Brake light switch

[BruceS]

I'll look up that relay but I'm wondering if it's (your brake lights) Earth switched as Terry mentioned?

Read this although it refers to lights the idea is similar.
SPECIAL CONSIDERATIONS FOR GROUND-SWITCHED SYSTEMS

Many Japanese vehicles, as well as a few others, use a "ground-switched" headlamp circuit. In these circuits, the headlamp and beam selector switch break or complete the ground side of the headlamp circuit, rather than the feed side. On these systems, it's imperative to use both negative and positive existing headlamp wires to trigger the relays. It is tempting to run the existing headlamp feed wire to relay terminal 86 (trigger feed) and simply find a convenient ground for relay terminal 85 (trigger ground). However, this will not work with ground-switched systems. Run the vehicle's existing feed wire to terminal 86, and run the vehicle's existing ground wire to terminal85.

Now, what are we going to do now that we've used-up our one and only ground wire on the 85 terminal of the low beam relay, but we still have to install the high beam relay? Go to the other side of the car and you have another ground wire! Remember, the relay trigger circuits can be as long as you like, because they take insignificant power. So, you can extend the vehicle's existing headlamp wires to your relay mounting location. It is fine to use this method regardless of whether you have a ground-switched system or not, so go ahead and use it if you're not sure.

Found info HERE... http://www.danielsternlighting.com/tech ... elays.html

[TigerMK1]

It is tempting to run the existing headlamp feed wire to relay terminal 86 (trigger feed) and simply find a convenient ground for relay terminal 85 (trigger ground). However, this will not work with ground-switched systems. Run the vehicle's existing feed wire to terminal 86, and run the vehicle's existing ground wire to terminal85. Isn't this the same thing?

[BruceS]

no.......... the first example is earthing to any where on the body of the vehicle whereas the second example is using the earth return wire right back through the brake light switch.
It's so hard to work out & explain. Do you have a wiring diagram for the vehicle?
Some vehicles have VERY complicated setups that will notice if one light fails etc.....
The great majority of older vehicles only really have live wires running around everywhere & simply use metal to metal to body to earth strap to battery earth (negative) to work.
Yours MAY have a complete insulated circuit so by using an earth to body may effect your lights. (not working?)

[TigerMK1]

Ok, the fuso does have an earthed chassis. Now if what you have mentioned about switching an earth to the brake lights,The wire I cut to fit to the relay would be the earth side that is then switched to the lights that have 24V on them. If that was the case, The relay would still work (and it does) and the lights should work. The circuit is complete however passing through the relay. I reversed the leads to the relay, no difference. It would also mean the 24V I am seeing on the other side of the brake switch is via the brake lights.

If this is the case how did the trailer connection work, I just connected to the lights via the normal active wire when they were turned on.

[BruceS]

That is strange if I'm understanding you properly.
With no relay & as it originally was there should have been 24V on one wire into the brake switch & nothing on the other wire out.
With key on & brake on you should have 24V both sides.
If you cut the wire on the 'nothing' side & connect both ends of the cut wire to your terminals on the 'coil' end of your relay do the lights still work & the relay click, click? Sorry if that's exactly what you just said!!
You might try a much simpler relay? Maybe borrow from headlight relay or horn?
Shouldn't really matter though.

[TigerMK1]

Had another thought, if the brake switch is switching earth, the other side where I am reading a full 24V would be from the lights. I will check the lights themselves tomorrow and see if thats the case. If so I would rejoin the earth wire I cut and tee in one side of the relay to the lead going to the lights and on the other side have 24VDC to the relay. The switch would put earth out to the lights and provide earth to the relay. Now just got to proove there is 24V on the brake lights.

Never thought it would be this difficult.

Edit, noriced your post while I was doing mine. Yes, in circuit the relay works fine but the lights don't. Short the relay and the lights are working again.

[TigerMK1]

Could.nt wait, must be the old tech in me. Yes the brake switch switches earth. I joined into that earth lead and ran it to the relay and other side to earth. Naturally no go. I then cut that wire and ran either side to the relay. Relay works, lights do not. Only reason I can think of is the relay has enough resistence to stop sufficient power to the the lights. So tomorrow I will rejoin the cut earth wire and join into the lead to the lights (24V shown). The other side of the relay will have to be hard 24V. When the switch operates, it feeds earth to the brake lights and to the relay that has 24V on the other side of the coil.
Sounds OK to me, will wait for any downsides as I will not be doing this till late tomorrow morning.

[T1 Terry]

Can you just use the earthing of the brake switch to close the circuit of a transistor or a mini 12v relay? I guess as you already have the 12v relay you want to use it, what if you power the positive side of the coil from the powered side of the brake light bulbs? I'm guessing that is only powered when the ignition is on, then just a wire from each side of the brake lights themselves, with a diode in one of the cables to stop the circuit through the bulb being enough to pull the relay closed.

[TigerMK1]

Can you just use the earthing of the brake switch to close the circuit of a transistor or a mini 12v relay? I guess as you already have the 12v relay you want to use it, what if you power the positive side of the coil from the powered side of the brake light bulbs? I'm guessing that is only powered when the ignition is on, then just a wire from each side of the brake lights themselves, with a diode in one of the cables to stop the circuit through the bulb being enough to pull the relay closed.

I think I can find a good 24V from under the dash where there is a very large fuse/relay panel. Going to the brake lights is another 7-9 meter cable run. I already did that running the blue electric brake lead from the dash to the back toad socket. I would rather not run another cable under the motor home unless neccessary. It took 3 days before my legs/back forgave me. I will have to get upside down on the drivers seat tomorrow, with a hot soldering iron in my hand to rejoin the earth lead to the switch, tap into the lead to the brake lights and reconnect the spade connections (loose) into the holder of the switch. After all that I will then decide to NOT run another lead or 2 down to the rear. I am not 20/30/40/50/60 anymore bugger it.

[T1 Terry]

Ahh... so you have already run the wire from the back to the dash, I was thinking you were making the relay block in the rear of the motorhome.