toad wiring question ... connect coach ground to car ground?

I just finished setting up my 2015 Sonic as a toad to pull behind our Winnie. I installed a blue ox base plate and am using a blue ox Alpha tow bar, and a RoadMaster magnetic light set. I wanted to get a custom light kit that I could just plug in (via T connector behind the Sonic's brake light assembly), but I had too hard a time getting hold of one, so I opted for the magnetic lights.

I ran a permanent 4-conductor cable from a 6-way plug mounted on the base plate to a socket in the rear of the car. I just throw the magnetic lights on top and plug the 4-way wire from the mag lights into the socked I mounted in the rear, and It's ready to go, without any wires outside the car to flail in the wind and mar the car's paint.

I have a Hopkins 7-way to 6-way tow cable (7-way coach to 6-way on car) to drive the mag lights from the coach. This all works great, but ...

I also ran a wire from the 12 volt feed on the 6-way socket (on the base plate) to a cigarette lighter socket that I mounted to the cars dash. This is to power a blue ox Patriot supplemental brake. The idea is to power the Patriot brake from the coach to avoid running the car battery down while towing.

I actually ran that 12 v wire to a switch also installed on the cars dash - the switch controls which vehicle supplies power to the cig lighter socket: either the car (useful when not towing) or the coach (useful when towing).

The problem is that the Patriot doesn't like it when powered from the coach - just says "Car battery low". But it works fine when powered from the car. Now, I measure 12 v on that cig lighter socket for either switch position (coach or toad) ... so I'm left scratching my head.

I'm beginning to think the problem is the ground connection. The negative lead on the cig lighter goes to car ground. So when switched to supply 12 v power from the coach, there is no electrical path back to the coach.

Also thinking that it's possible that the ground of the coach is at a slightly different level than the car ground. If so, it may be that 12 v from the coach (relative to the coach ground) is not a full 12 volts relative to the car ground.

I think that perhaps I should connect the coach ground to the car ground. That would level out the ground potential between the 2 vehicles, and provide a path back to the coach for the negative lead of the cig lighter socket when powered by the coach.

BUT I don't know if it's OK to connect the ground between the two vehicles, or whether that is even the problem.

Is that totally out of left field?? Got to admit I am confused and grasping for a solution.

Anyone have an idea about this? Connecting the ground between the vehicles may not fix my Patriot problem, but would doing that cause any harm? If not, I'd like to try it.

Thanks.

Do not hook a hot wire from the coach without using a toad battery tender. 

The two vehicle grounds are connected, more or less, through the metal hitch connecting the two vehicles.  By more or less, I mean it may or may not be a good connection all the time depending on the mechanical tolerances in the hitch as the car and motorhome bounce around.

You won't hurt anything by running a better ground connection through the trailer plug, in fact it will probably help. The 4th wire on the 4 pin connector is ground, BTW. (Left Turn/Brake, Right Turn/Brake, Taillights, Ground)

The low battery indication on the Patriot is probably coming from the voltage lost along the long charging wire from the motorhome to the toad.  When the Patriot draws current, the voltage drops, tripping the low battery warning.

You have to put a load on a circuit to test for voltage loss, a voltmeter by itself doesn't load it down.  That's why you read 12 volts at the socket, you have to measure the voltage while the Patriot is pulling power.

Thanks for the response Lou.

I think you may be right, but I think the various coats of paint and powder coat finishes on the base plate and tow bar may interfere with that connection. But I should have realized that since they are already connected, it would be safe to make a formal, solid ground connection through the tow cable.

So I did that, but it got too late to test it with the patriot. That will have to wait until morning!

Thanks again.

whokares2 said:

Do not hook a hot wire from the coach without using a toad battery tender.

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Thanks for your feedback.

I'm not sure I understand ... how will using power from the coach drain the toad battery?

I hadn't thought about it, but in principle, couldn't I connect the 12 volt lead from the coach directly to the positive terminal of the toad battery (through a fuse, of course). Wouldn't that keep the car battery charged?

Is this an example of what you mean by "toad battery tender"?
https://rvibrake.com/products/towed-battery-charger?utm_medium=cpc&utm_source=googlepla&_vsrefdom=adwords&gclid=CjwKEAjwytLKBRCX547gve7EsE4SJAD3IZV6qt3lE6HslrhkgwkVNls-5rDrjTIImm43Z2CMXx4jlhoCn2Hw_wcB&variant=8804388741

If I use one of these devices, can I then have the Patriot use the toad's power without risking a drain of the toad's battery?

