First off, sorry about the long title as I don't know a better brief of my situation.
Project:
I am converting a bus into a trailer to be pulled by a semi truck 5th wheel style.
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
Concept:
Here is what i think will work using a 5 pin relay. Your input will greatly help me.
Explanation:
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.
Application:
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?
Best Answer
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.