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

Classic Car Weekly (UK) - - Living With Classics -

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.

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