Thanks!

If you hook direct from the coach's  battery you could overcharge the toad and cook its battery.  Also the power cable from the coach could short out the Toad battery if it should ground out or have a parasitic draw.  It will keep the Toad battery maintained when using your Patriot system while driving down the road.

If you leave the Jeep running it will increase the odometer mileage I believe. 

Though the hitch does provide a ground path..... I'd not rely on it and if your towed vehicle connector has a ground wire option,, I'd use it. I would also use a fairly heavy wire.. Things tend to work better when PROPERLY grounded.

In fact, my current towed the "Towed lights" are independent of the vehile, but I had issues with 'em, turned out to be a bad ground at the RV end.. Improved the ground and all was good.

whokares2 said:

If you leave the Jeep running it will increase the odometer mileage I believe.

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Thanks WhoKares, but I'm pretty sure that the Sonic will rack up miles in any case ... that's just how it is - if the wheels turn, it registers mileage on the odometer ... I think!  Guess we'll see

John From Detroit said:

Though the hitch does provide a ground path..... I'd not rely on it and if your towed vehicle connector has a ground wire option,, I'd use it. I would also use a fairly heavy wire.. Things tend to work better when PROPERLY grounded.

In fact, my current towed the "Towed lights" are independent of the vehile, but I had issues with 'em, turned out to be a bad ground at the RV end.. Improved the ground and all was good.

Click to expand...

Thanks for the feedback John. I think you're right ... I made that ground connection last night, so I'll find out whether that fixes the problem right after I finish my morning coffee! As the DW would say, I'm useless before my first coffee! 

whokares2 said:

If you hook direct from the coach's  battery you could overcharge the toad and cook its battery.  Also the power cable from the coach could short out the Toad battery if it should ground out or have a parasitic draw.  It will keep the Toad battery maintained when using your Patriot system while driving down the road.

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All good points WhoKares. Thanks for your feedback.

All semi trucks running down the road have trailer(s) grounded through plug. Don't worry about over charging toad battery if you connect it to MH battery. They will equalize. I've ran a small battery (650amp) wired  directly into a 4 battery pack (950 amp each) for years with zero problems.

Blaise said:

Thanks WhoKares, but I'm pretty sure that the Sonic will rack up miles in any case ... that's just how it is - if the wheels turn, it registers mileage on the odometer ... I think!  Guess we'll see

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My toad does not rack up miles while being towed.

In any electrical system you can never have too much ground. In order for those mag lights to work you already have a ground from the coach to the lights tap into that somewhere and ground it to your cars frame or any part of the car that is bonded to the frame or even the negative battery post. 'As for the cigarette lighter you wired it will not cause any harm to your cars battery other systems since it is not wired into anything in the car. Make sure you fuse the hot wire to the socket the closer to the source the better. In the event of a break away your Blue OX internal battery should be up to the task. I like this approach in tow mode your towed is just a trailer with trailer brakes. There are wiring kits available from etrailer.com that isolate your car's battery from the coach using diode packs.

Modern cars will not register any mileage if towed with ignition off since there is no wheel sensor information being sent to the computer.

OK, for what it's worth, here's how it worked out ..

I connected the ground lead on the 6-way connector (this is the coach ground) to the car chassis. The Patriot brake was then happy to take power from the coach and it worked flawlessly.

My first trip pulling a toad was a success and a pleasant experience.  )

Thanks to everyone for the helpful feedback and pointers!

Blaise said:

Thanks WhoKares, but I'm pretty sure that the Sonic will rack up miles in any case ... that's just how it is - if the wheels turn, it registers mileage on the odometer ... I think!  Guess we'll see

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Blaise
I don't know about your Sonic....but most cars and light trucks with an electronic speedometer/odometer only report/record the miles when the key is in the ON position.

^^^  X2  ^^^

TheNewhalls said:

My toad does not rack up miles while being towed.

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Thanks for the feedback NewHalls. We just took our first trip with the toad and found that, like yours, our toads odometer did not advance while being flat towed.  )

mel s said:

Blaise
I don't know about your Sonic....but most cars and light trucks with an electronic speedometer/odometer only report/record the miles when the key is in the ON position.

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You were right Mel  8)
Our Sonic did not advance the odometer while being flat towed while in neutral and the key in the ACC position. Thanks!