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chevynut
01-05-2019, 04:49 PM
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. ;)

Rick_L
01-05-2019, 05:04 PM
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.

chevynut
01-06-2019, 08:25 AM
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.

55 Rescue Dog
01-06-2019, 03:40 PM
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/questions/16565/negative-grounded-vehicles-some-switches-on-positive-some-on-negative-why

chevynut
01-06-2019, 06:52 PM
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.

9470



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.

chevynut
01-06-2019, 09:26 PM
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.

55 Rescue Dog
01-07-2019, 06:14 AM
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.
9471

chevynut
01-07-2019, 07:18 AM
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. ;)

Rick_L
01-07-2019, 09:14 AM
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.

chevynut
01-07-2019, 09:26 AM
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? :?

55 Rescue Dog
01-07-2019, 12:56 PM
Probably because they could just use a simple one wire self-grounding switch, instead of a 2 wire switch like the glass bodied Vette which still needed 2 wires to the light too. Plus unlike newer cars you didn't need a computer just to simply light a bulb. It was too easy the old way, which I miss now. How many relays/switches/electronics/connections/wiring etc. do you want to troubleshoot when something doesn't work, and you have to rip the car apart to fix it? It would be my worst nightmare troubleshooting a newer
Chevy like my 13 Tahoe, or 97 C5 that can have many random issues. The simpler the better IMO, which is the beauty of a stock trifive.

chevynut
01-07-2019, 03:49 PM
Probably because they could just use a simple one wire self-grounding switch, instead of a 2 wire switch like the glass bodied Vette which still needed 2 wires to the light too.

Like a 2 wire switch is that complicated? LOL. They already had them tooled for the Corvettes, and they NEEDED to run two wires to the light on those cars to ground the dome light even if they switched power. They didn't need to on the steel bodied cars and it goes against electrical design conventions for everything else.....where power is switched.


How many relays/switches/electronics/connections/wiring etc. do you want to troubleshoot when something doesn't work, and you have to rip the car apart to fix it?

As many "relays/switches/electronics/connections/wiring etc." as I need to get the functions I want. My Nomad will have automatic on headlights, retained accessory power, and dome light soft dimming because I want those things. New cars have them and I like the features. And it's pretty simple to do with the DD module and the wiring isn't that complex. Plus I'll have a lot more "complicated" electrical things like EFI, keyless entry, fan control, electronic A/C control, power windows, power vent windows, power door locks, electric hood release, digital self-dimming mirror, and more. If you don't want those things stick to the simple stock stuff that you have the ability to wire and troubleshoot.

You keep drifting off-topic, which is WHY GM and others used switched ground to turn on dome lights. On a tri5 you save ONE connection by doing that. On a stock tri5 car you have one connection to each door switch, two connections to the dome light, and one to the headlight switch. That's a total of 5 connections. If you switched power you'd have one connection to the headlight switch ( if the switch was designed that way), two connections to each door, and one connection to the dome light for a total of 6. Getting one less connection while using more wire is hardly justification, imo, for changing the way the circuitry works for this one application. But maybe those push-on connectors were big bucks back then. :p

scorpion1110
01-07-2019, 04:53 PM
Like a 2 wire switch is that complicated? LOL. They already had them tooled for the Corvettes, and they NEEDED to run two wires to the light on those cars to ground the dome light even if they switched power. They didn't need to on the steel bodied cars and it goes against electrical design conventions for everything else.....where power is switched.



As many "relays/switches/electronics/connections/wiring etc." as I need to get the functions I want. My Nomad will have automatic on headlights, retained accessory power, and dome light soft dimming because I want those things. New cars have them and I like the features. And it's pretty simple to do with the DD module and the wiring isn't that complex. Plus I'll have a lot more "complicated" electrical things like EFI, keyless entry, fan control, electronic A/C control, power windows, power vent windows, power door locks, electric hood release, digital self-dimming mirror, and more. If you don't want those things stick to the simple stock stuff that you have the ability to wire and troubleshoot.

You keep drifting off-topic, which is WHY GM and others used switched ground to turn on dome lights. On a tri5 you save ONE connection by doing that. On a stock tri5 car you have one connection to each door switch, two connections to the dome light, and one to the headlight switch. That's a total of 5 connections. If you switched power you'd have one connection to the headlight switch ( if the switch was designed that way), two connections to each door, and one connection to the dome light for a total of 6. Getting one less connection while using more wire is hardly justification, imo, for changing the way the circuitry works for this one application. But maybe those push-on connectors were big bucks back then. :p

Serenity Now. Say it fast several times. Works well for me ;)

WagonCrazy
01-08-2019, 07:39 AM
I did 15 minutes of google searching "why did General motors use switched ground to turn on dome lights" without anything conclusive coming up.

I suspect its because they had a supplier (partner company owned by the GM parent) that made these 1 pole door switches from way back, and with the tooling cost...just consistently designed all their cars with this switched ground wiring plan. It was probably low hanging fruit to try and redesign it for a dual pole switch.

Just wire it the way you want to (with switched 12v power) and move on to funner things Laszlo.:)