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Old 02-19-2016, 02:16 PM   #1
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Using a relay to disconnect brake lights

Hi

Not sure if I can explain this clearly.

I have a Readybrute with a surge brake that pulls the brake pedal. This will override the turn signals on my 2005 Jeep TJ, so I need to stop it doing this.

I also want an indicator on the dash to show that the brakes are being applied (and more importantly, if they're stuck on).

My MH is wired to pull a trailer with a 7-pin socket.


A suggestion that I really liked in another thread was to wire a toggle switch (SPDT I believe) to the Jeeps brake switch. Under normal operation, the switch sends the power to the Jeep brake lights. When being towed, I toggle the switch and it sends power to the dash indicator in the MH.

I thought maybe I could automate this method by using the charge terminal in a 7-pin connector (Pin #4). This could be wired through the coil of a 12v relay to ground. When the toad is plugged in, the 12v will switch the brake light power to the dash indicator in the MH.

Question 1. What is the best way to wire this relay. Since I'm not using the charge line for anything else, I assume I would need a resistor in series with the relay coil to limit the coil current. Do I just use ohms law here and figure out the required resistance and power rating?

Question 2. I was thinking of using the center pin (#7) to transmit the return signal to the MH. I don't believe this is used, it's supposed to be a reverse light. Any problems with doing it this way?


Thanks
Steve
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Old 02-19-2016, 03:41 PM   #2
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Instead of powering the relay all the time, how about putting the relay in the brake light circuit on the Jeep. When the Jeep's brake are activated, it switches the relay on, takes the lights out of circuit and sends the brake light signal forward to the MH indicator instead? A simple DPST relay similar to a horn relay would work for this. Normal power off position would default to the Jeep's lights, when powered it would switch to the MH indicator light. Powering the relay all the time could build up too much heat and burn out the relay, unless you get a continuous duty relay.

The Mopar harness in my JK works similar to this. When the MH turn signals or brake signal is coming to the Jeep, small relays in the harness disconnect power from the Jeep to the taillights and transfers the taillight circuits to the MH. You can hear the relays clicking when the MH turn signals or 4way flashers are on.

As for using the center conductor, as long as you can locate it on the other end, then there shouldn't be a reason to not use it.
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Old 02-19-2016, 05:05 PM   #3
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On question 1. The charge line is nothing more then a 12 volt tap of the MHs battery system.

The only current it will carry is the current you ask of it. There is no reason to add resistors to it.

Qusston 2. The center pin, if unused, will be fine.

The manual switch would leave you in jeopardy of forgetting to switch it and having no brake lights while driving the jeep.

The little Bosch type realys are fine for what your doing. They can be switched on continuously.

There are probably a bunch of them in the MH and Jeep.

The come in SPST or SPDT configurations.
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Old 02-19-2016, 05:25 PM   #4
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The switch is the Kiss method. Perhaps just add a lighted switch so you have to turn it off to turn the lights back on in the jeep.
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Old 02-19-2016, 08:07 PM   #5
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Roadmaster sells a brake light relay designed for this purpose. Here's a link to the etrailer page for it. There's a wiring diagram on that page.

Some of us have used the "trailer electronic brake controller" wire to send the activation light signal to the coach's dash. If the coach is pre-wired for a brake controller, that wire is more likely to appear under the dash than the backup light wire you are thinking of using. Of course, if your 7-pin connector is not wired for either the brake controller or the backup light, either pin will do, because you'll be running your own wire from the hitch receptacle to the front of the coach.
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Old 02-20-2016, 07:03 AM   #6
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The JK factory Mopar light kit uses relays to do this exact thing. If you wire the brake light wire thru the relay and use the 12 volt charge lead from the camper to open the circuit at the brake pedal. Any standard Bosch 20 amp relay will suffice. The only thing I would check is if your 12V lead is hot in run or hot all the time. I noticed mine is hot all the time (older camper) so the relay is energized anytime im parked. While an over night most likely will not run the coach battery dead id rather be safe than sorry so plan to unplug when i stop for a night.
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Old 02-21-2016, 09:11 AM   #7
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Quote:
Originally Posted by l1v3fr33ord1 View Post
Roadmaster sells a brake light relay designed for this purpose. Here's a link to the etrailer page for it. There's a wiring diagram on that page.

