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Trailer/Toad wiring
I’m finally getting ready to pull a toad with our bus. I have a 6 pin connector below the rear bumper and a box labeled “Tow Relays” in the engine compartment. Did the ’97-99 XLV come prewired for towing from the factory?
I hooked up portable tow lights with a 6 pin to 4 pin adapter plug but I only got the turn signals to show not the brake lights. My guess is the culprit is somewhere in the “Tow Relay” box. I’m using a set of portable tow lights to rule out any ground issues between toad and bus. Does anyone have information on how to trouble shoot the “Tow Relay” box?
thanks, Gerald
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13 Replies
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Gerald,
The box in the rear allows you to change the tow settings. You have to set it up based on your toad lights, single amber for turn, red for turn, combined stop/turn, etc. I can’t tell you how to set yours up, there seem to differences from year to year with the chassis. Best bet would be if someone with a like bus could jump in.
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I found it easiest to verify coach wiring by CAREFULLY putting the pin on my multitester on the pins inside the plug at the rear of the bus to verify power to them for lights, signals and brakes. I then diagrammed it since I had verified which pin was hot and when.
I use the word CAREFULLY in caps because if you accidently hit the probe on the multitester to the case (ground) while it contacts a pin you have created a short.
And Tom has it correct. On my last two coaches I had a switch to go from one turn signal / brake light system to separate signal and brake lights. I don’t know if it was Prevost or Liberty that provided the feature.
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Yes, it sounds like your jumper is set for separate turn bulbs and brake light bulbs, like the coach uses. Most standard trailer wiring schemes call for the left and right turn signal wires to also be the brake wires. As I recall, this jumpre is labelled. On my coach, which is a 99 CC and 98 shell, the jumper and relays are in the big electrical panel above the engine.
Take a look, if not intuitive, let me know and I’ll go check.
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Finally had the coach at home to check into the missing brake lights on the trailer harness. I opened the electrical box labeled TOW RELAYS above the power steering fluid reservoir. See attached picture.
There is no obvious (too me anyway) way to set up the harness to combine turn and brake signals. I hear relays clicking when the turn signals are on. There is no sound when the brake pedal is being pressed.
I checked the relay labeled RB (rear brake). It seems fine. I checked the tow harness breakers/fuses in the main rear electrical box. They all seem fine.
There are two wires in the tow relay box that are just capped with wire nuts. I did connect them (with an inline fuse) just to see if this is the jumper I was looking for. No effect.
So, tow breakers are fine, tow relays are fine but no obvious way of switching between separate and common brake lights plus two mystery wires…..what am I missing?
Thanks for any pointers, Gerald
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Gerald,
Now that I have seen the photos it leads me to believe the trailer plug and associated relays and wiring are converter installed.
All my coaches have had a box from which the wiring to the trailer plug originated. The 87 coach did not have a switch to convert from separate brake / turn lights, but the 97 and 06 did have a switch that allowed either way to power the brake / turn lights. My set up does not look like yours so I think your answer needs to come from either Marathon or someone who has access to the Marathon wiring diagrams.
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Gerald,
Here’s a guess. Yellow and Green are the wire colors for the right and left turn signals. I would check the capped wires to see if they are the combined brake and turn signals. I checked mine and thought it was Prevost installed given its location, but discovered it was a Country Coach board.
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Gerald what happens if you connect a multimeter to the yellow and green wires and you hit the brakes?
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Thanks for all the ideas! It looks like the #2 connector is where one configures the tow relays. Connector #2 has 10 positions and 10 wires going to it, Wires and positions are labeled.
Current setup: #7 and #9 wires are capped and not connected. The #10 wire is on the #9 connector position and the #8 wire is on the #7 connector position. Connector positions #8 and 10 are open. All other wires are on their default position.
So my “smart” idea was to connect up all the wires the way the manufacturer probably intended them to be by default. That is, #x wire to #x connector position. Well…. this gave me brake and turn lights on the right side but nothing on the left/driver side. Close but ….
I’ll have to give Marathon a call and see if they can shed some light on this.
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One of the four 20A diodes in the tow relay board was bad.
The towing harness is protected via 15/20A thermal fuses but the 20A diode was still overloaded. I guess that the thermal breakers allow more current to pass for a while. Enough to kill the diode over time.
The CoachPro guys in Eugene said that the tow package wiring is broken/misconfigured on most of the buses they see.
CoachPro also found the problem with my entry door solenoid and door bell (wires pinched in hinge-common problem, seems a design issue) and lack of air horns (corrosion in solenoid valve).
Overall, a productive half day at the shop.
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Why the converters spent time to create an overly complex tow wire adapter is beyond me. None of the 4, 6, or 7 pin wiring standards allow for separate stop light wiring and turn signal wiring, referred to as 3 wire systems. The bus has a 3 wire system where it uses yellow lights for turn signals and red lights for stop lights. For less than $50 you can buy a self contained 3 to 2 wire converter. I guess the converters didn’t know that. BTW, if your toad has separate turn and brake lights you can install a 2 to 3 converter in the towed vehicle and still comply with 6/7 pin wiring standards.
Anyway, it’s good it’s fixed.
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Agree it could be done simpler. The advantage of the relay boards is that it completely separates the tow lights from the vehicle light system. Power drawn by the tow comes from the battery/alternator not the vehicle light harness. Sound engineering. The only flaw I see is that the thermal breakers in the tow circuits are too slow to protect the diodes in all cases.
Turns out our vintage Marathon actually came with two connectors, the European 10 pin for separate brake and turn and the US standard 6 pin for common brake and turn. That is why our box was not user configurable, it was set up for both type of cars from the factory. Along the way someone removed our 10 pin connector. Subsequently someone “customized” the 6 pin connector for a European car. Then I came along wanting to tow a US car….
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If easily changed, change the breakers. 10A is well more than any single lighting circuit draws.
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One problem all of us that buy used coaches faces is owner made modifications can be very puzzling.
I made a number of changes to my first coach, and while I recorded some of the information about those changes in my log books where I listed all my maintenance, I didn’t go into a lot of detail. So on my second coach when I made not-so-obvious changes I not only logged what I did, but I added some detailed descriptions of the changes on labels or cellophane protected papers located near the alteration.
That helped me remember the details several years after I made the change, but subsequent owners of the coach at least will know what was done and how to restore it to the original condition if they choose.
I had to modify the plug for the toad on my first coach because I used separate stop and turn lights, and while it was a simple change I now can see how someone could be scratching their head because it was not like that originally and except for a note in the logs there was no detail about what I did or how I did it.
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