How Stuff Works: Oil-grimed guru Fuzz Townshend sheds technical light on pop-up headlamps

1 Lights off and down

Vacuum is sourced via the inlet manifold when the engine is running, pulling past the check valve, thus creating a source of negative pressure in the vacuum reservoir and circuit. This allows atmospheric pressure to act upon any vented components, such as the atmosphere side of the diaphragms inside the actuators and relays. Here, the actuators are in lowered ‘park’ position.

2 Lights on and up

With the headlamp switch activated, the electrical circuit sends power to the bulbs. Simultaneously, in the vacuum circuit, atmospheric pressure is now acting upon the relay diaphragms, pushing the connected shuttle valves downward, thus allowing vacuum to influence the formerly atmospheric side of the diaphragms. Atmospheric pressure then acts upon the opposing side of the diaphragms, causing the connected headlamps to rise.

3 Lights off and up

There are times when access to the headlamps themselves is necessary, such as when changing bulbs. For this, the lights’ electrical circuit needs to be switched off and so the circuit features a manual override facility, which allows atmospheric pressure to act upon the relays and actuators in the same way as when the headlamp switch is on.