Some of us have used the "trailer electronic brake controller" wire to send the activation light signal to the coach's dash. If the coach is pre-wired for a brake controller, that wire is more likely to appear under the dash than the backup light wire you are thinking of using. Of course, if your 7-pin connector is not wired for either the brake controller or the backup light, either pin will do, because you'll be running your own wire from the hitch receptacle to the front of the coach.
I did the same thing with relay on brake lights and used trailer wire for light on dash.
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Old 02-21-2016, 10:15 PM   #8
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Stevemoors,
I was the one who setup the toggle switch for disconnecting the TJs brake light switch from operating its own brake lights while being towed. Through the toggle, I sent that signal to the dash of the coach so that, when the Ready Brake was pulling on the brake in the Jeep, that signal would notify me that the brakes were ACTUALLY being applied. Then, when driving the Jeep, I flipped that switch to the opposite side and, the Jeeps brake signal went to the Jeeps tail lights, as it should.

In the 5 years we towed it that way, neither the wife or I, EVER FORGOT to flip that switch to the proper position for towing or driving. It was part of the ritual for prepping to tow or, disconnecting from towing.

Now, you've been explained how to wire a relay to take the position of the switch. I was never smart enough to do something like that. The switch was my way of handling this issue and, it worked. But, a relay, properly wired, would do the job equally well. It would take the potential "Human forgetfulness" out of the equation.

I am learning about relays as we speak here. They're not hard, but, you just have to know what goes where and, for what purpose. Good luck on this project.
Scott
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Old 02-22-2016, 02:02 PM   #9
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Thanks for all the reply's, had a soccer tournament over the weekend, so didn't get chance to catch up.


I hadn't really thought about the relay being energized all the time, that is a concern I admit.


I think after consideration, I'll go back to the original method with the toggle switch and forget about trying to complicate things with a relay. I can always add the relay another time.


Thanks again for the tips. Much appreciated.
Steve
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Old 02-23-2016, 08:57 AM   #10
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I wired a relay into my Xterra. Thought about a toggle switch but figured sooner or later I'd forget and then be driving around with no brake lights. No bueno....
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Old 03-08-2016, 10:41 AM   #11
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I used the same relay and the +12 from the RV to do the same thing on my F150. This does 2 things, the trucks brake pedal doesn't override the signals from the RV for the TOAD lights. I get a light on my dash to let me know that the TOAD brakes are being applied.

I had to run an extra wire on the TOAD and RV to bring the brake light signal to the RV dash. Works like a charm, nothing to forget.
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Old 03-08-2016, 11:55 AM   #12
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I put switched flashing LED light behind toad mirror - facing front and tied to brake lights. Flashing light shows up on rear view camera monitor.
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Old 03-08-2016, 12:19 PM   #13
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I will toss in what I did using two relays.
I had LED brake/turn lights on the toad, and they were not connected to a common point.

So I used a DPDT relay for each tail light. Basically the relay switches the LED side of the brakelights to be controlled by the toad (orig connection), or by the wire from the motor home.

The relay is energized by the charge line in my 7 pin connector. On Freightliner, the charge line is controlled by the coaches ignition.
This gives 100% individual brake/turn indicators.

Then for the dash mounted activation light, I connected to the wire on the toad that goes to the brake lights, so the toad brake pedal lights the dash light.

Used diodes for the tail lights.

This way, there are no switches that can be forgotten.

Regards,

Dan
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Old 03-09-2016, 04:31 PM   #14
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I think I was over thinking this.


My concern was connecting the charge line direct to the relay then to ground would draw a huge current from the battery through the relay coil and I'd have to limit it somehow (externally with a resistor).


I looked online and found that most automotive relays have a coil resistance of around 100ohm, so at 12v, that's a current draw of around 100mA. Shouldn't be causing any fires.




Happy I cleared that up


Steve
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