Brake light switch

[TigerMK1]

Hi, I am attempting to connect a lead from the non active wire that goes to the motor home brake switch. I need to do this so I can connect a brake signal thru to the toad. The problem is that I cannot see the 24V on that lead when I operate the brakes. The motor home brakes are working fine. There is just no 24V out on the extra wire I attached. I have checked the continuity of the wire, all good. Just no 24V. I have been at this for over 6 hours and have no answer. Any auto sparkies out there that can advise why the brake lights on the MH work but no voltage out on the non live wire, even when I short out the 2 wires from the switch.
Steven

[T1 Terry]

It is possible the motorhome brake lights are supplied 24v when the ignition is turned on and the brake light switch in the motorhome makes the earth return cicuit causing the globes to light up. Easy test, with the ignition on, back probe the brake light bulb socket with a test light, the other end connected to a good earth. If the brake lights are supplied 24v, one side should light the testlight only and very bright, the other side will light the bulb and test light but rather dim.

[TigerMK1]

Hi Terry, I checked that 24V is supplied to the brake switch. It is not switching earth. I connected my wire to the other side of the switch. So,by my reasoning, when the switch closes and lights the rear brake lights, it must be switching 24V. The problem is my meter (i tried 2) does not see the 24V on the other side of the switch when closed. Thats why I am tearing what little hair I have out. I was even thinking of running a lead back from my rear trailer connection and see if that allows sufficient 24V to close the small relay. You see I need to feed the house 12V back to the toad to accuate the braking system and have a small 24V relay controlled by the motorhome brake switch to switch 12V (via a on/off switch) to the toad.

[T1 Terry]

What are the numbers on the pins of your relay? 85 - 86 - 30 - 87 are these the same as the relay you ar using?

[BruceS]

Did you test it THROUGH rather than wire to earth?
I mean on the + out side of the brake switch disconnect that wire and feed it into one of your probe wires and then connect the other probe wire back to that terminal on the switch.
Hope I make sense?

[T1 Terry]

Did you test it THROUGH rather than wire to earth?
I mean on the + out side of the brake switch disconnect that wire and feed it into one of your probe wires and then connect the other probe wire back to that terminal on the switch.
Hope I make sense?

That will only prove the circuit works, not that cable is a positive supply, a voltage reading will occur in the positive or negative leg of the circuit. The test to a good earth will either show positive voltage or close to zero voltage, this will confirm it is either a positive or negative supply, on a digital meter, a reading with the mV flashing beside the reading means you don't have a circuit at all.

[TigerMK1]

Did you test it THROUGH rather than wire to earth?
I mean on the + out side of the brake switch disconnect that wire and feed it into one of your probe wires and then connect the other probe wire back to that terminal on the switch.
Hope I make sense?

I replied to this before however it ended in never land. I like your idea of routing the brake circuit thru the relay coil (old term). it will either operate the relay and the brake lights or it wont. Worth a try. Terry, the relay is a Jcar unit with 2 x make or break switches on it. They are clearly marked on the case.

Tomorrow will be another go at it. The Blue Ox A frame from the USA arrived today, all up around $700 delivered to my door.

[TigerMK1]

Well today I cut the lead to the RV brake lights and connected the relay across the 2 wires. Yes the relay worked fine. Downside the brake lights did not work. Short the relay and the brake lights work again. Bugger if I know how to get the brake switch output down to the toad. Any other suggestions?

[BruceS]

Must be some weird way that those lights work.
Usually it's so easy to just connect more lights into any of the tail lights.
You see some buses have lights down low & another set up high. Usually just join in an extra wire & away you go.
I'll try to draw up a rough sketch.
Can you show us what your relay looks like.
4 pins or 5?
Numbered like Terry asked?

[TigerMK1]

The motor home is a Fuso with 6 rear brakelights. I also have it wired for a trailer and its brake lights work fine.
The relay is a FRA8PC-S2. It is shown as a 24VDC with 2 spade terminals at one end with a diagram etched between them showing a coill. At the other end there is 6 spade terminals with the images showing 2 sets of changeover switches. No numbers on any of the tags. I cannot comprehend how the standard 24V brake light circuit can power 6 globes as well as a pair of trailer globes as needed but cannot operate a soldi state relay and the brake lights at the same time. Tomorrow I will take the relay out, connect it to the trailer plug and see if it operates when the brakes come on.

Further options, a 24/12V converter? a second brake switch just for the 12V feed to the toad?