How the heck do the trailer relays work? (Schematic question)

[iroc86]

So, I'll start by saying that electronics is my hobby, so I'm not intimidated by wiring or schematics.

But I've been studying the trailer tow package schematic for late-model FSJs and I can't figure out how the stop light relay is supposed to work. I'm hoping that somebody could offer a different perspective and help me out. Please bear with my detailed explanation...

The following schematic is from Tom Collins' website and refers to the 1986 model year. I've marked it up to better explain myself.

On FSJs equipped with the factory towing package, there is a module that outputs to the trailer plug. Four relays inside the module provide an alternate voltage source for trailer lights. The relays are activated by the stop, turn/park, and backup light circuits. In the schematic below, the FEED connector is tied to the turn signal switch and stop light switch, while the OUT connector is tied to the rear body lights. Trailer connector C465 is located under the rear bumper.

Late-model FSJs had separate bulbs for the stop lights, whereas most trailers combine the stop light with the turn signals in a single bulb. The trailer module is designed to convert the output of the factory two-bulb wiring to single-bulb trailer wiring. The stop/turn wires for the trailer are shown in GREEN in the schematic. In order for the stop/turn bulb to illuminate when the brake pedal is depressed (RED wire), the GREEN wires must receive current.

When the brake pedal is depressed and current flows through the RED wire, the STOP LT RELAY is energized. The BLUE circuit is affected, but I don't really understand the purpose of its wiring. The BLUE wires are connected in a loop. Judging by the schematic alone, they don't serve a purpose: The stop lights will not be illuminated because the LH TURN and RH TURN relays have not been energized to allow current to flow through the GREEN wire.

Again, according to the schematic, the only thing that puts the GREEN wire in a circuit is when the turn signal relays are energized. That doesn't appear to happen when only the STOP LT RELAY is energized. My trailer tells me otherwise, because the trailer stop lights work just fine.

I hope this makes sense. At this point I'm thinking there's a typo in these schematics, but the 1990 wiring diagram shows the same configuration. Can anyone lend some insight?

[babywag]

They work best in a trash can...@ least that's where the last 3 boxes from late model wags I've owned seem to work best.

I wasted lots of time once trying to get one working, now I just unplug them and throw them away.

I replace them with the small 2 bulb converters available just about anywhere.
Haven't had a problem since.

Harvest the wiring & plugs from the relay box, and replace it with a modern reliable unit.

Just my .02

[Billygoat]

The relays are double pole single throw, meaning they have 2 in's and 1 out in this case. When not energized they allow the brake to come in and go out to the bulbs, when the single is activated they flop the flow away from the circuit to the single supply and out to the bulbs. I think the schematic should have a dashed line on the other side.
**edit**
Looking at the diagram again I think the signal is the normaly closed circuit and the brake activate the relay.
same process, I just had the circuits backward.
**

I built a similar relay panel for my CJ8 when the switch for the signals in the column took a crap on me.

[rollie715]

Your post intrigued me and I came to the same conclusions as you. If they are drawn incorrectly then I can only conclude that the 1990 version link was also wrong and the marked up version you posted was based on the same drawing.
This post was edited to remove an edited drawing I made which proved to incorrectly depict the relays and related post numbers.
See corrected drawing below in my next post.
Thanks Billygoat for correcting me.

[Billygoat]

nope yours is not a relay

"unactivated" post 30 and 87A are connected
"activate" post 30 and 87 are connected

Probably should have put it that way in my first post

[rollie715]

nope yours is not a relay

"unactivated" post 30 and 87A are connected
"activate" post 30 and 87 are connected

Probably should have put it that way in my first post

I believe my logic was sound, but I was not familiar with the standard relay post numbers.

With that said, I can see the desired outcome could also be reached if the connection wires were swapped between posts 30 and 87 on all relays.
Here's my new revised edit showing the reversing of wires 30 and 87 where they connect to the relays.
I did not make any changes to the back up relay, as it wouldn't have made any difference, but suspect all relays may have been shown incorrectly on the original drawing.

[iroc86]

Wow, thanks for the responses! I wasn't actually sure how many people were going to bother with this post.

I think you guys hit the nail on the head. I bet those diagrams were wrong from the beginning and the mistake carried over to the reprint. Admittedly, I don't actually have the gumption to crawl underneath the check the wires right now, but I think this is a good assumption. That blue wire in the original schematic just wasn't connected to anything, whether the relay was on or off!

