Part I       How the system works… If you own a C3 Corvette, you will eventually have to deal with headlight vacuum problems. While the bundle of vacuum lines and multiple devices in the system may seem complex, it is really quite simple. The first step in troubleshooting is to understand the components of the system and how each component works in this system. Headlight switch – This device controls both electric and vacuum. With respect to vacuum control, when the switch is depressed (off), it allows vacuum to pass from the engine to the override switch. Override Switch – This device is nothing more than another vacuum switch that, if the headlight vacuum switch fails, gives the driver a secondary method of controlling vacuum to the vacuum relay. Depressed is closed, extended is open. Vacuum Relay – There are two of these relays in this system, one for each headlight door actuator. The relays, based on the signal from the headlight switch/override switch, divert vacuum from the vacuum supply to one of two ports on its respective actuator, either opening the corresponding headlight door or closing it.   Door Actuator – Each headlight door is opened or closed by a dedicated actuator. The actuator consists of a body, diaphragm and shaft. The shaft is connected to the diaphragm internally and exits the body of the device on one side and connects to the door mechanism. There are two vacuum ports on the actuator body with one on each side of the diaphragm.  Vacuum at one port would pull the shaft into the actuator, while vacuum at the other port would extend the shaft outwards. Vacuum lines and components – There are 14 vacuum lines, one filter, one reservoir and one check valve. Each vacuum line is color coded for specific positioning within the system. The filter is to keep contaminates out of the system. The check valve and reservoir work together to provide reserve supply after the engine is shut down. How it all works in a properly working system…… Headlight switch ‘off’, override switch open, engine on – Engine manifold vacuum is created in the vacuum supply system, including the reservoir, which supplies vacuum to the supply port of the relays. Manifold vacuum is also passed through both switches and applied to the top (signal) port of each relay, pulling the relays’ plunger upwards and passing vacuum to the actuator rear port, pulling or keeping the headlight door closed.     Headlight switch ‘on’, override switch open, engine on - Or Headlight switch ‘on’, override switch closed, engine on - Or Headlight switch ‘off’, override switch closed, engine on - Engine manifold vacuum is created in the vacuum supply system, including the reservoir, which supplies vacuum to the supply port of the relays. Manifold vacuum is blocked at one or both of the switches and vacuum does not pass to the relay signal port. The relay, in its rest position, allows vacuum to pass from the supply port to the actuator ‘open’ port, opening the headlight door.   It may seem counter-intuitive that the elimination of vacuum to the signal port of the relay opens the headlamp doors. This is a fail-safe to enable the driver to have operating headlights in the case both switches fail, or vacuum is lost on the signal circuit for some reason. Next newsletter ……….. ‘Isolating problem(s) in the C3 headlight system’ Hope this article helps. Please contact us if any questions. Thanks. Rob service@usmracer.com