Brake pressure warning switch

[FC-V8]

Hi Guy. My engineer wants the brake failure light installed.
Im using a vp master cylinder which has a two pin plug. Im putting in a red led light in the dash. My question is how is this switch wired up?
Cheers
Glenn

[Harv]

G'day Glenn,

I have just gone through this drama with my EK wagon.

The brake fail switch is just a fancy earth. When the brakes fail, the switch allows current to flow (just like the idiot light sender for oil pressure).

On the early switches, there was only one pin. You hooked up 12V and a lightbulb, and the switch provided the earth.

On the later switches, there are two pins. The pins are connected together inside the switch. One pin gets wired up to the 12V and bulb, just like oil pressure idiot light. The second pin gets connected to a set of contacts in the ignition switch that are only closed just before starting the car. This means that as you are starting the car, the ignition switch provides an earth, and the lightbulb glows. This acts as a check to make sure the bulb is not blown.

For most early Holdens, there is no "contacts in the ignition switch that are only closed just before starting the car". You could bodgy one up with relays, but that would be painful. As long as you don't mind losing the "bulb check" process, you can wire both pins together i.e.:

+12V - lightbulb on dash - both pins on switch - earthed via switch body and vehicle shell.

Cheers,
Harv

[FC-V8]

Thanks of that.
Cheers
Glenn

[blownfc]

Hey Harv

Can you please explain how to wire this switch up again as I can see understand the way you have wrote it

Thanks

[Harv]

A) Connect a lead to +12v power from the accessory side of the fuse box. When the ignition is off, the lead is dead. When the car is running, it has +12V. Do this under the dash.
B) Connect your new power lead to one side of your brake fail bulb. If you are using an LED, make sure it is the + side of the LED. If it is an incandescent bulb, it does not matter what side.
C) Connect a second lead to the other side of your bulb/LED, and run that second wire out through the firewall (through a grommet).
D) Test your wiring: turn the ignition on, and earth the dangling end of your second lead to bare metal (under the bonnet). Your brake fail light should illuminate. Tuen the ignition off.
D) Connect the dangling end of your second lead to either (or both) pins of the brake fail switch.

Job done.

Cheers,
Harv

[blownfc]

Thanks for that
It’s what I thought was the way to go