5V is too low. Bulb will glow though. Sounds a though the current is coming through the other bulbs filament. It will act as a resistor and give you a low voltage reading and will not light the bulb you are testing.

 

1. Do you have 12v on the terminal of the brake light switch coming from the battery?
2. When the brake pedal is pressed, is the other terminal of the brake light switch showing 5v, or are you measuring it back at the rear of the truck? Measure it at the other terminal first.

 

Dual filament bulbs are in place and working?

 

Check the voltage that leaves the brake switch to a "good" ground. It should be 12 volts. If it isn't then soemthing is dropping the voltage (bad connection) on switch ?

If the voltage is good leaving switch, go to the wire at the light, without light connected is the voltage 12 volts , making sure you have a good ground connection on the meter. If it is then your light is not making good ground. If not you have a short or grounded wire from switch.

You have to have a good ground reference when measuring the voltage.

Hope this helps

Tom

 

Thanks for all the replies!


Problem solved. Pressure switch was changed out.