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Discussion Starter #1
Last night my son and I were messing around with his S-10 trying to get an idea of how to hook up some fog lights.

He wanted to be able to turn them on without having a key in the ignition, so I poked around his fuse box until the test light came on. Then we tried to connect the fog lights to the same places just to see if they'd work.

Ran the hot wire with its little fuse to the lights from the fuse box, and used the test light as the temporary ground wire from ground to the lights - no light.

Bypassed the lights - test light lights up.

Used a jumper wire for temporary ground - the fog lights came on.

Would the test light have enough resistance to prevent the fog lights from coming on when hooked to the lights and grounded to complete the circuit? That's the best guess I could come up with, but it doesn't make since to me - more likely just a bad connection when using the test light?

Thanks.
 

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The test light or any filament is a resistor. Putting that in series with another bulb divides the voltage between the positive connection and the negative connection. This lowers the voltage/current across the light you are trying to make work. So it's possible.
 

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Discussion Starter #4
John, I've been thinking about this - you said "...it's possible".

Is it possible, but unlikely? Would poor connections be more realistic.

Seems strange to me that a test light would prevent a fog light from working.
 

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Not knowing the value of either filament I hedged my answer with a "It's possible.". Easy enough to prove either way by lifting the ground wire and reinserting the test lamp again. This time you have a known working circuit.
 

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No doubt about it. John is correct. The test lamp would act as a voltage divider. A nice clean ground will allow your Son to operate his fog lamps with the engine not running. I hope you got him some nice jumper cables for Christmas ;) .

Mark
 

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Just wanted to add this.

The resistance of the test light is many times grater than that of the the fog lights therefore the greatest voltage drop will be across the test light, about 95% if the applied voltage. When using the test light to complete the circuit, the test light should light but the fog lights will not. Current is flowing through the fog lights just not enough to heat the filament.
 

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Discussion Starter #9
When we were fooling with it there was nothing, no light at all, from the test light and the fog light - might have been a bad connection, but we fiddled around enough where there should have been a flicker. Stuff like this puzzles me.

He's gone off with his cousin to save us all from the rising deer/squirrel populations, so it'll probably be Sunday before we reconvene.

Thanks for the help.
 

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Discussion Starter #10
He finished it up yesterday - I stayed out of the way and the experiment with the test light never happened.

But I wanted to share this with you.

Before he turned them on last night to demonstrate, he said "Dad, they're kind of bright". So, I'm looking across the road expecting the neighbors' entire yard to light up.

He flipped the switch and the power lines above his truck were easy to see. You had to be there...
 

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Word of warning guys! Probing around with a traditional test light on anything made after about '85 is playing Russian Roulette, and anything built after '95 is asking for trouble! There is a very good chance of putting too high of a resistive load on a circuit that ties to a computer somewhere. Zappo - some feature won't work, or worse the car doesn't run - and you won't have a clue as to what you did.

(I bought a new Fuel Injection CPU for a Porsche 911 Turbo during an alarm install - $2500.00 :eek: Test light went in the trash)

Use a multi-meter!
 
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