Tail light power issue

[richg]

Thanks for the welcome, great to join. I'm hoping the community wisdom will help me maintain an older camper.
Ran into an issue I am having trouble tracking in down. 92 Lance Squire, passenger side tail light. Tail light works, brake/ turn signal does not. Removed light assembly and tested on the battery. Bulbs and ground work. Re installed, power is good to the assembly, but as soon as I connect the turn signal wire, power disappears, dis connect and power from plug returns. Any thoughts?
Richg

 

[TheLooks]

When you say power returns, what power? The power to the tail light, marker light, parking light etc. It is a separate wire and should have no connection to the brake/turn signal other then sharing a ground. So check for a good ground.

 

[richg]

Sorry. I meant that that turn signal wire has power from the plug up until the time I connect the fixture. The meter shows no signal voltage through the wire while the fixture is connected. When I disconnect the fixture, the signal wire from the plug shows voltage again
Richg

 

[A-Texan]

Sounds like you have a voltage drop on that circuit. A couple things can cause this, first and foremost you may have corrosion in the circuit that reduces the voltage (uses it up before it gets to the bulb) when the connector is plugged in or you could have a wire that is rubbed through and only has a single strand or so of wire hanging on which could cause the same effect in my experience. I would rig up an old light bulb to load test the circuit (preferably a known good brake light bulb) attaching one wire to the ground side of the bulb and one wire to the power side of the bulb and load test the circuit with the connector disconnected from the lamp fixture. In other words you will be connecting the wires from the bulb directly into the wires of your vehicle bypassing the light assembly. If the bulb lights the circuit is good and the connector you are plugging into is likely the cause, on the other hand if it does not light when connected directly to the bulb your concern is in the circuit itself and you would need to trace the circuit moving back towards the input power source while the bulb is connected to the point where you find voltage present. Once you find where 12v is present you can either overlay that portion of the circuit (cut the wire and run a new piece from there to the light assembly) or repair it if you see an issue. Below is an image of a Bulb set up to load test a circuit (I made it years ago while still in the shop to check fuel pump circuits, etc. to be sure they could deliver the power needed for an accessory or device before condeming the component it operates). On many occasions in the shop I would find circuits that showed good voltage when unplugged, but once the circuit was loaded the voltaged went away (voltage dropped) and there ended up being a circuit problem (either corrosion in the wire itself causing the issue, high resistance in the lamp assembly connector, or damage to the wire limiting the integrity of the circuit - example - only a few strands of wire left connecting the circuit instead of all the wire strands being connected). I would take this approach rather than ohm out the circuit considering you show voltage when it is unplugged because this will quickly identify whether your circuit, light assembly, or the connector is at fault.

I also wanted to mention, I have found a lot of issues in the past on Fords (I don't know what kind of truck you have but if it can happen to a Ford it could probably happen to any make) with the 7 pin connectors getting corrosion in the male side of the recepticle causing voltage drops to the trailer lights, so if you have not checked it I would recommend doing so, usually a visual inspection would show a green cake looking material in it, especially in the backside of the connector due to water intrusion. I would check there first before going too far with checking the camper if you have not already done so. Also check the camper side female end too because you might find an issue there right away (you can load test the circuit on your truck at the trailer connector to rule it out quickly if your camper isn't hard wired to that circuit and uses the trailer connector by connecting the bulb to your effected circuit, use a test light to verify voltage at the correct terminals first, then use the bulb to load test it before moving on to the camper).

If what I stated sounds confusing just Google videos on how to load test a circuit. Unloaded circuits can have voltage present when checked with a test light, but that doesn't mean they have the integrity to power something that draws much higher amps.

Hope this helps.