Power to trailer while towing

[KGs2108DS]

New Lithium batt not charging during towing. Is wiring harness supplying power to trailer during towing? Using Victron with truck running, I got the following: with 7pin harness plugged into tow veh, -6W & -0.29A. With harness disconnected -14W & -1.06A. Only Refrig on 12VDC running in trailer. Seems to me given sig. difference in Watts that harness IS supplying current and power to trailer when plugged in and truck running. About 8W and 0.77A. Seems to indicate truck supplying power decreasing power drain on battery. Is this a correct conclusion? (BTW, I am having new WF-8955-AD-MBA circuit board installed in WFCO 8955PEC power center tomorrow).

[powercat_ras]

You might measure with 12 V refrigerator off. If it's 12 VDC compressor is running that alone should be pulling a lot of amps from the 12V system. Suspect at least 10 amps, maybe somewhat more. Of course there is a duty cycle between when the compressor runs and it is off, so it's going to toggle back and forth between that and less than 1 amp.

[KGs2108DS]

Thanks Randy. More importantly, does it appear from the measurements of A and W that electricity is flowing from truck to trailer battery? My concern is that last year the harness became d/c from truck during towing, bounced along pavement for about 100mi, and i have to cut and splice with butt-end connectors. All other tow vehicle powered items working just fine, but the question of whether electrical flow from truck to trailer battery/power center may not be making contact in splice.
Draw of refrig when on LP has always been very, very low. Thanks!

[Morich]

I might ask a basic question as it seems you are doing things in a very hard way! Are you trying to work this problem without a meter to test voltage and resistance?

Even the best trained folks do need the tools to do a job!

[KGs2108DS]

No, I’ll admit I have a pretty nice multi-meter, perhaps smarter that I :-)
I’d thought to do a conductivity check (ohms) from the 7pin (on the power pin) to the + battery cable in the trailer. If no conductivity, then the splice may be the problem. Sorry, I just wasn’t sure exactly how to do the conductivity test, and where the leads needed to go. I did do a voltage test w/engine running and not running, across the trailer battery posts and could not tell there was a difference in voltage. Thanks!

[Morich]

Okay, if meter is on hand, checking for continuity is somewhat easy as there is not need to worry about which probe goes where! One of the easier tests but also one big thig to avoid. Try to never stick the probes on anything that has power as it may blow a fuse in the meter!
But if you want to check for good connections, like across a splice to make sure it is good, put one probe on each side of the splice! This can be really close to the splice or as far away as you can reach!
There are often three holes in the meter to connect the probes. If there is common (COM?) one will go there and then there are often two holes left. One will be to measure resistance and voltage. You chosse this hole for the second probe and then turn the knob for what setting you want. Often there is a setting for various levels of DC voltage. Marked with a - sign meaning a steady level, not up and down as alternating.
But if you want to test alternating like 110 AC, move to the hole that is often marked with an up and down crooked line that represents the up and down voltages of the AC that we can't see without better equipment as it changes 60 times a second!
It rises to 110 positive (above the center) to 110below at 60 cycles (times) a second!
So our simple meters just look at how far away from zero (ground?) it gets. Lots of fussy high tech stuff involved but we usually don't need to know how and why it makes those decisions, just go with calling it good if it's kinda close to 110-120!

Most of the time if we use a digital meter (no needle to read!) we can leave the probes in those two holes. There is likely another hole for measuring current flow but I rarely use it. It takes moving one probe to that hole and leaving the other in the COM hole but most meter will not test above a very low level without blowing a fuse. Something low like 3 amps or so.
I don't use it enough to remember what the limit as it is so low that I rarely find it any use on auto stuff!

Digital meters make it easy to forget lots of the techy reasons for where each probe goes. Resistance it doesn't matter at all and if we are reading voltage and get the red and black reversed, it will just tell us we have negative battery instead of positive and since it comes from a battery, we can be sure that we really don't.
Unless we put the battery in backwards!

One trick to make sure we have the meter connected and working right before we really try to test voltage or resistance is to set the meter on any of the resistance scales and touch the probes together. If we have the meter set to a really high scale it may give us a crude reading like 500 ohms but if we want to know closer to how much resistance we have from one hole out to the probes and back, we can set the scale down lower and lower. If you have good probe wires and they touch to make good contact, it may come down to saying there is only3-5 ohms resistance and that is a pretty close answer as the wire does have "some" low amount of resistance!

A good way to build up some confidence in using the meter is to start with testing really safe things like a piece of wire that is not connect to anything. Then move to testing voltage on something like a 9 volt battery.