CHMSL / 3rd Brake Light Install on a trailer

[weebl]

My wife is going out of town for the weekend, so I'll have a nice break from the routine and get several vehicle related projects done. Since I'll need to make a trip out to Princess Auto (a bit like Northern Tools for Americans not familiar with them), I figure I'll get a few things for trailer upgrades as well, which I won't be able to do until I get the trailer back out of storage once the spring weather arrives.

One of the upgrades I want to do is add a CHMSL 3 3rd brake light. The current connector from the tow vehicle is the standard RV 7 pin. So the available signals I get out of there is: - brake controller (voltage varies based on braking intensity), positive always on, negative ground, marker/clearance, combined brake/left turn, combined brake/right turn. The 7th pin is not currently wired, but I will be using that for a reverse light I'm installing (the unused pin is normally for the reverse light).

Searching online, I see some people talk of various kits/circuits they're using to illuminate a brake light only on the trailer side, or more commonly, run an additional brake light wire from the tow vehicle to the trailer. Some people don't even bother, and just mount 2 brake lights, and let them also blink left or right when the signals are on.

The route I want to go is to mount just one brake light in the middle, and have all the wiring done trailer side, and keep it low cost. That means I don't want to tap an extra line in the tow vehicle for an additional brake light signal, and I don't want to do the two brake light thing.

My initial thoughts are there must be some way of doing one of the two:

1. A relay connected to the brake controller signal. The relay would then energize the circuit into the constant positive to light the brake light when it's applied. My concerns here are, the brake controller signal is variable voltage, and I am concerned that something else on that circuit may cause the trailer brakes not to function properly. (If some systems are picky while others aren't, then the brake controller is a Tekonsha Prodigy, and the axle / brakes supplied from the factory are Dexter).

2. Some sort of a signal combiner (I don't know of a specific product on the market, or if there is a way to fashion one myself using common parts), that takes the left signal/brake and right signal/brake, and only light the 3rd brake light if the brakes are applied, and not flash. Seems like the more complicated method of the two.

I suppose if either method were not within reason in terms of effort or cost, I'd just do what most others did: an additional wire from the tow vehicle or do two brake lights.

 

[Dave Sherman]

If you don't mind having the brake light flash if the hazards are on, then you could use two relays, each coil energized by the turn signal circuits, and have the contacts wired in series to energize the brake lamp. That would probably be your simplest route, short of trying to build something to sense different levels on the brake controller line.

 

[eljefino]

^ with the relays in series if either turn signal flashes the CHSML will flash. Probably need some diodes in the mix.

 

[Dave Sherman]

Originally Posted By: eljefino
^ with the relays in series if either turn signal flashes the CHSML will flash. Probably need some diodes in the mix.


No, with the contacts in series, then BOTH relays have to be energized (i.e., both signals would have to flash simultaneously) to complete the circuit. I'm an electrical engineer.
relay left relay right
+12V ----/ --------+----/ ----------> to brake light

 

[weebl]

Dave, I'm interested in your idea. I don't mind if the CHMSL flashes with the hazards on. So if I understand correctly, the constant 12V will go through the relay for the left side, then straight to the relay for the right side, and finally on energize the CHMSL? With this wiring scheme, the CHMSL will be on (and not flash) in the following conditions, right?

- Brake applied, no turn signal
- Brake applied, left turn signal
- Brake applied, right turn signal

But the CHMSL will flash in these conditions:

- No brake applied, hazard on
- Brake applied, hazard on

Am I correct?

 

[Dave Sherman]

Ah, maybe I see what eljefino was thinking of, one scenario I hadn't considered. With the above setup, the brake light will flash if either signal is on and the brake is applied. So, here's what happens

Nothing on - brake light off
either signal on, brake off - brake light off
either signal on, brake on - brake light flashes
hazards on, brake off - brake light flashes
brake on, signals off - brake light on solid
brake on, hazards on - brake light on solid

No easy way around that without a dedicated brake signal. The electric brake signal could be used to turn on a transistor to energize the brake light. I wouldn't connect it directly because the variable brake signal will make the light vary in brightness, and I wouldn't connect the relay coil to it because the relay might not energize until the brakes are heavily applied. At least with the brake controller I have, once the brake switch closes it puts out a minimum brake signal that could be used to turn a transistor on.

