Electrical problem. Need some help.

weedpharma · Dec 8, 2014

skyhook said:
You have an open ground somewhere, when you test with your meter, you are effectively creating a ground with it when you ground the test lead giving the voltage a path to ground through the meter. Remove the meter and you have now, removed the ground. A test light will give you the same results.

jix is correct!

You have a high resistance in the supply lead. Think of the supply lead being 100 ohms.
Put a meter to ground and it will measure 12v as the resistance of the meter is millions of ohms so does not load the circuit.
Now put your lamp in. This is only a few ohms. You have 100 ohms in series with a few ohms the voltage across the lamp will be minimal.

Think of a normal circuit where the wire is an ohm. You have an ohm in series with a few ohms. You will get a voltage across the lamp. As the filament heats the resistance increases and you will measure 12v.

Weedpharma

BobRip · Dec 8, 2014

weedpharma said:
jix is correct!

You have a high resistance in the supply lead. Think of the supply lead being 100 ohms.
Put a meter to ground and it will measure 12v as the resistance of the meter is millions of ohms so does not load the circuit.
Now put your lamp in. This is only a few ohms. You have 100 ohms in series with a few ohms the voltage across the lamp will be minimal.

Think of a normal circuit where the wire is an ohm. You have an ohm in series with a few ohms. You will get a voltage across the lamp. As the filament heats the resistance increases and you will measure 12v.

Weedpharma

You have a bad connection which may be in the ground or in the supply. It is not completely open, but higher than normal resistance. With the light turned on and bulb installed, measure the voltage between the battery ground/negative terminal and the negative on the bulb. If it's more than a volt the problem is in the ground circuit. Measure the voltage between the battery positive and the positive on the bulb. If it is more than 1 volt then the problem is in the power or positive side.

aczlan · Dec 8, 2014

BobRip said:
You have a bad connection which may be in the ground or in the supply. It is not completely open, but higher than normal resistance. With the light turned on and bulb installed, measure the voltage between the battery ground/negative terminal and the negative on the bulb. If it's more than a volt the problem is in the ground circuit. Measure the voltage between the battery positive and the positive on the bulb. If it is more than 1 volt then the problem is in the power or positive side.

Per the post below, the problem is the ground:

jbwrangler said:
I connected a wire from the Positive wire on the bulb socket to the positive terminal on the bulb and another wire from the frame to the neg terminal on the bulb and the bulb lights up fine.

Aaron Z

Soundguy · Dec 8, 2014

The input impedance on your meter is likely in the megaohm range and does not load the circuit, the lamp assy, on the other hand is quite low, single digit ohms.

I'd also say you have a bad connection, but perhaps at the switch or feed line, if 12v ref disappears on the meter with lamp in, meter referencing feed line to ground

jbwrangler said:
Why would connecting the bulb cause the voltage to go from 12v to 0v on my meter?

I connected a wire from the Positive wire on the bulb socket to the positive terminal on the bulb and another wire from the frame to the neg terminal on the bulb and the bulb lights up fine.