I'm hoping you guys will be able to troubleshoot this for me as I am totally baffled...Background:-In our former TV cabinet, we had a 12V outlet on the same plate as the coax jack. It didn't work. Took it out and found there were no 12V wires attached whatsoever. Bummer.-Beneath our former TV cabinet, we had the standard coax jack with the antenna amp button. No 12V outlet there. I saw the combo plate online and ordered it several years ago. But, when I removed the existing antenna amp plate the 12V wires only read ~8V on the multimeter. Huh? Abandoned that plan and just lived for a few years with no 12V outlet in coach.Current:-Getting geared up for solar install. Would like to install some 12V outlets (plus my control panel battery readout stopped working, so would like a 12V outlet to use with my cheapy 12V plug in meter until the TriMetric is installed). So, I revisited the 12V wiring situation, and found the following:--->The antenna amp plate still reads ~8.xxV (we've never used the antenna or a TV).--->The light above the sink (just a few cabinets down from antenna amp plate (maybe 5 feet max) reads 6.xxV????? The LED light I installed works.--->Decided to check voltage at charger/converter (WFCO WF8955AN) and found the ~8V circuit. Pulled the fuse and it was blown! Have no idea how/why a blown fuse was passing partial voltage. Replaced fuse and now have 12V to the antenna amp plate. Yay! Installed and now have one functional 12V outlet in coach!My Question:-Any ideas about the 6V at my light? I didn't unscrew any other lights to check them...I kind of threw in the towel for the night. All the other circuits on the charger/converter seem to be okay (i.e. match the battery output which was about 12.9V if I remember correctly).-I did the above testing on battery power only. Should I plug in and see if there is a difference? Any other ideas?

LED's will work with lower than printed voltage.. I have, not far from me, an indicator lamp assembly.. The body of the assembly says 12 volt.. The volt meter says 5, works fine.Some LED's are "Current limited" and more or less ignore supply voltage, some have built in voltage regulators and thus can take far less (or more) voltage than what you think they can.. I use a test LAMP (old fashion hot wire kind) to load test wiring.

Thanks for the input. I should probably clarify my question a bit... I'm trying to figure out why I only have 6V at that light? That doesn't seem right. What sort of troubleshooting should I be doing to track down the problem?

Is the fixture a regular RV fixture that takes classic 1156 bulbs and you have installed LED direct replacements with the 1156 style lamp base or is it a modern LED fixture and you removed the LED lamp and have tested the LED lamp socket?

It's the original fixture with eBay plug-n-play LED lights. And I unscrewed it from beneath the cabinet and am metering the incoming power wires.

What meter are you using? Checked the battery in the meter recently?

Weird 12V question for you electric gurus... | Good Sam Community

17 Replies

PaulJ2
Explorer
May 23, 2014
I believe you are seeing 6 volts because LED lights are an oscillator. Your meter is reading a chopped signal instead of straight DC.
cruz-in
Explorer
May 22, 2014
If the light (and other lights on the circuit)is on when making the measurement, I would bet the missing 6 cvolts is being lost in connections and wiring losses. It is indicative of a wiring problem.

Wire, fuses, connectors, etc. are not perfect conductors. They all have resistance. That resistance under load (the light(s) on) results in a voltage drop across the wiring, connectors and fuses. This drop reduces the voltage one would read across the load (the light in this case) to something below 12 volts.

Having said that, 6 colts is pretty low. I suspect you have a higher than typical reisitance somewhere in the circuit.

A trick I use to find these is an infrared temperature gun. Take the temp of various points in the circuit. The places it is warm/hot is where you high resistance point is.

Also I suspect if you turn nearby lights off (as they are most likely sharing the same feed wires), leaving the one you are measuring on, you will see the 6 volts increase. This is because you are reducing the current (and hence the voltage drop) thru the wiring, connectors, and fuses.
alliemac9
Explorer
May 22, 2014
Well, the mystery appears to be solved! Whoever suggested that the LED bulbs may be the culprit was correct (I'm replying from my phone, so it's difficult to check)! When the light was on, I was reading 6-7V (depending on whether the coach is plugged in to shore power, I believe). Turned the light off and I got 0V. This made me realize I was taking readings from behind the switch of the light fixture. I grabbed an old 12V bulb and put it in - and was reading 14V or so (while charging) so it's all good and I don't need to tear the RV apart looking for loose connections (thank goodness!).

