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Old 01-31-2018, 04:53 PM
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Electric current takes the path of least resistance. <--take this to the bank.

1157 bulbs use a common ground wire between the stop/signal filament and tail light filament.

So, if your ground (or return path) is missing, current will travel through the other filament which is probably connected in parallel to another bulb which has a better ground.

Notice, I didn't stipulate which filament was powered because it doesn't matter. Power is always seeking a return path.

Let's run through this: You step on the brake and one brake light shines. The other does NOT because its ground is missing. No problem, current simply goes into the parking light filament in the same bulb, to another bulb's parking light filament because all the parking and license plate lights are connected in parallel (and they should all have a good ground). This is so typical in all classic cars that depend on 50 yr-old rusty spot welds and anodize aluminum housings, for current to make it back to the battery. Some classic cars have stop lights in the trunk lid, that depend on hinges for electrical continuity.

Modern cars never have this problem. Most modern lamp holders have their own ground wire. The associated wire harness also has a ground wire leading back to the battery. The age of plastics brought this on as most lamp housings are not metal.

House wiring is the same story. Our service panels used to have a ground wire connected to the nearest water pipe and the meter had a short jumper or shunt wire around it. Later, folks replaced metal water pipes with plastic so the National Electrical Code mandated; the ground wire must connect between the service panel and the 'street side' of the meter with a continuous wire with no splices. Now, the code wants two ground rods, sunk in the earth, six feet apart with an un-broken #6-AWG copper wire connected between both rods and the service panel. That is how important grounds are. We don't use pipes any longer, not even copper pipes.

In automotive, we take grounds for granted when in reality, your 12-volt system requires a better bonded ground than your house.

Questions? - Dave
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