Powering Marker Lights with an Isolated Ground System

[Jon R]

Hi All,

One of the last electrical items I have to figure out on my new Grandby is getting the marker lights to work when the truck marker lights are on. I found the truck marker light fuse and have connected the camper lights positive wire to that circuit. No lights. The lights work if i connect the wire in the camper to the camper battery. I tested the wire to the truck for continuity and am good there. After thinking about what could be wrong with the ground side for a few minutes, and mentally tracing the ground path, the light bulb came on, so to speak. I remembered that I decided to isolate the truck and camper grounds, and used the isolated version of the Victron Orion. The only connection to the truck ground is the 6 gage wire running from the truck battery negative to the negative input side of the isolated dc to dc charger.

So now I need to choose one of a few ways to tie the marker light ground to the truck ground. One way is to give up the isolation altogether. I’d prefer not to do that. Another would be to take apart the camper ground bus bar and test every wire to see if it’s the ground wire for the marker light. However, I’m not sure they didn’t just use the chassis as the current return for small circuits like those lights. If I could find that ground wire I could run wire to the truck ground and connect it to that. A third way, which would solve the problem if the camper chassis is the return, would be to run a small truck ground wire to the camper and use the two wires from the truck to operate a relay that connects the marker lights to camper power.

Am I failing to think of a better solution to this? I would really prefer to maintain isolation of the camper and truck grounds.

Thanks

 

[2Z Bundok]

"I would really prefer to maintain isolation of the camper and truck grounds."

I don't understand this? Is this common practice? Doesn't the 6ga battery cable connect them?

As I try to make everything work on older equipment I have always been guided to verify/repair grounds first and figured more clean grounds is better!

On our 97 pop up I replace all marker bulbs with LED...None worked! Turns out the original fixture was pop-rivited as positive ground. The aluminum rail was the ground. Replaced all of them with sealed LED fixtures and added ground jumper wire at the (replacement?) plastic corner pieces.

 

[Jon R]

When I was deciding about how to set up the ground system for the camper, I did not have it yet. I asked on this forum about whether the turnbuckles electrically connected the truck and camper bodies. The responses indicated they did. Based on that, I chose to use an isolated dc to dc converter so that, during charging from the truck, I wasn’t driving any current through the structure, and was only driving it through the cables (“avoiding ground loops”). There are good reasons to avoid driving significant current through the structure for long periods.

Now that I have the camper, I have found that the turnbuckles, at least currently, do not electrically connect the truck and camper. Assuming that remains the case (and it should because the anchor points in the camper appear to be attached to plywood structure where there is no aluminum frame), I could just tie the camper ground system to the big negative wire coming from the truck, but I'd prefer to keep it isolated.

 

[rando]

I am curious what your motivation is for an isolated ground is?

There is a potential to run into problems with this configuration, even without considering the running lights. There is pretty good chance you will have an 'incidental' ground path between the camper and truck, which is generally not a good idea. If you do end up with current flowing on this path, you don't know how or where it is flowing, and what the resistance is, which can lead to hotspots and flakey operation and you won't be isolated anyway.

In my FWC the metal frame is tied to the battery negative bus and is thus 'ground'. I suspect even if this were not explicitly so, it would end up that way due to 12V appliances (heater, water heater, lights) which have their metal cases tied to ground, so it is better to explicitly tie the frame to ground so you know it is well bonded. Same thing with the camper to truck connection - without an explicit ground path, there is a good chance you will find an alternative path - through the turnbuckles, or through the siding where it has rubbed a bare spot etc.

If everything is working right then the extra ground path probably doesn't matter, but if either ground wire to the DC-DC converter comes loose, then 30A will start flowing through some other ground. Also depending on the resistance of the DC-DC converter some of the current may flow through the frame even under normal operation.

Sorry for the long winded reply - the short answer is that unless there is an important reason to isolate the grounds, your are likely better not to so you know what is going on.

 

[rando]

When I was deciding about how to set up the ground system for the camper, I did not have it yet. I asked on this forum about whether the turnbuckles electrically connected the truck and camper bodies. The responses indicated they did. Based on that, I chose to use an isolated dc to dc converter so that, during charging from the truck, I wasn’t driving any current through the structure, and was only driving it through the cables (“avoiding ground loops”). There are good reasons to avoid driving significant current through the structure for long periods.

