Hard time understanding backfeeding

[RobVG]

I had (supposedly) a bad ground causing my backup alarm to go off when the left signal was on and the brake applied. Wiggling wires solved the problem.

Someone here told me that the current was backfeeding. I'm trying to get a handle on just what that means. After doing a little reading I guess the common conception is current flows from negative to positive be cause of "potential". Using the water analogy, does this mean current is flowing "downhill" to the positive?

I drew a diagram so no one would have to. With both switches closed, the load with the bad ground is getting it's ground from the other load?
I'm just not seeing the path that the electricity is following?

 

[digger doug]

Both loads are still hooked to the common ground.

Now draw a wire from the common ground back to the negative lead
of the battery.

Now cut that wire.

 

[Delmer]

You had three loads, the turn signal, the brake light and the backup alarm. With the ground disconnected from ground but still tied to each other, you had current from the turn signal going to the ground wire, back through the brake light (or is it reverse light?#$%#????) and then back toward the reverse switch which is open, but connected at that point to the backup alarm which has a good ground.

 

[lantraxco]

Yep, one brake light and backup light must have had a bad common ground, so with no ground for the brake/turn lamp power would feed through the brake light, then back through the backup light lamp, and then through it's common backup alarm power wire to ground. These newer backup alarms don't take much voltage or current to work, so there you go...

 

[RobVG]

I forgot to include the alarm in the diagram. Also, both the turn signal and the brake light had to be on at the same time to trigger the alarm. I guess I'm a little dense but how is the circuit for the alarm being being completed- the "switch" is open? Where's the hot coming from? Is ground and hot coming down the ground wire, i don't get it. Can't be.

 

[John C.]

You wiggled the wires and it started working? I'm thinking about a short in the wiring harness feeding power from that other function.

 

[RobVG]

Yeah that's what I thought John, they come through a tight grommet in the bumper but I looked either side and where the grommet is and didn't find anything. They split the wires about a foot upstream. Didn't take that apart. Maybe a loose splice on a ground?

 

[RobVG]

Or maybe a short there.

 

[Delmer]

From the positive battery, through various switches etc, through the brake and turn bulbs, to the ground connection that isn't grounded, back through the ground side of the back up light to the reverse switch which happens to connect to the positive side of the back up alarm which has a good ground. The "backfeeding" is the reverse light being run backwards

 

[RobVG]

I need a few minutes to digest that Delmer

 

[lantraxco]

If the brake and turn are separate lamps or filaments, then you probably have a bad splice or corrosion causing a resistance in the ground circuit. It's like choking off a hydraulic return line, the pressure has to go somewhere.... so it goes back upstream following the path of least resistance.

 

[Delmer]

This show the theory of how a bad ground causes backfeeding. Your case is a little more complicated because the brakes and turn signal are causing the back up alarm problem.

 

[kshansen]

Electricity is funny how it will do its best to find its way home, some times that includes using your body which can be anything from a little tingle to major smoke if volts are high enough. Try not to let the smoke out of your body, not a good thing!

 

[FSERVICE]

the best way I can put it & make it easy for anyone to understand is... it takes the path of least resistance!! be it a ground wire, throttle cable, screwdriver, wedding ring, ETC!!! you get the idea. if the ground is growing funky green stuff somewhere it will seek a better path to ground

 

[John C.]

Franklin theory says electricity flows from positive to negative while electron theory says the opposite. I am like Rob and have a hard time believing this to be a ground problem solely because wiggling the wires seemed to solve the problem. I don't know that the backup alarm is wired with the light circuits but I could see it being that way in a truck. I could propose a little different explanation of the problem though.

About thirty years ago the factories starting using plastic braided covers for loom on the wiring harnesses and at the same time used plastic instead of rubberized insulation on the individual wires. The loom was porous and let any fluid through which included water but especially oils. The oils degraded the insulation on the wires which lowered their resistance to current flow. We began seeing plenty of cross talking in wire harnesses. Where the problem showed up the worst was in electrically and electronically controlled transmissions. We started seeing plenty of machines with three clutch packs engaging at the same time. Think Komatsu and Cat wheel loaders and Terex articulated trucks from my experience. I saw the same in some of the high mileage field trucks that worked in lots of mud or ran over plenty of salted highways. In recent years I've heard of water wicking up wiring harnesses and contaminating electronic controllers. Some Komatsu excavators had harness connectors not covered from the weather and mounted vertically where the water took out the computers.

At any rate I would say that yes you should check all your ground connections but there is a good chance that won't solve your problem permanently. If moving the harness around a bit made the problem go away, I'd be looking real hard at opening up that loom and looking inside. Regular electrical tape will seal it back up just fine if you find nothing. Faded colors and soft or sticky feeling wires are an indication of problems.

 

[lantraxco]

One thing I am sure of is it doesn't matter a damn which way the electrons flow, but if anything makes heat, sparks, or smoke, you can be sure they are flowing!

 

[Delmer]

I don't know that the backup alarm is wired with the light circuits but I could see it being that way in a truck.

I'm not there, so I'm not diagnosing it, but all you need for the backup alarm to be backfed from the lights is one reverse switch controlling both the alarm and back up lights with no relays. I've never seen a relay in that circuit or two reverse switches on a transmission, so I'm still betting on the grounds, not that I've looked for the relays though. You still might need to take the harness apart to get to where the ground wires come together.

I'm not arguing with anything else you've said. Computers, salt, oil soaked wiring, what a mess.

 

[kshansen]

While talking about back-up alarms and electricity. They will do strange things and the alarms sometimes need very little power to cause them to work when they shouldn't. I know we had at least on machine that every time you washed it the alarm would start beeping at low volume even with key off till it dried off.

 

[RobVG]

Delmer, If I get your diagram right, current is flowing through the common ground on the left turn and goes to the right turn? The thing that's puzzling is, the circuit to the alarm is broken by the switch, it should only be getting ground. I unplugged the backup light and still had the problem.

The easiest explanation would be a short like John says. But there was the other symptom which I may not have mentioned. The side marker lights were flashing dimly with the turn signal which I've heard is a symptom of a bad ground.

For now, if it ain't broke.......

 

[lantraxco]

The marker lights flashing do indicate a bad ground, current is flowing back through the unpowered taillight filament and through the marker lights and probably the opposite side taillight filament to their ground.