The Inertia Switch sits in the corner of the engine compartment (see picture). It's function is to disconnect the power to the fuel pump when the car is involved in an accident, thus preventing a serious fire.
click for bigger pic.
It works like this:
The plunger on top of the switch has a brass ring which bridges the two electrical contacts at the bottom. A steel ball sits in a bowl held by a magnet which stops any unneccesary movement. When the vehicle receives a hard shock as it would in an accident, the inertia in the ball pushes it up the side of the bowl hitting the bottom of the plunger, pushing it upwards. This moves the brass ring away from the contacts, disconnecting the power to the fuel pump.
click for larger pic
To restore power to the pump, simply push the plunger down.
It is possible to activate the Inertia Switch in other ways. If the vehicle is carried on a trailer, it is possible that it can be activated by the shock when it is returned to the road. When my car was resprayed the bodyshop ran a wire from the fusebox to the pump to get the car to start.
Corrosion can also prevent the switch from operating the fuel pump.
The switch can be tested by applying a meter or test lamp to the contacts on the bottom. If plunger is pushed down and there is no connection through the switch, it will be necessary to clean it. This is a very simple repair.
Remove the switch from the car. At the bottom of the switch body sides there are two small holes into which pins on the base of the internals locate. Press in the pins and pull the internals down. The plunger will have to be removed from the body. The top of the plunger is simply pulled off of the part inside.
You will now see the workings of the switch.
Clean the contacts with a piece of wet & dry and reassemble. The plunger can be held in place with a screwdriver while the top is pushed on.
Test again with a meter and you will find that the switch is working again as it should. To test the operation, hold the switch vertically then give it a sharp tap on the side. You should find that the plunger has popped up. Pushing it down again restores continuity.
Thanks to Mike Taylor for the Engine bay photo.