Am I Supposed to See 12V at the Tail Lights?

If you are using a digital voltmeter and no bulb in the socket your voltage measurements may be useless. Sorry for the bad news.

Without a load (bulb), in the circuit you cannot determine if you have a bad ground side. Yes the input voltage is a bit low but if it goes even lower with a bulb in place that is an issue too. Not sure why you have a green/ground and a white/ground wire. Is this a dual filament circuit? Be absolutely sure you are using the same bulb as in the side that is working.

A digital voltmeter can read full scale and not have enough power to run a bulb once installed. I like to use a cheap 12 volt test light as a quick backup. It is a simple go, no go device and visual.

dougrainer wrote:
WHAT is the voltage at the front 7 way Bargman. What is the voltage at the other Tailight or any of the clearance lights? Garbage IN Garbage out. If the input is below 12 volts, then you have something wrong with your truck/tow supply. The VOLTAGE at all your lights comes from your Tow vehicle. NOT from the RV. You should have at minimum 13.1 volts with tow vehicle running at all lights. Doug

I'm with Doug on this one. You need to use your meter with someone's foot on the brake and measure the voltage from green (brake/turn hot wire) to white (negative/ground).

If you have the same voltage at that point, then while the brake is still held down measure the voltage on the other side of the camper.

If it's the same there, then go to the front at the output of the truck and when disconnected, put your meter leads into the proper places on the plug and if you still have the same there you are getting closer to the problem.

With electrical things, first you prove you actually have the problem you think you have, then you systematically troubleshoot either forward or backward until you isolate the specific area where the problem really occurs.

WHAT is the voltage at the front 7 way Bargman. What is the voltage at the other Tailight or any of the clearance lights? Garbage IN Garbage out. If the input is below 12 volts, then you have something wrong with your truck/tow supply. The VOLTAGE at all your lights comes from your Tow vehicle. NOT from the RV. You should have at minimum 13.1 volts with tow vehicle running at all lights. Doug

was the truck alternator spinning?
is the trailer battery fully charged?

"There is no negative, only a ground"


Perhaps defensible but in an alternating current setting, where a zero crossing point is referenced. Also technically involving when "0" battery volts becomes peak inverse negative during a transient voltage event. I interested, read the following: An automotive DC transient voltage event of greater magnitude may involve a positive oriented peak of say, 150 volts, and a negative oriented gulch of (please don't ask why) 80 volts. Any semiconductor exposed to such a transient gets whacked from both sides of "0".

I use the term chassis negative to denote a connection to a minus supply voltage while avoiding the term "ground" which I reserve for earth ground.

However for the sake of simplicity and comprehensibility, in a DC automotive circuit, ground means battery negative.

The above is only useful for nerds :D

Back from my appointment and have set up my meter to test this circuit again.

Status: (1) Ridgeline and 192RBS connected via 7-pin bargman; (2) truck and trailer NOT hitched; (3) shore power disconnected; (4) running lights powered up at truck's dashboard.

Picture of exposed wiring:



Note: Black wire is "hot;" Green wire is "negative;" White wire is ground

Applying my meter's probes to the Black "hot" and White "ground" wires I receive a measurement of 11.42V. (Edit): I just received a call from the tech at our Forest River dealer, and he suggested seeking a ground measurement at a point on the frame. Doing so yielded slight improvement at 11.51V.

My ultimate question is: will this amount of voltage be sufficient to charge my Swift Hitch SH04 camera? It mounts nicely on my rear bumper with its own magnetic base - augmented by two sheet metal screws to secure it for travel. I've used it a couple of times in this way and the image on either an iPhone or iPad is remarkably stable and clear (this is a wi-fi unit). The camera's internal battery is rated for 5 hours, but I would like to keep it charged underway for any extended trips. The best answer will come from the Swift Hitch people themselves, but when I discovered what I perceived to be a low voltage reading at those tail lights that started a whole new area of concern.

Final thoughts, anyone?

