trailer lights - finding a short

[GoodToGo]

I have been trying to find a short in my trailer lights, which seems to have appeared after I replaced the lights with the same brand lights and re-wired and re-decked the trailer. All of my wires coming from the lights appeared to be shorted to each other. Going all the way back to the lights themselves, using an ohmeter I find continuity between the tail, stop, and ground circuits when the bulb is in. When I remove the bulb the continuity goes away. A guy at work says I have to remove the bulbs in order to find shorts, that the bulbs themselves provide continuity. Can someone verify this, and explain how the lights can work when there is continuity between both the circuits and the ground?

[rmrakt]

The filament within the bulb is what you are seeing (very low resistance). The bulb must be removed.

[Giant_Jackpot]

I agree, you must remove the bulbs. Is it possible that when you put the deck on you could have put a screw into the wire somehow? Just a thought. Trailer wiring can be a pain in the a$$.

[lawdog]

I had that same thought about the screw Giant...come to think of it, I have the same thought about trailer wiring being a pain too!!!

[Surface Tension]

" Can someone verify this, and explain how the lights can work when there is continuity between both the circuits and the ground?"

Correct the bulb has to be removed. So from you post it sounds like the lights are working but you were concerned about continuity and thought you had a short?

[Jeremy airjer W]

They are correct. The both of the bulb filaments share the same ground. So any combonation of wire you check with the bulbs installed will show continuity. with the bulbs removed there shouldn't be any continuity between any of the wires or any of the wires and the trailer frame with the exception of the ground wire if it is attached to the trailer.

[GoodToGo]

Okay, I can see that the two ciruits share a ground, so have continuity through the bulb. Why doesn't that shared ground cause tail light voltage to go "backwards" through the stop light circuit? Does the filiment use up the voltage to the point that it can't cause problems to the other circuit?

I did have a short in a clearance light. It was while trying to find that short that a found this continuity issue and went down the wrong path. Last night I disconnected the clearance lights, removed the bulbs from the taillights, and had no remaining continuity issues. So problem solved, but still don't quite understand the electronics of it.

[hydro]

Did you have a problem with the actual operation of the lights? If not, then you're OK per the comments above. The nasty problem is finding a grounding fault when the operation of the lights goes goofy. That's when the interconnection of the run/stop filaments causes everything to light up at once (sort of....)

[Mr. Bear]

The reason you have to take the lights out is that the resistance of the bulb is low enough to still trigger the continuity checker. I've found it's better to use the ohmeter when I can. That way you can tell the difference between a true short (somewhere close to 0 ohms) and the inherant resistance of your circuit (something bigger than 0 ohms). It's nice to listen for a beep, but you get a lot more information from the ohmeter.

Your question: "Okay, I can see that the two ciruits share a ground, so have continuity through the bulb. Why doesn't that shared ground cause tail light voltage to go "backwards" through the stop light circuit? Does the filiment use up the voltage to the point that it can't cause problems to the other circuit?"

To explain this, I'll try an analogy that helps some people with more of a mechanical background: think of voltage as pressure, current as flow, and resistance as a constriction in a pipe. Voltage does not flow for all practical purposes. Voltage is electrical potential to do work. Kind of like when you pump water into a water tower. The water in the tower creates pressure to drive water through the pipes, but if you don't open a valve the pressure will be maintained. This is like a battery with nothing attached, it will maintain a voltage for a long time if nothing is attached (of course they aren't perfect). If you turn on a faucet the water pressure will cause water to flow through the pipe. This is like closing the switch to turn on your lights, which allows the battery voltage to drive a current through the light bulb. A big pipe would be like having a very low resistance in your circuit. A lot of current would flow through it. A tiny pipe would be like having a high resistance in the circuit and less current would flow through it. Now, a ground (a good one at least) is an infinite current sink. It will always be at zero volts. This is why current flowing in one circuit can not "jump" to the other circuit and effect the other light. Assuming the stop light is off, it will not be connected to the voltage source and the voltage on the incoming side will be zero. On the ground side of the light, we know the voltage is zero (it always is if it's a good ground). Thus, the difference in voltage across the light bulb is zero (0 incoming - 0 ground = 0 across). That means there is no potential (no pressure) to drive current (water flow) through the bulb (the faucet) and it does not light up.

I hope this makes sense to people, I typed it pretty fast. I know electrical stuff can get a little abstract, but this analogy helps a lot of people visualize things and get the terms right. And it really isn't too bad once you get the basics down: at least until they ask you to determine the poles and zeroes of the frequency response of a 3rd order band pass filter

[Jeremy airjer W]

Or you could just say that electricity is lazy and chooses the path of least resistance. Power goes through a wire to the bulb filament and then to ground. The ground is easier to go to than another bulb filament.

[Mr. Bear]

airjer,

That works too, but I find that statement can be misleading. If you have two resistances in parallel, the electricity will not go just through the branch with the least resistance. In fact, the combination of the two branches gives the least resistance. Thus, the current divides between the branches proportionally with more going to the branch with less resistance. In the trailer light situation, the comment works because the branch to the 2nd light is actually an open circuit (near infinite resistance) and the ground has very near zero resistance.

[Jeremy airjer W]

Mr. Bear,

Your are correct! A little technical , But Correct! Its funny how you used the water analogy. Automotive electrical is one of my stronger assets. it just comes naturally. When I'm explaining to a customer what is wrong with there vehicle and they are really interested in trying to understand what I'm saying, I use the same thing.

[Phred52]

Hey good to go, Do you have a battery charger at your disposal?? What I do when I have trouble is hook up the ground wire of the charger to the ground pin and the positive to a small phillips screwdriver that I can use to touch the other 3 pins. The battery charger will break the circuit if you have a direct short. Or it'll light the light.

[GoodToGo]

Took a break from the trailer for deer hunting...The clearance lights are badly corroded and bashed up, they all need replacing, I'm sure I will find the short when I do that. Thanks for the help.

Thanks Bear, that was a good explanation, now I get it. I should have been able to figure it out for myself, must have somehow killed those particular brain cells.