What tool for 12v troubleshooting?

[Steverino]

Hoping someone can point me in the right direction here... Our motorhome has 12 volt light fixtures in the basement compartments. Several (but not all) have stopped working - the bulbs are fine, and I think it is a bad positive wire or connection somewhere. The problem is that there are no schematics and I have no idea how to efficiently trace backwards from the lights to the source. All wires are wrapped up in plastic loom which makes it seriously difficult - one could unwrap the wires, but as this is a 36' rig, that's a whole lot of work and/or guessing.

Is there a tool / tester that one can use to "tone" out the circuit (i.e. clip on at the bad light and then "wand" over looms or wires in various places to find the circuit)? I have seen some things on Amazon, but reviewers say that if wires are bundled there are lots of false readings.

Thanks for any help!

 

[kbeefy]

Power Probe ECT.
I have the 2000, it takes some practice. I have no experience with the 3000.

 

[Steverino]

Thanks @kbeefy - I'll check it out!

 

[mrjaw14]

Basic power and ground tests are in order. Have you made sure all the fuses are good? The issue may be in the wire loom, but I would bet that the fixtures are daisy chained and a failure at one point right at a fixture or other shared point is the cause. Do the fixtures share a ground circuit, or are they grounded to chassis individually? Issue can only be an open circuit(ghost voltage/no continuity), short to ground (true 0.00 volts), or high resistance. A multimeter can help you check for the first two easily from the fixture with it switched on. A test light will help with the third. In the case of high resistance, a multimeter may show 12v but the light won’t light. without current flow there’s no voltage drop. That’s why I advocate for an incandescent test light. it‘ll load the circuit enough to see if there’s an issue passing current. Don’t bother with resistance checks. A single good strand will show continuity but the circuit won’t pass current and bulbs won’t light. A loaded voltage drop test is good, but you’d need to understand more theory to make use of it and may need longer than standard test leads since I like to go straight to battery. I would be willing to bet the issue is at a corroded or loose/bad connection at one fixture. start at the fixture closest to the 12v distribution panel for positive leg. Usually wiring is really easy to determine without a diagram on RVs

 

[Steverino]

Yep, done some of the basic tests with a multimeter, but I hadn't thought of your test light / resistance test idea. Thanks for that!
Problems include not knowing where the 12v source actually is - there are multiple fuse panels for the house and the chassis. Could be any of them, as the circuits are not well labeled (surprise, surprise). So it is anybody's guess which light is closest to the source. I was hoping with some use of a circuit finder-type tool, I might be able to get a better picture of where the wires are running and narrow things down.
I agree - a corroded or loose connection is my suspicion too.

 

[joshmodelskidoo]

Check your coach fuses not the chassis fuses. They should be pretty close to where the shore power comes out/hooks up. I have seen them under beds, refrigerators and under sinks. A regular test lite will probably do the job for you. I have gotten away with a test light most of my life. If it’s bright you have lots of power and if it seems dim its low on power. It’s not scientific by any means but it has worked for me in the past.

 

[RTM]

I love my 12v idiot light* for car and trailer light trouble shooting. The load, as mentioned above, beats a meter in so many ways.
The nice part about the load, you can hook up to the battery, and push voltage to things, and if you hit a ground, no biggie, it just turns the light on. If you hook up to a good light, your probe will dim a bit.

* used to belong to grampa, incandescent bulb, not LED.

 

[mrjaw14]

The issue with wire tracker you may find is that:
1. They bleed to adjacent circuits through induction which will send you on a wild goose chase.
2. if any part of the circuit is shared, all the wires carry the tone. Tone generators usually are two wire, and for longer circuits you have to use both. Short runs you can leave the ground off to tame it a bit, but the tone gets weak fast and you lose it. With two wires of the tone generator hooked up, because the coach Ground is shared, every bulb on the chassis may pass the tone through the filaments back to positive bus which is usually worse than issue #1 because the tone will appear almost everywhere. I have several tone generators and I’ve given up using them for most automotive applications. I see your delima, and it’s up to you if you want to spend the cash and risk it not being usable in your application.

Can you remove the lights and inspect the connections? You might also try removing the battery and back feeding 12v to a non-functioning light Similar to a power probe. If the lights are chained, you’ll see a bunch light up, and if you’re lucky find one that still won’t light. You might get an idea of where the issue is, but use a fuse and be cautious. I’m assuming your circuit can’t pass current or is open, but if the fault disappeared your coach may try to power up. Only use that if you’re sure the circuit is dead and isolated to find where the issue is (likely) not to aid in testing the lights and eliminating runs of wire between them.

if you have slide outs and you suspect a wire loom issue, it’s usually at the ends of the slide out or at a flexing joint. May try to energize the lights and operate the slides and see if the fault changes.

 

[Bubba Fett]

It's gonna take a while, I'm afraid.

This might be helpful:

Amazon.com

 

[The Cobbler]

first I would do is determine if it's a ground or supply issue. don't rule out running a new circuit to the lamps. might be easier in the long run

 

[Jlarson]

We've got a couple voice/data toners and even a real 600 volt live circuit tracer but I've never had much luck with them on heavy trucks/equipment. Usually a Fluke, power probe and a good incan test light are my go to 12 and 24 volt t-shooting tools along with a lamp for load test/short finding.

The 12 volt house lights on most coaches and trailers are run off the fuses in the converter. Probably grounded to the frame, single power wire run up to a switch somewhere? Start at that known control point and work both ways on the circuit.

