Thread: warning light when brake pedal applied

I just inserted that small detail about the earth path without being very descriptive and i admit it wasn't the best choice ... my theory is that in fact the resistor is there to protect F25 from blowing when the diff is locked and solid earth is given to that line then when the fuse is blown and there's no voltage earth through the bulb's filaments gets to that path when the brake is applied through the closed brake switch and resistor cos it's no need for solid earth for the warning to come on it can be an up to 1.8V return on the earh path.

How ever it is the gist is that it's made so on purpose to warn the driver about the lack of brake lights due to the blown fuse and it's nowhere mentioned, the only desription i found for the diff-lock warning is this(and i'm not ure it's correct):

"The power input for the LED is supplied by the instrument pack via fuse 13 in the engine compartment fusebox. The
differential lock switch or switches provide(s) the earth path illuminating the warning lamp.
The voltage on the differential lock switch or switches to instrument pack earth path produces the following warning
lamp functions:
< 1.8 volts = warning lamp on.
> 7.7 volts = warning lamp off."

Originally Posted by sierrafery

I just inserted that small detail about the earth path without being very descriptive and i admit it wasn't the best choice ... my theory is that in fact the resistor is there to protect F25 from blowing when the diff is locked and solid earth is given to that line then when the fuse is blown and there's no voltage earth through the bulb's filaments gets to that path when the brake is applied through the closed brake switch and resistor cos it's no need for solid earth for the warning to come on it can be an up to 1.8V return on the earh path.

How ever it is the gist is that it's made so on purpose to warn the driver about the lack of brake lights due to the blown fuse and it's nowhere mentioned, the only desription i found for the diff-lock warning is this(and i'm not ure it's correct):

"The power input for the LED is supplied by the instrument pack via fuse 13 in the engine compartment fusebox. The
differential lock switch or switches provide(s) the earth path illuminating the warning lamp.
The voltage on the differential lock switch or switches to instrument pack earth path produces the following warning
lamp functions:
< 1.8 volts = warning lamp on.
> 7.7 volts = warning lamp off."

In other words, it's a pull up resistor

A pull up resistor will be something like 5k ohm, so the current to ground with diff locked will be a few milliamp.

It's a very common technique when interfacing swirches to micro controllers.