First off, sorry about the long title as I don't know a better brief of my situation.


I am converting a bus into a trailer to be pulled by a semi truck 5th wheel style. enter image description here

The bus has it's own electrical systems that I don't want to take away, but i need to control the stop lights, etc from the semi truck when it is attached. The semi truck by design of course can power lights on a trailer so I will be using it in this way to control the lights on the bus.

I want to have a bypass switch. If the bus is powered on by the master disconnect switch, then I need it to lift the ground from the semi truck power feed...thus turning off the control of the lights from the truck battery


Here is what i think will work using a 5 pin relay. Your input will greatly help me.groundliftcircuit


I'm thinking to tap the bus positive, post master disconnect switch, to feed relay point 86. 85 goes to the bus ground. This same ground is where 87a, via 30 allows the semi truck to complete the circuit to control the lights.


When the bus master disconnect switch is switched off (normal usual operation), the ground from the truck connects to the bus allowing the truck system to control the bus lights.

When the bus master disconnect is switched on (rare but to operate the bus systems independent from the truck), the ground from the truck should "lift". This will assure that both battery systems are not powering the same circuit.

Is this a good way to do it even though I'm only working with 12v on either side of the relay? Am I missing something?

  • \$\begingroup\$ I've read your question twice but I'm missing something. By "lights" I presume you mean the trailer tail, stop and turn indicator lights. These should, of course, remain under control of and be supplied by the tractor unit electrical power. Everything else on the trailer can be powered by the trailer battery / generator. There would be no need to isolate either set of batteries. \$\endgroup\$ – Transistor Jul 14 '18 at 18:14
  • \$\begingroup\$ Yes you are correct. The tail lights, stop lights, turn rev etc. is what i'm going to be controlling from the semi truck just as though the bus was a normal trailer. My situation is that the bus has it's own system that we will be using sometimes as we use the bus as a classroom and want to run the AC, and Video monitors that are part of the bus itself. I do understand what you are saying about the systems one runs is different from the other...i'm just cautious about the ground from the Truck that is also terminated on the bus frame. the"ground lift" is to make sure nothing crosses over. \$\endgroup\$ – David Jul 14 '18 at 18:27
  • 1
    \$\begingroup\$ Then rewire the tail lights directly to a trailer plug that goes into the socket on the tractor unit. \$\endgroup\$ – Transistor Jul 14 '18 at 18:30
  • \$\begingroup\$ that's what i originally was thinking, but there are a LOT of lights and was figuring that if i connect to the lights at their switching source would be easier considering the limit of 7 wires from the standard semi Truck electrical connector. Seven wires being only positive feed for stop, positive feed for running lights, positive feed for reverse, positive feed for left turn, positive feed for right turn, ground and one extra not normally used \$\endgroup\$ – David Jul 14 '18 at 18:33
  • \$\begingroup\$ It doesn't sound easier to me! You will probably spend a lot of time tracing backfeeds and trying to fix them. \$\endgroup\$ – Transistor Jul 14 '18 at 18:39

You are imagining a problem where there is none and likely to create headaches for yourself. The solution is to wire a multicore cable with regular trailer plug on it along to the back of your trailer, ground the negative to the trailer chassis and wire the cores to the lights, disconnecting and insulating the original tail lamp wiring.


simulate this circuit – Schematic created using CircuitLab

Figure 1. It should be clear from the schematic that the only connection between the two systems is the common battery negative chassis connection. Without a positive connection between the two systems no current can flow between them.

The result is a system that will work on any tractor unit, doesn't rely on trailer battery condition, is simple, standard and reliable, and can be maintained by any decent mechanic.

  • \$\begingroup\$ Thank you for your comments. So basically the trailer/bus will share the ground "circuit" but my isolation of the positive to just the lights from switching keeps it ok to use only the positive from the truck to actually switch the required lights on. do i have it right based on your suggestion? \$\endgroup\$ – David Jul 14 '18 at 19:20
  • \$\begingroup\$ "... but my isolation of the positive to just the lights from switching keeps it ok to use only the positive from the truck ..." seems to have been garbled between your brain and your keyboard. Can you re-phrase the question, please? \$\endgroup\$ – Transistor Jul 14 '18 at 19:22
  • \$\begingroup\$ In your schematic. Is the Chassis ground the shared termination ground of the -12v truck and -12v trailer/bus. Got it...i guess i was over thinking that i needed a safety because of the shared chassis ground \$\endgroup\$ – David Jul 14 '18 at 19:27
  • \$\begingroup\$ Thank you for your feedback and help. I think the fog has lifted from my brain and i can proceed forward. \$\endgroup\$ – David Jul 14 '18 at 19:36

Assuming that both are chassis ground it would be risky to disconnect the power systems by trying to disconnect ground. You really should disconnect the +12V connection.

You haven't shown how the +12V will be connected to the bus lights. I assume that you intend to connect the +12V from both batteries together? This is also a bad idea. If one battery is lower than the other you will draw very large current over that connection. Using a relay as you have shown is a make-before-break, meaning that both +12V are connected together for a short time until the relay switches. Not a good idea.

I would recommend a heavy-duty SPDT switch, break-before-make, that lets you connect the +12V in the bus to either the bus battery or the truck battery, but never both at the same time.

  • \$\begingroup\$ The electrical system of the bus is still maintained as it's own separate unit. I was hoping to just tap into the lighting systems when the truck is hooked up to the bus. The bus master power should never be operated while the truck is connected...but since is this will yield a product to be used for years...i wanted to assure that there was a safety "lift". The +12v from the truck is to only operate the lights as mentioned. I don't want the +12v from the bus to be connected to the light system but it's hard to establish that without re wiring everything \$\endgroup\$ – David Jul 14 '18 at 18:46

You're not going to be able to reliably separate the trailer and tractor grounds while the trailer is hitched, any system that relies on that is likely to fail with lots of smoke

But there is no need to isolate the grounds if the live wires are not interconnected.

Disconnect the wires for the signal lights from the bus electrical system, and connect them instead to the trailer signal socket.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.