Now that I have the camper, I have found that the turnbuckles, at least currently, do not electrically connect the truck and camper. Assuming that remains the case (and it should because the anchor points in the camper appear to be attached to plywood structure where there is no aluminum frame), I could just tie the camper ground system to the big negative wire coming from the truck, but I'd prefer to keep it isolated.

I think you hit the nail on the head with the last comment - the best way to avoid this is to provide a solid low resistance path to ground through the 6AWG cable. Trying to electrically isolate two mostly metal objects that are physically in contact is going to be tough.

 

[fuzzymarindave]

Today I was planning to replace the factory negative battery bus (distribution block) with a larger unit.
https://www.amazon.com/gp/product/B08RRYD6J3/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
I took a quick look last night and the factory negative bus appears to be electrically connected to the metal frame as it is attached to the front camper wall by metal screws. I am installing a battery monitor and need to get the negative connections off the battery an onto the new bus bar with a shunt in between.

 

[Jon R]

Hi Rando,

One of the advantages of remaining isolated is the result if the failure you identified, but we are coming to different conclusions about what would be best because of different assumptions about incidental electrical connections between the truck and camper.

First, on my camper the negative bus bar is well connected to the frame with at least one dedicated bond wire, so that is the same as you described for your camper. However, I believe my camper is quite well protected against incidental bonds to the truck. The bumpers are rubber and contact bed liner material. I have three inches or more clearance to the lined bed and rails all around except at the tailgate opening, where I have an inch clearance to thick sprayed on bed liner. The anchors are attached to the wood structure of the tub only. So the only place the truck ground gets near the camper is the 6 gage wire I installed to the source side of the dc to dc converter.

With the camper isolated, if that wire were to fail, I’d pick up on it pretty quickly due to lack of charging from the truck. With the camper not isolated, I better have my other ground connection to the truck designed for long term 30 amp operation because I may never get a clue the wire has failed.

If you assume a high likelihood of incidental bonding, then I agree you should just put in a real, adequately sized bond, but you will drive some charging current through structure whenever the truck is charging the camper. I’m not seeing incidental bonding as likely for my installation, so I’d prefer to avoid creating that structural current path.

I agree this really isn’t a critical decision and either approach has merits. I just want to stick to my original approach but solve my running light problem separately. I’m going to call FWC and ask if they used structure as the current return for the marker lights. If so, that will limit my choices and I may decide to give up on isolation.

Thanks for your well-considered responses.

 

[Jon R]

FWC just told me they run a dedicated ground wire for the marker lights back to the ground bus bar in the cabinet, so I think I will identify that wire, disconnect it from the bus bar, make sure it is isolated from structure with an ohm meter, and connect it to the truck ground.

 

[Jon R]

Well it turns out that, contrary to what FWC told me, my new Grandby does not have fully separate grounds for the marker lights. I bought the rear flood lights option, and for some reason the right flood light has its own dedicated ground wire to the bus bar, but the left light has it’s ground tied to the ground for the marker lights somewhere in the rear wall. I don’t think it was supposed to be wired that way according to FWC tech support, but I don’t want the camper taken apart to correct it.

So I’m ending up tying the truck and camper ground together via the 16 awg ground wire that was intended to be the marker light ground from the truck. I’ll put an in line fuse holder in that wire where it connects to the bus bar in case some sort of fault causes high current in the wire, in which case it will go back to being an isolated system and the markers lights being out will tell me something abnormal happened.

 

[craig333]

If they are running a dedicated ground for the lights thats new. Mine is just grounded to the camper.

 

[Jon R]

I’ve pulled the wires off the ground bus one at a time to identify which wire does what. It definitely has dedicated ground wires for the various external lights, but the person who wired it tied the ground for one of the rear floods to the ground for the marker lights, so there’s no way to run the marker lights with their own isolated ground. I gave up and tied the truck and camper grounds. All the lights now work and the markers are switched on with the truck markers via the trailer wiring system.

What I just discovered is that, when a “trailer” u is sensed by connecting to the trailer light circuit, my new truck disables the parking assist rear proximity warning and the rear cross traffic alert warning. It has a trailer menu selection for an accessory powered from the trailer circuit rather than a trailer, but the warnings are still disabled in that setting, which makes no sense. I don’t want to cut into the truck wiring, and GM uses all kinds if unusual connectors that can’t be purchased separately.

I get the camera that will take the place of the tailgate camera this weekend. Hopefully the camera won’t be disabled when a trailer is sensed. If it is, I will definitely need to do something different.