Oh, and I found one other mistake in the schematic (at least on my rig): Both the '86 and '90 diagrams show a single-filament 1156 bulb used for the stop light, whereas a dual-filament 1157 handles the tail/turn. My '89 is opposite: the 1156 is used for the turn signal, while the 1157 is used for tail/stop. These old Jeeps are goofy.

The trailer module actually works great on my rig. I was looking over the schematics to see if converting to the old-style single-bulb lamps would affect the trailer wiring. Turns out it won't, because the current will pass through the relays as if the turn signals were on. The conversion is handled in the steering column and is an easy one-wire modification. A few other modules are set up the same way, e.g. Ford Explorer.

[The Colonel]

I believe my logic was sound, but I was not familiar with the standard relay post numbers.

With that said, I can see the desired outcome could also be reached if the connection wires were swapped between posts 30 and 87 on all relays.
Here's my new revised edit showing the reversing of wires 30 and 87 where they connect to the relays.
I did not make any changes to the back up relay, as it wouldn't have made any difference, but suspect all relays may have been shown incorrectly on the original drawing.

The factory diagrams for the trailer relays are wrong. I built one today from scratch and the brake lights didn't work- as mentioned. I rewired my relays to match the diagram above and everything works great. BTW- that back up relay shown is actually the tail light (or running light) relay.
I bought a 6 pin weather-pack connector and wired as shown and it plugged right in between the factory harness- no cutting or splicing of the stock harness

[iroc86]

Thanks for posting -- I'm really glad this thread helped you out!

You're right about the "backup light" relay being for the tail lights. I believe I pulled the relay names directly from the service manual, so Jeep had that wrong, too.

[Daddio]

I finally tackled my trailer light relay box so we could tow our camper last weekend. The box sort of worked, but it was very weak and the lights barely lit up with it. I started by cleaning the plug for the heavy gage red wire about a foot away from the box. It didn't fix the brightness, but it did change it to a steady power source. Next I checked the weather pack connectors and they were all fine. Finally I took the relay box off and removed the cover.

The problems I found were:
1) a bad ground connection (What, a bad ground on a Wagoneer?)
2) and the main problem was a bad circuit breaker.

I replaced the circuit breaker, ground the rust off for the ground and put it back on. It worked like a charm and the trailer lights are bright!

[710 Burner]

I had a few problems with that circuit drawing. The first was, IIRC the unit that was on my former '87 had 3 circuit breakers, one for each function. I had to replace one.

[itselliot]

Seems I'm behind everyone here...

I was googling for replacement relays for the trailer wiring box and came up with this thread.

I have 2 relays that went bad on the PO and the trailer wiring is a little hacked up. I simply need to find some relays that will plug into the OEM plugs that are inside the relay box.
Can anyone post a link to where the relays that will work with the OEM plugs might be found??

Thanks
Found this:
But I can't tell if its an exact match

[jsinajeep]

I was googling for replacement relays for the trailer wiring box and came up with this thread.

I have 2 relays that went bad on the PO and the trailer wiring is a little hacked up. I simply need to find some relays that will plug into the OEM plugs that are inside the relay box.
Can anyone post a link to where the relays that will work with the OEM plugs might be found??

Thanks
Found this:
But I can't tell if its an exact match

Found mine on ebay. Looked under relay . That looks good to me.

[itselliot]

relays

gonna give this a shot, they were $3.77ea

[blakeusa]

Relays are just electronic switches.

They are used most often - to allow the switching of power (on/off) - in this case 12v to the circuit (lights, turnsignals etc) - without having to actually run the power back and forth from the main switch (i.e. brake pedal, light switch, turn signal etc or battery. The 12v power source is there - and then is put into action when the trigger is tripped.

Download this Bosch Relay Guide. It is old and out of print - and by far is the best guide on how to use relays. Will show you how to wire a relay for doing just about anything on your rig.

http://www.dlcparts.com/images/BoschGuide.pdf

** There are a lot of very cool tricks you can do like trick starting where you gotta say (Press the brake to get the car to start) -or a hidden switch.

I don't know how to save this on the site so if someone knows how it should be in the tech archives.

The relay you show is an industry standard 4 or 5 pin automotive relay. I stay away from the Chinese brands and at least use made in Mexico. They are the same, but the 5 pins have an extra out.

Also be wary of the relays sold on ebay etc as most are not good quality and have random wire colors and pin outs.