Brake system power with Battery disconnected

bayoubelle wrote:
I have a 2016 Ford Focus 2.0 L Hatchback, the dealer installed the StaY n Play Duo braking system. Unfortunately he did not install a battery disconnect, he insisted the brake would not work if he did.

We know that we have to disconnect the black cable. So we called SMI (Brake co) and they said to disconnect the positive terminal, with a turn off switch. That the brake system needs the ground from the black.


Any advice?

I went back and re-read your original post after I had already offered a comment and think I misunderstood your question. I had assumed you were concerned about the SMI unit staying active when not towing which I think was not your concern. You are were concerned about the toad onboard systems being powered up during towing since you must have to have the key in an "on" position to unlock the wheel? You've got the problem covered it sounds like. I apologize for my adding to the confusion.

I did not have the same issue with my Jeep as Chrysler allowed an unlock position on the TJ with the key but I really did not like leaving the key in the ignition when towing and was concerned that I might forget to unlock the wheel sometime anyway so I disabled the steering wheel lock on the Jeep like many do so I would not forget.

Different ways to make the same thing happen so I decided to put a disconnect on the black wire that runs from the operating unit to the GFCII instead of on the 12vdc wire. The black wire provides the ground to the system when in operation and it already was running to my dash on the Jeep and so I simply installed a switch in an unused place on the dash. It is very convenient to simply reach up and flip the switch as I am putting the Jeep into the correct gear position to tow. The same thing is accomplished by using the switch on the GFCII but, with it on the kickpanel below the dash, it just was not as easy to reach.

I might add that I have used the same SMI unit on two different coaches now with the first one being a gasser and now our DP. Even though our ISL Cummins has a very aggressive compression 2 stage brake, I can still adjust the GFC to apply the braking as needed. The Air Force One would have been great but, when we already had the Duo, I thought let's try it. So far, works just fine with our DP.

Thanks for feedbackl

We brought the car back to the dealer and got the disconnect switch on the positive terminal. Seems that putting it on the negative would not shut the vehicle down, due to power being provided by the braking system.

Not sure about the diodes on the lights, so I will have to talk to my Dad about that. I think we might invest in the Roadmaster Auto Disconnect at a later date.

When you are ignorant of these things, they just put it on and don't bother to give you a lot on info.

We also have a Ford Focus and have the SMI system. You need to install a battery cut off switch on the positive cable only. The mechanic must also install diodes in the rear of the car in the original wiring for the tail lights. Diodes only allow power to flow one direction like a check valve. Installing diodes in the wiring will prevent current from back flowing from the newly installed wiring for the rear lights to the rest of the vehicle.

If your mechanic does not know what a diode is or what the term back flowing means, find another mechanic. It is essential they be installed correctly. The battery MUST be completely isolated from the SMI circuit that powers the brake vacuum pump.

We have towed several thousand miles now with this setup with no problems and love it.

I recently installed a SMI Stay n Play DUO system in my truck to replace a 10 year old 4 Wire Stay N Play system. Since I already had the 4 wires from the MH to the truck I utilized it even though my truck's battery doesn't have to be disconnected. I felt I'd rather get power from the MH. The breakaway switch is connected to the truck battery. The 4 wires from the MH provide 12V+, a 12V ground, a connection the the MH brake light switch, and feedback from the truck's brake light switch.

As far as removing power from the car itself, it doesn't matter which cable you disconnect from the battery. It sounds like your dealer installed the Stay-n-Play power so it's directly wired to the battery positive terminal, so as long as you don't disturb that connection and leave the negative connected, it'll work ok. Just install a battery switch on the positive side that disconnects everything else.