Simple answer to switching ground is:

Run power from the + side of the battery directly to the load. The other end of the load runs to a switch and then to the - side of the battery.

Complex answer is:

Run power from + side of battery through a fuse in a junction box to one side of the load. The other end of the load runs to the switchable part of a relay, and when closed goes to ground. The switch on the dash may have voltage directly to the ECU or use CAN BUS as a controller to the ECU. The EBU connects to the + solenoid in the relay, and then runs the - side to ground.

So, as you push the button on the dash, it sends a signal to the ECU, which connects the solenoid of the relay which switches the load to ground causing the item to come on.

or, at least that's the way I heard it explained.

Jeff

 

I think the general "rule" is to interrupt the positive pin (86) on a common 5-pin relay (or 4-pin too). But just flip it to pin 85, the coil ground. Now you can just jump positive from 30 to 86 resulting in only three wires to the relay versus four. In the case of one relay setup I will be doing, I will switch the ground but to the positive side of the coil I will tap headlight power. So when high beams are on, positive from headlights will energize the coil but with the ground disconnected by the switch, nothing will happen. That way I can ensure my light bar will only operate when the high beams are on. And of course when I turn off high beams the light bar will go out even when the switch is on.

 

Thank you both for the explanations.

 

Here is a link with images to help even further. The set on the left of the page is what one does for a switched ground setup.

https://www.the12volt.com/relays/con...g-polarity.asp

 

Strike some of what I mentioned before, including source for switches. At least for now. My switches came today. Two I will be using for LED bars that have two light modes. So these switches are OFF-ON-ON and look like they will work for ground switching of the relays. Except the upper icon of the switch may not work when needed (middle position has no illumination of upper icon). The lower icon is directly driven by the dash source. But the upper light on the switch seems it may need some positive power when in that position of the switch itself. These two switches are six-pole.

In fact, all the switches I have seem to integrate the lighting and switching. My naivety expected the lighting to have its own pins (maybe a shared ground at best). But the other OFF-ON common switches I have do not appear to work at all without some positive power on the switched side. These are three-pole switches.

I need to wrap my head around things before I continue. I would still prefer ground switching if possible to simplify things. Also if the wire harness from the switches to relays gets damaged some how, it will just be ground to ground most likely. I may just end up buying more switches that have more poles and swap these cover plates over.

 

Don't give up yet.

It may seem wrong, but the wire coming from the relay solenoid to the switch and then to ground IS the 12 volt power source.

My plan would have the wire run from the + terminal of the battery (power), through a fuse (safety), through the relay solenoid (the load), through the switch (switch) and then to ground ( well, you get it).
So the wire would have power when the switch is thrown.

I am still struggling with the whole switching the ground thing, but I am slowing figuring SOME of it out.

But then, I could be wrong.

Jeff

 

I see what you are saying. In DC is all a matter of what side things are on. But the problem with the switches I bought is that being 3 pin they need a constant 12v source to light either top or bottom portion. Neither light can be lit at the same time. So they really would be a no-go for ground switching combined with dash illumination. The 6-prong, two-stage switches I mentioned before I can make work.

What I did now is order 5-prong switches, I can modify them easily enough to make them work. And I will just switch covers over to have the labels I want (they are the laser etched type).The lower text portion will be on dash illumination with the icon "on" portion possibly being on relay power as you mentioned. But I still have to modify them a little.

The reason you can not use switched they way you describe is that you still have the internal switch mechanism. So you have the "input" pin. This would be your negative side of the coil to ground. Or as you say, since its coming from the coil, it can be seen as positive in a DC circuit. But now, when switched on, that pin is closed straight to another pin - which for ground switching means to ground. This thus completes the coil circuit and it energizes . There is no opportunity for the light to get that positive input. Now IF the switch was designed that the circuit when from relay, to that "input" pin, to the positive side of the tiny LED, then out to another pin to ground - sure, it would work. But the switch design, in the case of ground switching, will not let the LED illuminate. Does that make sense?

So my plan is to take the tiny little switch PCB, turn it around (you would have to see one to understand why) and jump a positive wire from the lower light to the upper "on" light. Then on the board, cut the ground trace. Then take another tiny wire and solder it to the un-used pad (#6) - with the board turned around, pad 6 is not in contact with the switched ground. Basically, modify the upper light on the switch to ground switching where as right now the LEDs share a common ground on the PCB and are 12v+ switched.

Now, with all that said until the 5-pin switches arrive, this is all theory. Will work on my 6-pin off-on-on switches and the PCB boards for the 3-pin are identical in every way to the 6-pin switch. So I assume the 5-pin light PCB boards are the same too.

And my head hurts...

edit: I may even setup my entire design on a bench, relays and all, to really make sure this all works as planned.

 

Can you send me a product description and a schematic of the switch? I may not be able to solve your problem, but I will at least have an understanding of it.

I have a feeling I may be planning my project using the same switches and if so I need to change direction now while it is still in an early stage of design.

Thanks

Jeff

 

I am not even going to bother with the 3-pin switch, it flat out will not work they way I wish. Even if used by design, the dash illumination may not have a high enough voltage in a dimmer state to excite the relay coils. So its flat out.

So here is the 5-pin switch setup as designed. The lighting wiring on the PCB criss-crosses. So here pins 7&8 are both grounds, one for each light. And as noted, since they are both ground they can simply be jumped. Pin 6 will be positive input from the dash, as so noted. This means voltage enters pin 6, to the LED, then grounds to 8. For the other light, positive comes in at pin 3 (sorry, I see I painted over 3, but its the lower left pin) and grounds to pin 7. Pin 2 and 3 are the physical switch part of this. None of the other pins have any mechanical contact when the switch is applied. So normally you have positive coming in at pin 2. Pin 3 "outputs" positive from pin 2 when switched ON. When OFF, pin 3 is dead. Pin 3 also contains the pick-up, internally, for the upper light. So if setup as designed, it works. For ground switching, pin 2 to 3 will just be grounds and basically the upper light will just have a ground on each side of the LED.