Engineering tomorrow

[sambo]

Originally Posted by LT65

The second wire goes to your ignition switch so the brake light comes on at start up to check the bulb.

Just to clarify, your ignition switch is positive, so connecting the "second" wire to the ignition switch would result in positive going to both sides of the brake fail lamp when starting - lamp wouldn't light. Also, if your brake fail switch was earthed (eg, low fluid pressure), you might have 12v pos from the ignition shorting through the brake fail switch - yikes.

So, the "second" wire on the XF brake fail plug is just another potential earth for the brake fail lamp. To get it to work, your ignition start circuit should activate a relay which causes the second wire to connect to earth (chassis), causing the light to go on while the starter cranks. Remember the other side of the lamp always has 12v pos.

The tricky part (to me anyway) is that the ignition starter relay activates an earth circuit to the lamp, much like the brake fail switch, whereas we normally use relays to control positive circuits. Eg, headlights, fans, etc.

Hope this makes sense.