12v charge line fuse blown from tow vehicle

[Jetstreamer]

the few times I have checked the 30 amp fuse that protects the 12 volt charge line running from the tow vehicle to the trailer, it has always been blown. So it makes me wonder why that’s happening. Must be from running the jacks down while the umbilical is still connected maybe? I’m mean unless you’re occasionally checking trailer battery voltage while connected to a running truck, you really wouldn’t know would you?
It’s making me rethink keeping the truck hooked up while running any large electrical loads such as the landing gear or slides.
Wondering if anyone else has ran across this issue or let me know what I may be doing wrong thanks.

[time2roll]

You are telling me the airline mechanic goes down to the local auto parts store and picks up a few of these short stop breakers to put in an airplane?

Could this be from a 747?



AFTER

[dougrainer]

time2roll wrote:

dougrainer wrote:
Now, if you have a Short, you do NOT install a Auto reset Breaker. ALL that will do is burn out the Auto Breaker. Doug

So you are just waiting for a problem to burn the thing out? No thanks, if there is a fault I want power off until the issue is resolved.

Have seen posts of these things fail in a closed position or burning up. Does not sit well with me as a safety device. Sure they work fine as long as there is no fault to protect.

There will always be a naysayer, no matter what the facts. I can't believe you have ever flown on a commercial airline with your negative outlook:B They have used Auto reset 12 volt breakers for YEARS on RV's. A LOT of Diesel Motorhomes have at least 30 Auto resets on there panels now. What type of breakers do you think they use on Airplanes??????? Doug

[time2roll]

dougrainer wrote:
Now, if you have a Short, you do NOT install a Auto reset Breaker. ALL that will do is burn out the Auto Breaker. Doug

So you are just waiting for a problem to burn the thing out? No thanks, if there is a fault I want power off until the issue is resolved.

Have seen posts of these things fail in a closed position or burning up. Does not sit well with me as a safety device. Sure they work fine as long as there is no fault to protect.

[dougrainer]

time2roll wrote:
I do not like auto-reset breakers except maybe for diagnostics.

Also, explain your reasoning. Yes, they are great for diag, but there is no problem using in place of fuses. Doug

[Jetstreamer]

So after a few recent road trips I rechecked the 30 amp fuse and all is well. The one change I made was to disconnect the umbilical from truck before running any big 12 volt loads.
I’m pretty sure that’s what was causing the fuse to blow and if there were any shorts in the system I would have seen it by now.
Thanks all for the support.

[time2roll]

I do not like auto-reset breakers except maybe for diagnostics.

[YC_1]

Times two on the breaker. You can get them to plug in place of the fuse.

However your TT batteries should be parallel to the tow vehicle batteries and hence draw equal amounts of current. Of course there are wire sizes and lengths etc. That said, 30 amps from each power source and actually over from the tow vehicle would put the current demand somewhere around 60-70 amps and I don't think that is the case.

I suspect you have an intermittent short or the TT batteries are very low when you connect and then start the engine.

Time to take apart each connector along the line for a good inspection.

[dougrainer]

GordonThree wrote:
I'll hold my tongue on "auto reset" atc fuses

No, explain your thought process! I have used them for 30 years and various RV makers use them both mini and regular ATC auto reset breakers for various 12 volt circuits. NO PROBLEMS. Now, if you have a Short, you do NOT install a Auto reset Breaker. ALL that will do is burn out the Auto Breaker. Doug

[GordonThree]

I'll hold my tongue on "auto reset" atc fuses

[Jetstreamer]

dougrainer wrote:
I advise customers to install a Auto reset 12 volt breaker in place of the ATC fuse. That way, when you have momentary overloads, it trips and then resets within a minute or so and you do not have to keep replacing fuses. They make Auto reset in ATC type. Doug

Thanks Doug I think that’s a Great idea.

[dougrainer]

I advise customers to install a Auto reset 12 volt breaker in place of the ATC fuse. That way, when you have momentary overloads, it trips and then resets within a minute or so and you do not have to keep replacing fuses. They make Auto reset in ATC type. Doug

[Jetstreamer]

time2roll wrote:
30 amp fuse seems light. I thought most were 40 amp. Check the owner's manual.

Clamp-on DC ammeter will tell the tale if that 30+ amps is actually getting to your pair of AGM. The way people talk about the charge line passing a trickle charge at best of 5 amps would point to a short somewhere. I think the power could very well be getting to the batteries but need to put a meter on it to know.

If the trailer battery is fully charged do you still have the issue?

I don’t have enough of a track record established with this issue so no, I don’t know.
I just thought I would check it the other day since it’s happened to me in the past but from now on I’m going to watch it to try to determine the cause. I’m going to buy a better multimeter to check dc amps.
The plug itself looks to be in great shape and I have no other issues with any trailer lights.

[time2roll]

30 amp fuse seems light. I thought most were 40 amp. Check the owner's manual.

Clamp-on DC ammeter will tell the tale if that 30+ amps is actually getting to your pair of AGM. The way people talk about the charge line passing a trickle charge at best of 5 amps would point to a short somewhere. I think the power could very well be getting to the batteries but need to put a meter on it to know.

If the trailer battery is fully charged do you still have the issue?

[BFL13]

If the 7-pin were all corroded inside, then some of the other pins would not work either, most likely. So if it is only pin #4, keep looking for the cause.