Last Train wrote:

Artum Snowbird wrote:
OK.. to be clearer perhaps... and this might make more sense. If you are looking at a double filament bulb, one is for the brake/turn and the other is for the running light. There will be two feeds to the bulb, hot for the brake, and a hot for the run. There is no negative in the fixture, only a ground. So, my estimate of what you are measuring might be the voltage drop across the two filaments.

Have you taken the lens cover off yet to see what is going on with the actual filaments?

Yes, absolutely. All of this has been done with the lens cover removed and the positive and negative wires exposed as they emerge from inside the back wall of the trailer. I am looking at the source of the power from the wiring harness itself.

Coachmen has provided a positive black wire and a negative green wire (green is a non-standard color in this application, isn't it?). And there is a very short white wire - which I mistakenly ignored in the past - which appears to attach to the base of the bulb socket. Should this be the ground?

I've got to run to an appointment in a few minutes, but when I get back on task I will post a picture that will surely be worth the proverbial "1000 words."

Thanks again.

Trailer wiring

I hope this diagram helps. You can see that on one side green is the positive used for brake/turn and on the other side yellow is used for brake/turn. White is the negative return/ground. Most trailers now take the white wire to each bulb fixture instead of relying on just the frame of the trailer to provide the ground/negative return.

Your running lights are all fed with a brown wire.

Your voltage measurement should be taken between the positive and the negative wires. The negative is the same as the ground. The negative is,....or should be grounded to the frame in one or more places.
If you measure between the positive wire at the bulb and the frame, you should see 12 volts !! If you don't, then the negative does not have a good connection to the frame, in one or more places.

This is something that is so simple, but like always, some are making it so difficult.

Artum Snowbird wrote:
OK.. to be clearer perhaps... and this might make more sense. If you are looking at a double filament bulb, one is for the brake/turn and the other is for the running light. There will be two feeds to the bulb, hot for the brake, and a hot for the run. There is no negative in the fixture, only a ground. So, my estimate of what you are measuring might be the voltage drop across the two filaments.

Have you taken the lens cover off yet to see what is going on with the actual filaments?

Explain to me your statement, "There is no negative, only a ground".
Negative AND GROUND is the same thing as it relates to RV 12 volt wiring. To the OP, IF you measure less than 12 volts at the light using the NEGATIVE(ground) wire at the light and the Hot, odds are it is a loose/bad/ ground(negative connection in the trailer. The 7 way bargman at the tow vehicle and on the RV SHOULD have a 14 gauge GROUND(negative) wire connection to complete this from the Tow to the trailer. IF not, then the trailer ball will NOT be adequate to supply Ground(negative) to the trailer. The purpose of the previous posts to use a long Ground(negative) jumper from the RV frame is to verify that the ground(negative) UP to the taillight is not the fault. Also, some Trucks/Tow vehicles REQUIRE a Powered tow adapter to get correct 12 volt plus voltage to the RV light system. This means the Truck requires a 12 volt powered adapter that boosts the trucks lighting voltage to adequately power the RV lighting system. Very few trucks need this type system It is usually needed when using a Car or Smaller SUV. IF all the lights on your RV are bright and only the one taillight is dim, then the Ground(negative) is the fault just for THAT light. Doug

Artum Snowbird wrote:
OK.. to be clearer perhaps... and this might make more sense. If you are looking at a double filament bulb, one is for the brake/turn and the other is for the running light. There will be two feeds to the bulb, hot for the brake, and a hot for the run. There is no negative in the fixture, only a ground. So, my estimate of what you are measuring might be the voltage drop across the two filaments.

Have you taken the lens cover off yet to see what is going on with the actual filaments?

Yes, absolutely. All of this has been done with the lens cover removed and the positive and negative wires exposed as they emerge from inside the back wall of the trailer. I am looking at the source of the power from the wiring harness itself.

Coachmen has provided a positive black wire and a negative green wire (green is a non-standard color in this application, isn't it?). And there is a very short white wire - which I mistakenly ignored in the past - which appears to attach to the base of the bulb socket. Should this be the ground?

I've got to run to an appointment in a few minutes, but when I get back on task I will post a picture that will surely be worth the proverbial "1000 words."

Thanks again.