 

[weebl]

So if I were to use a transistor connected to the electric brake signal, how would that work? Would the transistor be then used to switch a relay, or straight to the brake light?

I also found this online, and wondered what your opinion of this circuit is. It is to wire an old MGA car to have a CHMSL. Apparently, the wiring on these old cars are similar to what I'm working with on the trailer. He's added a third relay in the mix, as well as some diodes it appears:

http://mgaguru.com/mgtech/electric/et240b.htm

 

[Dave Sherman]

Looks like he spent some time coming up with that, but I see how it works. Once BOTH lights are energized the third relay stays latched through either CR1 or CR2, so it doesn't matter if one of them starts blinking. Yes, that would work, although it would still blink with the hazards. It would alleviate the problem of my simpler circuit.

The transistor could drive the light directly, but a heat sink would be worthwhile. It would be a little more crude than what this guy came up with because it would depend on having that minimum signal from the brake controller, which I'm not sure all brake controllers would do. Also, you (I) would need to include something to keep the flyback from the brake magnets from doing unpleasant things to the transistor.

If you're comfortable building the circuit he described, I'd recommend it over the transistor circuit. The double pole relay should be a Radio Shack item, unless they are getting rid of those now. Don't expect the counter robots to know what a double pole, single throw, 12 volt relay is though. The other two would single pole, single throw. The diodes and relay contacts should be rated for at least 2 amps, and since I don't think Radio Shack has anything with less than a 50 PIV voltage rating you should be fine there.

 

[weebl]

Perfect, thanks! Looks like that guys circuit is the way I would go, since I can see funny things happening if I tap into the brake controller signal. I'm sure mine has a minimum signal, but I could see the flyback from the brake magents being a potential problem source.

 

[eljefino]

Off the wall idea, but try an LED CHMSL just straight wired into the brake controller. They have really high impedance so electrically they're barely there. Obversely they take so little current to run, it'll come on when the brakes are lightly applied.

Note: I know nothing about brake controllers, so double check this idea with your good stopping in mind.

Or you could get a Ford cruise control kill switch and wire this into a Tee in your hydraulic brake lines. If the kill switch engages when the brakes are OFF you can use a relay to flip it around. If you set this up a certain way and drill a tiny hole in the back of the light, you can see it activate with your rearview mirror as a confirmation that you're getting trailer brake pressure.

 

[ffhdriver]

Back in the '80s when some cars didn't have a CHMSL and brake light wiring was simpler,you bought a kit to add it. You had to attach a wire from the CHMSL to the left brake light and another wire to the right brake light (plus a ground) or the turn signal flasher would make the CHMSL flash. Attaching to only the left or right brake light would cause the CHMSL to flash. IMO a flashing CHMSL defeats the purpose that you installed it for.

 

[weebl]

Okay, so before I even thought about attempting to build that circuit, I figure I should have and understanding how it works, so I printed some off so that I could trace out the electrical paths as various conditions were thrown into the circuit. I don't claim to be an electrical engineer (or any engineer for that matter) so maybe there is a flaw in my understanding, but I traced the first condition I thought of.

Assume brakes off, and a power pulse of the left signal. Brake light should not illuminate:

12V+ comes in through the lead and branches into 2:

1. Branch into "relay 2". Coil is energized, goes through the lower pole. Enters "relay 1" but doesn't go anywhere as there is no energized coil. It also branches up to energize the coil on "relay 3". Coming out of there, on one branch hits a diode going the wrong way so end of circuit there. Also branches up to lead into the CHMSL, and therefore it is illuminated.

2. Second branch from the 12V lead, goes through a diode and through "relay 3" as this has been energized by the first branch. This also powers the lamp.

So as you can see, I haven't got very far, and it appears there is a flaw in the design. I didn't bother tracing any further.

Thankfully, I have found some online sources for a "3rd brake light logic module" with a cost similar to what all the parts would be trying to build this myself. I just need to find a brick and mortar source if I can now (or low / free shipping, possibly if I add in other things I want to get).