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Mark7RV · 05-29-2023, 04:52 PM

2006 Monaco Diplomat, ISL 400 Cummins
Was driving in the motorhome towing a 20’ tandem axle cargo trailer with 4 wheel electric brakes. Have done thousands of miles over the years with this motorhome and trailer together. Had just left home on a 1,000 mile trip and made it about 30 miles.

I was going down a moderate downhill with the engine brake on at about 50mph. Suddenly the dash goes dead, no power, no power steering, engine off. Luckily I was in a spot where I could pull safely off the road. Tried restarting but turning the key did nothing. Not a good feeling. Started checking everything. All batteries and connections good. Generator starts and runs. Full tank of Fuel. Then I found a blown 5 amp fuse labeled “eng/tran/ign” in the exterior bay under the driver seat, in a small black fuse box called “Monaco 2”.

I replaced the fuse with another good 5 amp fuse, and it started right up. Continued driving again and went about 10 miles until I needed to use the engine brake again. Then same thing – coach went dead, had to pull over again. Replaced the same fuse again with a 10 amp (all I had with me) and it started again and I just drove back home carefully without using the engine brake and it ran fine and I made it home without any problems.

When I got home I noticed that one wheel of the trailer had the electric brake wires broken and shorted out. I disconnected the trailer, and took the coach out by itself for a test, using the engine brake repeatedly and everything was fine.

So the coach seems to run fine now, but my wife is leery of heading out again without some solid resolution to the issue.

I have a Tekonsha brake controller that has been in there for years and always worked fine. When the engine brake (a PacBrake unit) engages, it applies the trailer brakes and automatically downshifts the Allison transmission. Coach has always been reliable and ran great since I bought it new. Have about 76,000 miles on it.

QUESTION - Is it possible that a short in the electric trailer brakes could blow that “eng/trn/ign” fuse when the engine brake is activated?

Please help with any input or ideas of what else to do or check. Thanks

Arch Hoagland · 05-29-2023, 05:02 PM

Is the 10 amp fuse still in there or did you replace it with a 5 amp once you got home?

rpaldrich · 05-29-2023, 05:29 PM

First -- Occam's razor states that you probably found the issue and that once corrected you'll be good.

Second -- sounds like a weird wiring thing but perhaps when your trailer was shorting out your brake system, it was also shorting out that fuse.

Mark7RV · 05-29-2023, 05:34 PM

Still have the 10 amp fuse in there. I will go get another 5 amp and put it in and see what happens.

gsgriffin · 05-29-2023, 06:12 PM

When I first got my 04 (used), I found that when I was driving at night, my pacbrake would be stuck open and I lost all power...barely enough to move since compression was just being blow out. With a friend looking the wiring, we thought it had the possibility of being a bad circuit in the taillights. Sure enough, found the rear passenger light socket corroded. Replace it, put in dielectric grease, never had another problem. Added 70k since then and still no problems.

There are connections between the breaks and lights in the circuit.

gsgriffin · 05-30-2023, 05:48 AM

Quote:

Originally Posted by gsgriffin

When I first got my 04 (used), I found that when I was driving at night, my pacbrake would be stuck open and I lost all power...barely enough to move since compression was just being blow out. With a friend looking the wiring, we thought it had the possibility of being a bad circuit in the taillights. Sure enough, found the rear passenger light socket corroded. Replace it, put in dielectric grease, never had another problem. Added 70k since then and still no problems.

There are connections between the breaks and lights in the circuit.

If you want to confirm, for the benefit of your wife and bring peace to her travels, go ahead and purposefully ground the wiring you found and see if it blows the fuse again. If you blow it with the grounded wire and don't blow without the ground, you've confirmed it.

Grounding a positively charged wire SHOULD blow the fuse. That is exactly what the fuses are for. Everything connected to that circuit will cease to function....on purpose. In this case, you have identified where the ground is, which is often the biggest challenge in a rig this size and with wires hard to reach throughout most of the vehicle. You should consider yourself VERY lucky that is was fairly easy to spot and fix.

153stars · 05-30-2023, 06:43 AM

I don't believe the engine brake circuit should drive the trailer brakes. It doesn't seem like a 5amp fuse could handle it from day one anyway. Unless some pilot relay and the short cooked it shorted as well.
Not sure if that is any help at all. Glad you were able to get off side of road and home.

RichardE · 05-30-2023, 07:14 AM

IF, and that’s and IF, your coach is one that activates the brake lights when the engine brake activates, then you have found the culprit.

The short is in the brake light circuit as you have identified. Good detective work on your part. Without having a schematic of your coach, it’s hard to explain exactly how that would blow the fuse it did.

Easy enough to check in the driveway. Identify the brake light circuit at the trailer connection. Short the brake light connection to ground. Crank the coach, press the brakes.

YC1 · 05-30-2023, 07:36 AM

Great ideas on shorting the wire in the driveway. With a 5 amp fuse in place.

The short might not have been sufficient to blow the 10 amp.

I would look carefully at where the brake controller picks up power. Whoever installed it could have picked out a wire just willy nilly that has ignition controlled power.

This would be easy to confirm with a voltmeter and then removing the fuse to see if the voltage goes away.

The broken trailer brake could just be coincindental. I have too often seen more than one problem while diagnosing an issue on a vehicle like this. But shorting out that wire you suspect might be the culprit should eliminate that.

jacwjames · 05-30-2023, 07:44 AM

Quote:

Originally Posted by 153stars

I don't believe the engine brake circuit should drive the trailer brakes. It doesn't seem like a 5amp fuse could handle it from day one anyway. Unless some pilot relay and the short cooked it shorted as well.
Not sure if that is any help at all. Glad you were able to get off side of road and home.

I looked at my chassis wiring diagram and front #2 Monaco fuse box. I have the same 5 amp fuse. From what I can tell it goes to the Cruise Cancel relay but also is associated the the brake signal and the PacBrake Cruise Cancel relay.

But my chassis wiring diagram does not show a trailer or tow wiring harness/hookup.

My house wiring diagram does show the tow harness, but simply shows the brake circuit tied into the rear brake lights, turn signals etc tied into the 6 wire harness located at the hitch for tow hookup.

On my rig I have a 6 wire tow harness, it does not have a (+) charge wire. I was chasing a problem and found that the tow harness wiring was a short ~5' pigtail taht plugged into another 6 wire plug but this was all buried in a bundle of wiring loom and I never went any further.

So my guess is that the wiring damage on the trailer could have caused the fuse to blow by back feeding power through the harness.

With the trailer harness disconnected and the fuse doesn't blow that pretty much confirms it. But I'd check to make sure the cruise and PacBrake work and not other fuses in that circuit are bad.
I would also go back to a 5 amp fuse to protect that circuit.