12 volt to 120 volt ground

[Tinstar]

I've got a 12-volt light above the passenger seat that does not have a good ground. The positive side of the light is getting good voltage and it works when I manually ground it. I removed the light and the negative is no good coming to the fixture. I pulled the bottom out of that cabinet thinking the problem was there and it would be an easy fix. As my luck would have it, the wire disappears into an inaccessible cabinet toward the front AND there is NO other ground in there to tie the fixture. I do have easy access to a 120-volt plug. Is there a problem of supplying the 12 volt light fixture from the ground of the 120-volt plug? I know it will work but would there be any negative effects to permanently wiring it like that. I really don't want to start tearing out cabinets.

[wolfe10]

Were it me, I would just use a magnet to locate the nearest chassis member and run a new ground wire to it. Should be one within a foot or two.

[Sparksvonrou]

I would think that wisdom would suggest never connecting any DC connection to any AC connection ever. Mainly because though it would work, what happens when someone (or in my case me) later forgets what was connected where and thinks it something to do with AC or shore power.

[Hank_MI]

As mentioned previously, multiple times, the 120VAC ground is connected to the chassis. The 12VDC negative(ground), from both the chassis and coach batteries are also connected to the chassis. So obviously both systems share a common ground.

wa8yxm wrote:
...The 120 volt ground often sits at 60 VAC believe it or not (I can explain why but it's not important).

Not important, oh it's definitely important. It's both wrong and hazardous. I wouldn't want to stand on damp ground and touch your chassis, 60VAC will give you a good tingle. Bare feet and could be deadly. If the chassis ground is floating at 60VAC then there is most likely a fault in supply wiring.

I have seen this situation before and got a shock from the screen door. It was due to mis-wired supply.

[midnightsadie]

its ok we all have those senior moments.glad you woke up before you tore things apart.

[wa8yxm]

What the other poster improperly called "The converter panel" is in fact the Fuse box. the panel with the 12 volt fuses on it. there may or may not be a converter there, the proper name is 12 volt distribution panel but we normally just call it the fuse block, panel or box.

I agree with the "I would not tie to the 120 volt ground" you could end up in a very bad way doing that. The 120 volt ground often sits at 60 VAC believe it or not (I can explain why but it's not important).

Better to find a chassis ground spot if you have to.

[Tinstar]

dougrainer wrote:
There are some smart people on these forums. But to think sharing a GROUND between the systems would ever cause a problem is just crazy. Especially a VERY low volt draw from a map light. Doug

As I understand they are tied together anyway. Where seem irrelevant. Thanks for your insight Doug.

[dougrainer]

There are some smart people on these forums. But to think sharing a GROUND between the systems would ever cause a problem is just crazy. Especially a VERY low volt draw from a map light. Doug

[darsben1]

You may have looked dumb but it did not cost you anything.

[time2roll]

You don't need to follow the ground through the cabinet inch by inch. Wire is probably fine. You just need to find the other end and fix the connection.

No, I would not share with the 120v fixture.

[Tinstar]

Well, it's a mute point now. As always dougrainer was right,,, AS USUAL. I checked my dash switches, drivers side panel switches and nothing. I happen to think there are 3 switches beside her arm rest and one is labeled "MAP LIGHT". Surprise, suprise, I flip it and the light comes on. DUH, my DW said the light used to work but I didn't listen or didn't ask how she turns it on. In my defense, that light is the only thing controlled by the switch. My map light works by flipping the switch on the fixture.

That's what I get for thinking; I know what I'm doing. Lesson learned,,, until the next time I do something dumb. It wasn't bad taking the fixture and bottom of that cabinet out then putting it back together but it was a lot more trouble than it should have been.

[DrewE]

Electrically, it should work fine if everything is wired properly. Interconnecting the two systems that way is not the right way to do things, though, and I would not do it or suggest it. If the bond between the motorhome chassis and the 120V ground bus in the distribution panel were faulty, you could conceivably have some nasty voltage potentials present between the two.

I'd concentrate on figuring out where the ground lead from the 12V light leads and verifying that the other end of it is tight. The wire may go through an inaccessible cabinet, but the chances are pretty good that the problem is at one end or the other (or at some intermediate connection that presumably is more accessible).

[ScottG]

Electrically you could connect the 12 light to the 120V ground (not neutral) and it would work but I would never do this. There's always the posibility that high voltage could end up on that wire and the resistance of the wire could be enough that the HV could end up going through the 12V side of the converter.
That would be catastrophic and could even result in fire. At a bare minimum it would fry everything in its path.
So while it would work, I would not do it.

[Mandalay_Parr]

dougrainer wrote:

Mandalay Parr wrote:
I would Not connect those two grounds together. For 12 volt ground I would use the chassis if you can not find the ground wire.

Where do you think the 120 Ground circuit has its final ground? AT the chassis frame. What do you think could possibly be a problem with using the 120 ground for a 12 volt light? Nothing! Doug

I've seen cases on other equipment where someone connected the grounds together and shorted out the rectifier. Not sure if a MH converter would do the same but why take a chance.

Also the ac neutral is not to be bonded to the chassis ground in an rv.

Jerry Parr

[Tinstar]

dougrainer wrote:
Are you POSITIVE that that light is not part of the Dash Map Light Circuit? That the Map light switch or part of the headlight switch does not have to be engaged for the ground circuit to be completed? Doug

I'll go check right now. I do have the same fixture over the driver seat and it works on its own and it is not part of the dash map light circuit.