom Cars, LLC MadMooks
Page 1 of 2 12 LastLast
Results 1 to 10 of 14

Thread: Dome lights

  1. #1
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    9,963

    Dome lights

    Does anyone know why auto manufacturers switch ground to the dome lights, rather than switching power? I've been trying to see a benefit of doing that and haven't been able to see one. They did this for decades, and maybe still do, but I don't know why. The reason I ask is I have lights in my doors that take two wires and I really don't want to run power AND ground to the lights, so I'll probably ground one wire inside the door and use a relay to invert the signal to switch +12V to them instead. That's what got me wondering about this.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax

  2. #2
    Registered Member
    Join Date
    Apr 2012

    Member #:571
    Posts
    3,867
    Putting power on one side of the light and switching ground on the other side lets you have multiple switches with less wires, which even a stock trifive has. The door switches only need one wire with the other side grounded to the body. Same with the switch that's part of the headlight switch.

  3. #3
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    9,963
    Not sure I see how it's less wiring. Chevy ran power to the dome lights then each dome light has a ground wire that goes to each door and to the headlight switch. If you applied power to the lights instead, you would just need power to each door switch and headlight switch (power already there) and from there the wire would go to the dome light and the housing could be grounded to the body like all the other lights are. Seems like if you reversed it you might even have less wire since you could power the door switches and headlight switch with shorter lengths of wire instead of two long runs.

    C1 Corvettes used door switches that have two terminals on them so you could use them either way if you wanted to. I'm not sure if they switched power or ground on them. Also, I believe some cars (Mopars??) switched power to the dome lights instead of ground. They mention that in some wiring instructions for dome light controllers and keyless entry/alarm systems.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax

  4. #4
    Registered Member 55 Rescue Dog's Avatar
    Join Date
    Nov 2015

    Member #:2775
    Posts
    929
    Never gave it much thought, but a good question. As this article states and show in the schematic, if you switch power it takes a wire to and from the switch. But switching ground you only need to run one wire to the switch.
    https://mechanics.stackexchange.com/...n-negative-why
    Last edited by 55 Rescue Dog; 01-06-2019 at 03:50 PM.

  5. #5
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    9,963
    Quote Originally Posted by 55 Rescue Dog View Post
    if you switch power it takes a wire to and from the switch. But switching ground you only need to run one wire to the switch.
    Yes, but by switching ground you need to run two wires to the dome light.

    Here's the circuits I sketched out....the top one is the way GM does it. Looks to me like switching power is a lot easier. So I still wonder why they do it this way. I have a hard time believing they did it to make the switches simpler, because the Corvettes used a 2-wire switch so it was already designed and tooled.

    20190106_001.JPG



    I never gave it much thought either until I realized that my Dakota Digital PAC-1300A headlight and dome light controller controls the dome lights with a ground trigger signal from the doors to turn them on by slowly grounding the dome light, and turns them off by un-grounding them. That's all fine, except that requires me to run two wires into the doors to control those lights the same way since I don't have power accessible in the doors. I have a spare wire but don't really want to use it for this. The only solution I can see is to use a relay energized by grounding the relay coil with the door switches, then switching +12V to the door lights. They won't dim like the dome lights will, but that's fine with me. I think DD should make their unit work with either a positive trigger or a negative trigger for more flexibility.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax

  6. #6
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    9,963
    I thought it was amusing how that guy on the website linked by RD purposely drew a more complicated wiring diagram for the positively switched dome light circuit. I can draw a lot simpler one by just relocating the light and not looping power like he did.

    In reality, it's all about proximity of the switches to each other and to the light. Seems like it's fairly easy to get power to switches near the a-pillars and the headlight switch and make one long run to the light on the roof. The light is further from the fuse box than the switches are.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax

  7. #7
    Registered Member 55 Rescue Dog's Avatar
    Join Date
    Nov 2015

    Member #:2775
    Posts
    929
    If you want to run 2 power wires to and from your door switches that will work fine, but this is the way it's typically done with just a ground wire to the switches.
    IMG_4919.JPG
    Last edited by 55 Rescue Dog; 01-07-2019 at 06:18 AM.

  8. #8
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    9,963
    Quote Originally Posted by 55 Rescue Dog View Post
    If you want to run 2 power wires to and from your door switches that will work fine, but this is the way it's typically done with just a ground wire to the switches.
    RD, I know how it's "typically done" but even in your diagram it would be just as simple to put all the switches between the +12V and the lights. I asked WHY they did it that way because I see no benefit. Also, running power from my door switches doesn't solve my problem because my dome light controller is looking for a ground signal. I'll need a relay to invert the signal no matter how I do it. The question isn't necessarily how to solve my dilemma, but why car manufacturers switch ground for only the dome light.

    In modern cars the lighting is typically controlled by a BCM. For most electronic control boxes you see the inputs needing to be grounded instead of supplying a positive signal to them, and outputs often are active when grounded. That's because the electronics typically use "pull-up resistors" on inputs and "open collector" transistor outputs. It's design convention and the best way to design I/O drivers. Some drivers go to relays in the boxes so they can switch either way. For example, all of the input signals for my Eldorado seats require the signal to be grounded to activate that function. The DD 1300A drives the headlight outputs "low" (to ground) to control a relay for the headlights. So when using an IC in a module it makes sense to use a grounding dome light switch in modern applications with a BCM, but not on older cars imo.

    The other problem with switching ground is the entire wire from the +12V source to the dome light itself is powered even with the key off. Most circuits are not powered like that, so the dome light circuit is unique in this way.

    So I still don't see WHY they do this. Even if it's to save a couple feet of wire (which I don't necessarily believe it does in a tri5) it seems strange to go against conventional electrical design. There must be some other reason.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax

  9. #9
    Registered Member
    Join Date
    Apr 2012

    Member #:571
    Posts
    3,867
    You don't save wire length, but if you use single terminal grounding switches, you do save on the number of pieces of wire, as well as the number of connections to make.

  10. #10
    Registered Member chevynut's Avatar
    Join Date
    Nov 2011

    Member #:115
    Location
    Fort Collins, CO
    Posts
    9,963
    Here's where I'm coming from on refuting the "shorter wire" claims specifically for a tri5.

    Let's say the power for the dome light originates at the driver's kick panel. It has to run about 4 feet back to the b-pillar, about 4 feet up the b-pillar, then 3 feet to the center of the car. That's 11 feet. The ground wire on the passenger side has to run from the dome light, right 3 feet to the b-pillar, 4 feet down, then 3 feet forward. That's another 10 feet. Add ten more for the ground to the driver's side using the same routing. For this calculation I'm ignoring the wire to the headlight switch. You have 31 feet of wire.

    Now let's switch power instead. Again assume the fuse panel is at the driver's kick panel. Power has to run about 1 foot to the driver's switch door switch and about 6 feet to the passenger door switch. That's 7 feet. From there power runs the same as ground in the prior example, 10 feet per side. So you end up with 27 feet of wire.

    The switched power design would eliminate 4 feet of wire in this example. I know the lengths aren't exact, but they're consistent for each scenario. The point is that switching ground doesn't save any wire length.

    So why did they do it that way?
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension

    You can see my 56 Nomad build here http://www.picturetrail.com/chevynut

    For affordable C4 Corvette Suspension conversions for your car, visit http://www.classicedgedesigns.com

    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax

Page 1 of 2 12 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •