I've been having trouble, on and off, with the classic ABS/ASR/Hill Holder Error - generally thought to be caused by the Brake Switch. I have a Grande Punto 2009, which has the blue/brown bayonet fitting brake switch - looks like this:



Generally, I just wiggle the switch and the problem goes away - but not this time. So, before I bought a new switch, I thought I would look at the old one - it's a switch after all, so it can't be difficult to understand - can it?

It lives just behind the top of the brake pedal, in a little plastic housing, with a heating vent right across the front of it. Take off the heating vent (1 Torx) and it's easy to remove the switch - a 1/4 turn clockwise does it. Taking off the connector is harder - push a little section of the white connector in and it comes loose.

The switch disassembles easily with a little levering from a screwdriver. And what's inside looks complicated! There's one switch which connects the two outer pins in a 'normally closed' state, and a pair of sliding contacts which connect the two inner pins in 'normally open'. When the switch plunger is depressed, the two outside pins open, and the two inner ones close.

But the complicated bit is the switch plunger. When you take the switch out, the plunger is free-sliding in the switch body. This is because the bayonet fitting has a couple of little black plastic stubs which engage in holes in the mount of the switch, and rotate when the switch is turned to engage the bayonet fitting. These stubs connect to a ratchet clamp inside the switch body. When the switch is removed this clamp opens, and the plunger is not connected to anything - when the switch is mounted into its housing the clamp closes, and the plunger is held at just the right length to operate properly.

I cleaned the contacts, then re-assembled the switch and re-installed it. The error was still on the dashboard when I turned the ignition on, but went away when I drove the car for a few feet.

I suspect that one possible failure mode of this switch is that the plunger gets pushed up inside the switch body somehow, and thereafter fails to work. To make it work, don't try to pull the plunger back down - just remove the switch, check that the plunger is free-moving and the little locking stubs are in line with the bayonet, then re-install the switch, making sure it is pushed firmly onto the housing before rotating.

Let's see if the car is still error-free in the morning...! F.I.A.T...