Battery - 12.8V idle; 14.3V engine running

Auto trans in "R", backup lights showing 11.65 volts each socket using multi meter. NSS appears to be good and truck will only start in "P" and "N" as it should.

Insert bulb in both sockets and no lights. Leave one bulb in (either side) and test other socket and get 0.00V on meter. Somewhat confused on why I'm showing no voltage on either socket when a bulb is inserted on the other socket. I had equal voltage on both sockets before adding a bulb to the scenario.

Multimeter shows 11.8-12.0V on black/red wire (female plug trailer plug connection) going to backup lights. So, I know I'm getting power back to the sockets.

Brake / turn / emergency / side lights all operational.

Started truck, reverse, getting 12.8V at each socket to ground. Put in one bulb and lose all volts on other socket.

looking for recommendations on what to pursue next?

 

where is your multimeter grounded when taking the voltage measurements? Have you confirmed the grounds are good for the backup lamp sockets?
it's possible you have a bad connection between the bulbs and the power source that causes high resistance when a load is applied

 

Grounded mulitmeter several places to compare readings -
- actual bulb socket ground
- rear bumper
- ground screw on bed (same ground for side marker, brake / rear turn signal lights - they all work)

I am showing good voltage from the black/red wire coming from the trailer 4 pin plug going to the back up light sockets when auto in "R". Thats both with engine shut off and running.

Background
- replaced backup light sockets with Jeff Bronco units (units in China) one year ago. Had a ground problem.
- new units and new ground wires resolved my issue one year ago
- still using existing rear lighting wiring harness = 44+/- years old.

I'm suspecting maybe my back up socket(s) / or ground wire could be a problem.

May have to disconnect my light sockets and test with truck battery

 

the reverse lights share the same fuse as the windshield washer pump, just curious if it works?

 

Used multimeter with ignition key on and auto trans in "P" and was reading 12.8V

I had installed a new bussman 15A fuse initially just to rule it out

 

Put the bulb in the socket check ground between actual ground and brass side of bulb. Just to eliminate a bad socket, the ground contact is a 2 piece connection in the construction of the socket. Just throwing that out there.

 

Appreciate all the technical advise thus far.

Checked each socket with bulbs at battery (multimeter 12.8V) - both sockets are good with bulbs illuminating. Bulbs rated at 12.5V

Checked Black/Red wires that split off from trailer plug and provides power to each socket. Both showing around 11.8V, so I'm losing approx 1 V between battery and backup sockets when wired to rear lighting harness.

Rewired one socket to backup lighting wiring harness and no illumination.

Even though I'm getting V in the backup lighting harness wondering if my NSS is defective. Replaced it last time I fixed my backup lights.

FYI - I recently replaced my gage board circuit card and reinstalled in dash, but I have reviewed my electrical schematic and cannot see any connection points in the dash that would cause my current problem.

 

You said in post one that the NSS was good.
In post 3 you tested light socket contacts as good, then you suggest your BU sockets or ground is /are defective.
Now post 7 has me confused royally.

Have you checked current on the BU light wire back at the rear of the truck using the ground wire too?

 

Auto shifter in "R" with key "ON" I am getting 11.8V indicated with my multimeter at BU BLACK/RED wire going to each back up light socket. With engine running I'm getting 12.8V.

My point about my NSS - could I be getting voltage loss that could be causing this problem?

I have voltage at -

- Battery = 12.8
- Fuse = 12.0 = auto in "R", key on, and reading from outlet side of fuse to ground
- BU wire coming from hopkins trailer plug male side = 11.8V
- BU wire coming from hopkins trailer plug female side BU BLACK / RED wire going to each back up light socket = 11.8V

- BU light sockets with bulbs proved good at battery terminals

- Ground at rear good = supports by side, tail and blinker lights being illuminated.

- I have a BU light jumper inside my engine bay compartment driver side, when jumped, it used to keep both my BU lights illuminated. Now that's not even working anymore.

The only thing I have worked on with this truck before this issue showed up was installed a new circuit board, resistor, LED light bulbs for instruments on the back of my gage panel. Which resolved my gages no working = they all work now (gas tank (both tanks); ALT; Oil press; Eng temp).

I have to tell you that I AM REALLY CONFUSED.......missing something simple I am sure. I will retrace circuit and double check voltage drop = battery, ignition key, fuse, NSS, BU wire, sockets and ground

 

11.8 volts will light a bulb if enough amps are available. 12.8 volts with engine running sounds low, should be up in 13s. Just 11.8 volts from a battery is like ... dead / drained / kaput / discharged. It'll give a reading on a meter, but it may not provide enough amperage to even light a bulb through long runs of wire with many connections.
A good battery should be up in the 13 volt range at least, a 100% charged perfect battery can show 14.1 or higher. Any good battery and charging system will show 13.2 volts or higher, even into the 14s ... when running.

You mention the Hopkins plug. You have BU lights on trailer too? Are you testing voltage on the wires at the BU lights at the very rear of the bed? I've had hidden rodent damage there before. Some was between the lights under the tailgate, and more recently was in the few wires bundled running past the gas tank hidden behind the frame rail.

I wish you luck, but really I think you have a defunct battery. Take it and the truck to an auto parts store and let them test it all ... free.

 

Battery is old, installed in 2018. I will check it. Thank you!!!