Blowing Fuse When In Reverse

[CVD]

I have one fuse for turn signals and back up lights, which is blowing when I put it in reverse (takes 2 – 3 seconds, I then hear a faint pop). I’ve started looking at the wires coming out of the shifter box, trying to figure which carries the reverse signal (which would be where I’d look for something causing a short).

However, thinking more about it, aren’t the reverse and neutral sensors (NSS) physically on the transmission? If so, my problem isn’t in the wires coming from the shifter (by the dash), but on the transmission (or somewhere between the transmission and back up lights)?

Either way, any suggestions on tracking down this problem? Coach is on 98 Freightliner chassis, ISB engine, MT643 trans.

Thanks,

[gfs1943]

On the round 6-pin plug, the center pin is often the backup light pin. At least, that's the way the two motorhomes I've owned (one Itasca, one Monaco) are wired. There is no aux power wired to mine. I've often thought of disconnecting that backup light wire, and wiring aux power (for toad battery charging) to the center pin.

If this doesn't help, please disregard. And a big thanks for posting your solution; that always helps others to find and fix problems.

[CVD]

Update – I did a number of things, one of them seems to have eliminated the problem (two different 10 min test drives, reverse lights now work, and the fuse doesn’t blow).

I took apart both tail light housings. No rust, no frayed wire insulation, no loose connectors, the push-in bulb receptacles looked fine, bulbs looked fine (filament intact).

I noticed the ground pin on the tow plug (on RV bumper) was pushed in; was not the same length as the other 5 pins. Don’t understand completely, but replacing that tow plug seems to have fixed my problem. That ground pin must have been hitting some other circuit and popping the fuse when in reverse. The odd thing is this coach has 5 wires going to that round 6 pin plug (L signal, R signal, tail, aux power, & ground), no reverse circuit. Again, don’t understand, but doesn’t matter – seems fine now.

Thanks for the suggestions.
Cliff

[DSDP_Don]

I would take a look at your trailer harness at the hitch. It should have a wire in it for the back up lights. Typically the back up light wire is purple. Remove it and see if the issue still persists.

[tropical36]

MrWizard wrote:
why do you think its a switch

i would open the backup/reverse lights and check the lamps and wiring at the lights

(2-3) minutes in reverse, sounds like a light full if water to me

I agree and for disconnecting the light fixtures first, even if it is on a bet.

[enblethen]

Do you have a camera that comes on in reverse?
I would remove all the reverse lamps first. Inspect them and sockets. Replace the lamps while you have them opeen.

[CVD]

Thanks for the suggestions.

The reverse light bulb & sockets look fine (no water or rust, filament intact), I’ll try pulling them both out, then replace one at a time to see if I can narrow it down.

Then, will try to trace the reverse signal wire from the trans back to the back-up light, looking for damage that could cause short.

I do not have a back up alarm. I think the only thing the reverse circuit does is activate the back up lights. Given I flat tow 90% of the time (and don’t use reverse), I can live without back up lights. I just need to fix the problem so my turn signals don’t go out.

Had to put RV away; will pull it out after holiday next week and investigate.
Cliff

[hoosiermark]

why not pull the fuse and put a tester on it to see if there is a grounding issue. That is the first simple way to check it. It sounds like there is a ground issue and when reverse is activated, the lights come on and it blows the fuse. I have found it does take a few seconds at times to blow a fuse. I would bet the wires have rubbed something and are bare. How do I know? I probably pull a trailer behind my truck twice a week and have dealt with mice, road salt, vibration, etc all causing a simple bare spot. Yes I know he has a class A but light wiring is pretty much the same. Fixed one trailer 5 times before I finally rewired it and found I had a mouse apartment house in the trailer frame and I was catering their meals with the wiring.

[ROBERTSUNRUS]

🙂 Hi, do you have a back-up alarm?

[Executive45]

I'll chime in here and second that it's most likely in the light circuit and not a switch. Easiest way is to start removing light bulbs. If you don't want to keep replacing fuses, remove ALL the bulbs and start putting them back in one at a time until the fuse blows. Bulbs being cheap, you could also just replace them all and be done....BOL .. Dennis

[ScottG]

TNGW1500SE wrote:
Take the bulbs out and see if it still blows. If not install a new bulb in one side. See if it blows. If not replace other side bulb with new bulb. May be that simple.

Or maybe replace them both with LED bulbs!
Just a suggestion!

[TNGW1500SE]

Take the bulbs out and see if it still blows. If not install a new bulb in one side. See if it blows. If not replace other side bulb with new bulb. May be that simple.

[ScottG]

The trans switch usually just connects the wire to ground, completing the path for the negative side. A failed switch usually means either no b/u lights or they're always on.
So it's probably not the switch.

Has someone connected bigger, brighter b/u lights? If so they could be too much for the circuit.

[MrWizard]

why do you think its a switch

i would open the backup/reverse lights and check the lamps and wiring at the lights

(2-3) minutes in reverse, sounds like a light full if water to me