Thanks for everyone's help!!
johndeerefarmer
Explorer III
May 21, 2014
Easiest way to to use your voltmeter. Several ways to do it. If you can find the common ground connection for the fuse panel put your black test lead there. Then with the red lead read the voltage on each side of the fuse. If you have 12V on both sides of the fuse, it's a good fuse. If you have 12V on one side and 0V (or any other reading other than 12V it's a bad fuse).

Alternatively you can check the voltage ACROSS the fuse. If the fuse is good you will read OV across it. If the fuse is bad you will read 12V. I know these readings sound wrong if you aren't experienced in electronics but they are correct. A good fuse has near 0 Ohms resistance and will therefore not drop any voltage- thus the reason it should read OV

The third method is to remove the fuse and check it with the meter on resistance. A good fuse should read near 0 Ohms resistance. An open fuse should read infinity (some meters show "OL", others show a "1" for infinity).

The first two methods are the best as you are testing the fuse under load (actual working conditions). I have had fuses that checked good with an ohmmeter but were actually bad- when the power hit them one end of the element would get hot and disconnect itself from the fuse. This doesn't happen often but one time is enough to make you want to do it using the first two methods. When I was an inexperienced tech, way back when I had this problem and wasted several hours troubleshooting it. I have never forgotten this and therefore use the first two methods.

Sorry if I sound like an Electronics Instructor- that's because I was. After being an electronics tech for 10 years, I taught Electronics for awhile

Good luck
alliemac9
Explorer
May 21, 2014
We have 2 12v batteries.

Can you tell me how to test the fuses? Thanks!!
westend
Explorer
May 21, 2014
Have you removed all the blade fuses and checked them for continuity and resistance?
PaulJ2
Explorer
May 21, 2014
Do you have by chance two 6 volt batteries in series powering your rig? Did someone by chance wire one of multiple wires to one battery only? Just a thought.
johndeerefarmer
Explorer III
May 21, 2014
alliemac9 wrote:
@JohnDeereFarmer - Thanks! I'll see if we have a 12V bulb to test with tonight. In answer to your question - I assume the fuse that we changed was not the same circuit as the light that is reading 6V. There were two strange 12V occurrences, and I described both, even though I believe by changing that fuse we solved the 8V at the antenna amp plate issue.

However, changing that fuse had no impact on the 6V reading at a nearby light. I'm not sure how to check the connections along the path. I do have a wiring diagram, but I guess I'll need to disassemble the fake bottom of the interior cabinets to get to the dead space? And maybe take off vent covers to see if I can see the lines in the ceiling? Blah...

If you have a wiring diagram see what else is connected to the circuit that is reading 6V. Check every other light or device on that circuit. If all of the others are low then it has to be at the source, but if only some are low it's a bad connection. If none but that one are low then I would start at that bad voltage and start working back towards the fuse.
You will have to manually trace the wires to each junction. They may have used crimp on connectors, or wire nuts. Just go to each connection and make sure it's tight. If it's a wire nut, take it off and make sure that all of the wires are twisted together and then replace the nut. If you are able to, you can stick the tip of your probes into the back side of the wire nut and check the voltage at each junction. If they used some type of connector that you can't get a reading on you can take straight pins or needles and push them through the insulation into the wire. Then take your voltage readings on the pins. When you remove the pin the hole should "heal" itself. If it doesn't just put a piece of elec tape on it.
Hope this helps.
BTW my dad taught me the "straight pin trick" about 40 years ago and I still use it several time a year.
alliemac9
Explorer
May 20, 2014
@JohnDeereFarmer - Thanks! I'll see if we have a 12V bulb to test with tonight. In answer to your question - I assume the fuse that we changed was not the same circuit as the light that is reading 6V. There were two strange 12V occurrences, and I described both, even though I believe by changing that fuse we solved the 8V at the antenna amp plate issue.

However, changing that fuse had no impact on the 6V reading at a nearby light. I'm not sure how to check the connections along the path. I do have a wiring diagram, but I guess I'll need to disassemble the fake bottom of the interior cabinets to get to the dead space? And maybe take off vent covers to see if I can see the lines in the ceiling? Blah...
alliemac9
Explorer
May 20, 2014
I'm metering at the attachment point of the wires on the light. The light has a switch on the face of it - it is not one on a wall switch.