 

[theoldwizard1]

Hoping someone can point me in the right direction here... Our motorhome has 12 volt light fixtures in the basement compartments. Several (but not all) have stopped working - the bulbs are fine, and I think it is a bad positive wire or connection somewhere.

More likely a bad ground at/near the bulb.

12V incandescent (not LED) test light is the best tool.

 

[2ndGearRubber]

Wire diagram and a test light. Use some 30ft retractable test leads to connect straight to the battery.

 

[qqzj]

delete

 

[Steve W.]

I'm with theoldwizard on this one. I used to work in an RV shop and have owned a few trailers, and have found that simple and basic are the best. Just use a test light. Does not really matter if it's incandescent or LED.

Start at the fuse panel, make sure that power is going out to the fixtures. At a fixture that is not working, clip your test light to a good ground, touch the probe point to where the wire comes into the fixture. If it lights, you have power. With the fixture's switch ON, touch the probe point to the ground point in the fixture. If the light still comes ON, you have a ground problem.

.

 

[2ndGearRubber]

^ LED will not load the circuit. No more valuable than an unloaded meter reading.

 

[californiamilleghia]

a small 12v horn with clips ,

one to ground the other to the wire you are trying to trace ,

a trouble light is nice if you can see it , but I like the horn for testing turn signals etc

 

[Steverino]

Wow! Thanks for the replies everyone - really appreciate all of the ideas. Time to roll up my sleeves again and give it one more shot...

 

[Steverino]

Quick update: Found an incandescent test light in my electrical drawer - forgot I had it!
Confirmed this is definitely a 12v positive issue, not ground. Unfortunately so far, each bad light pair of wires disappears back into a much larger "trunk" bundle / loom that runs along the chassis. Still trying to locate where they get power from... And I wonder where my time goes

 

[Wrench97]

The manufacturer has to have made a wiring diagram for the unit.
Are there different wire colors or numbers on the wires?

 

[Steverino]

Nope, no diagrams available. This is not unusual in the RV world, especially in recent years. Older stuff, sometimes you get lucky. I am using colors to try to follow, but because all looms / bundles are run inside of the black corrugated covers it is very difficult.

 

[kbeefy]

The issue with wire tracker you may find is that:
1. They bleed to adjacent circuits through induction which will send you on a wild goose chase.
2. if any part of the circuit is shared, all the wires carry the tone.

^^ I agree. Very frustrating. I'm about 50% success with mine.

You might also try removing the battery and back feeding 12v to a non-functioning light Similar to a power probe.

I like that idea. Use a test light or Power Probe and see if you can find connections between non-functioning lights.

 

[shawhite]

Quick update: Found an incandescent test light in my electrical drawer - forgot I had it!
Confirmed this is definitely a 12v positive issue, not ground. Unfortunately so far, each bad light pair of wires disappears back into a much larger "trunk" bundle / loom that runs along the chassis. Still trying to locate where they get power from... And I wonder where my time goes

Just curious how you determined it to be a 12v power issue?

 

[RTM]

Take notes. I helped a buddy with an RV once, and our notes as to what went where served him well 5 years later after I left the area.

@shawhite Usually you know its a power issue when you have a good ground at the dead lights.

 

[shawhite]

Take notes. I helped a buddy with an RV once, and our notes as to what went where served him well 5 years later after I left the area.

@shawhite Usually you know its a power issue when you have a good ground at the dead lights.

Yeah I understand that but the OP never said he verified the ground. your test light will act the same if you have no voltage or a floating ground.

 

[garfunkle24]

a small 12v horn with clips ,

one to ground the other to the wire you are trying to trace ,

a trouble light is nice if you can see it , but I like the horn for testing turn signals etc

I use a 12v back up alarm the same way, bit less obnoxious than a horn. Hook it up and flex the harnesses etc to find the fault.

 

[Wrench97]

We used low air buzzers from trucks, made noise and even less obnoxious

 

[Milton Shaw]

How about just running a new supply line to where you can access the wiring and repower the whole circuit. Make sure you are using at least an inline fuse. Problem solved, since they probably all tied together, just one power feed will power it up.

 

[Steverino]

How about just running a new supply line to where you can access the wiring and repower the whole circuit. Make sure you are using at least an inline fuse. Problem solved, since they probably all tied together, just one power feed will power it up.

That's actually a great idea. I've been so focused on finding the problem (cause I really like closure ) I think that may well be the direction I go... Thanks!

 

[Steverino]

Just curious how you determined it to be a 12v power issue?

Well, I supplied 12v from another source to the fixture and the fixture light worked... Therefore the fixture ground is fine. This worked the same for two non-working fixtures that I tested.

 

[shawhite]

You can use a telecom tone and amplifier as a cheap circuit identifier but don’t hook it up to ground or you will pick up the tone everywhere as all the grounds are tied together. Once you know the route of the wires you can test continuity on each section. You may also be able to locate the source.

 

[Steverino]

Thanks for the telecom idea - gonna keep that in mind for future.

Today I jumpered from a good 12v source to one of the light fixture positive leads. As predicted, fixed the problem. All of my bay lights have been switched to LED, so the additional draw on whatever circuit I tapped will be nominal. I did make sure that the source was on the house side (not chassis) and was switched by the power disconnect. I did not want to accidentally bridge the two systems and have it try to backfeed from one to the other with the jumper. Viewing this as a temporary fix, though in reality I may never get back to it since there are always too many projects calling my name...

Thanks again everyone!