Weird electrical question, getting 8 volts?

[bpu699]

I am placing phone chargers and USB chargers into my 2000 Winnebago ultimate advantage.

Easy to do, just wiring into the 12V system.

Put several on the dash (off cigarrette lighter wiring).

Several on the passenger wall (off the wall lights).

Tried to do the same on the drivers slideout. Wired to the wall lights, wouldn't work. Weird. Checked voltage, and it was 8 volts?!

Both chassis and house batteries 12.8v+. All other 12v lights, at 12v+...

But, lights on slide out at 8v. They work fine as they are LED, and you cant tell its a lower voltage by looking at them. But the USB charger needs 12+ volts.

How in the world is it 8 volts? No other lights on in the RV. Nothing else running. No light dimmer switch that I have ever seen. No wiring in series that I see that might lower the voltage.

How is that possible??? Is there some kind of voltage regulator? If so, why just for those 3 wall lights???

Only other electrical thing on the slideout is the microwave, that's off 120v, and works fine...

[Persistent]

As you suggested, it must be something in series. Maybe a corroded or loose connector. Maybe a crack in insulation followed by corrosion. Maybe loss of ground and now passing through another appliance.

[cdc76]

It’s possible you have a bare wire slightly grounded somewhere and that’s where the other 4 volts are going. You could eliminating some of the lights on the circuit and see if for some reason one of them is introducing resistance into the circuit.

[Unplanned Tourist]

Try running a separate ground wire. Bad grounds do strange things.

[bpu699]

Hmmm. Good ideas so far. Like the idea of a new ground. It is on the slide out, so perhaps a grounding strap somewhere...

[cdc76]

Originally Posted by bpu699

Hmmm. Good ideas so far. Like the idea of a new ground. It is on the slide out, so perhaps a grounding strap somewhere...

Keep in mind, until you eliminate the grounded supply wire, adding a ground strap won’t fix the problem. You could cut the supply wire at a convenient location and attach a 12v battery to it and help narrow the problem area. If it is no longer grounded after you splice the battery into the circuit then you know you’ve cut the ground out of the circuit. if nothing else, run a new supply wire if it’s not too big a deal.

[bigb56]

My vote is for a bad ground (negative). When there is a poor or open ground connection it can cause downstream loads to back feed thru the upstream loads creating a series circuit as the current finds it's way back thru the loads. It shows up as dimming on lighting loads Run a temporary ground (negative) to the loads in question to prove.

[cdc76]

To check for a grounded circuit unhook both ends of your circuit from the termination points and use a multimeter to check from one end of the the circuit to ground. Meter should read OL or .1. This number needs to be non existing or very low. A high resistance means that circuit is touching ground somewhere.

[Domo]

Originally Posted by cdc76

To check for a grounded circuit unhook both ends of your circuit from the termination points and use a multimeter to check from one end of the the circuit to ground. Meter should read OL or .1. This number needs to be non existing or very low. A high resistance means that circuit is touching ground somewhere.

I didn't understand this, sorry.

If you disconnect both ends of the pair of wires (positive and negative/ground) from the appliance AND from the source (battery or last connector in-line), they should be floating. If a reading from either wire to "real" ground is 0 or very low that would indicate that wire has a short to ground somewhere along it's disconnected length. A high resistance would mean it is not shorted to ground.

As I said, I didn't understand the instruction.

[bigb56]

Originally Posted by cdc76

To check for a grounded circuit unhook both ends of your circuit from the termination points and use a multimeter to check from one end of the the circuit to ground. Meter should read OL or .1. This number needs to be non existing or very low. A high resistance means that circuit is touching ground somewhere.

A LOW or zero resistance means a short or a heavy load, a high or infinite resistance means little to no conductivity. A moot point anyway for the ground (negative) wire since if it shorted to ground on a 12 volt 2 wire DC system it wouldn't affect the circuit operation. If the positive side shorted to ground it would normally take out the overcurrent protection device, a partial short is almost an impossibility unless it is passing thru a load with enough resistance to keep the current below the trip point of the OCPD, then it becomes a series circuit like I described in my earlier post.

Simplest and quickest way to check is a temporary wire from negative to the connection point where the low voltage problem exists, if that fixes it you know the ground (negative) path is poor and you can route a new wire from a known good location.

[BigSkyBob]

Try holding the switch down in the on position. My lights will dim or brighten by holding the switch down. Unsure if your system is similar.

[cdc76]

Originally Posted by Domo

I didn't understand this, sorry.

If you disconnect both ends of the pair of wires (positive and negative/ground) from the appliance AND from the source (battery or last connector in-line), they should be floating. If a reading from either wire to "real" ground is 0 or very low that would indicate that wire has a short to ground somewhere along it's disconnected length. A high resistance would mean it is not shorted to ground.

As I said, I didn't understand the instruction.

No, leave the appliances hooked up. Unhook the positive and negative from the battery. Ensure neither end is touching anything. Make sure you have your meter set to read ohms. Stick one lead to either wire (positive or negative side) and the other lead to a known ground. (A good paint, rust free spot of trailer frame should do it) This should be OL (open line) or will possibly 0.1 -0.5 (if it’s higher than I would call it a grounded circuit.) any reading other than OL means the ground and the wire are sharing a path somewhere. when I take lunch I will sketch up a quick diagram of what I’m trying to explain and post it.

Or like someone else mentioned if you have spare wire you can just connect a new wire from your negative terminal to the appliance and see if it corrects it.

[Archer2]

Just FYI USB voltage is 5V, not 12.

[bigb56]

Originally Posted by Archer2

Just FYI USB voltage is 5V, not 12.

I'm sure the OP is using pre-made USB assemblies that connect to 12 volts and produce 5.