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The secrets of the vacuum switch exposed

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  • The secrets of the vacuum switch exposed

    The vacuum switch has been a troublesome component on the SU HIF44E since it was fitted to early Maestro Montego models until the introduction of the ERIC (electronically controlled ignition and carburetion) system in 1989. The vacuum switch is basically an economy device allowing the engine to run leaner on choke compared to a manual set-up, as it was capable of automatically supplying the extra enrichment required during acceleration. The vacuum switch only has an effect on the fuelling whilst the choke is operating because it bleeds air into the chamber around the rotary choke; its operation is as follows.

    Manifold pressure acting on one side of the diaphragm through pipe Z is taken from the manifold side of the carburettor throttle butterfly. There are 2 ports in the valve X & Y connected to the carburettor via short rubber pipes, Y is connected via a drilling to the atmospheric side of the carburettor and X is connected via a drilling to the outer chamber of the rotary choke, the tubes X & Y are also connected together via a small air bleed restrictor in the switch body itself. When manifold pressure is low (yellow) the diaphragm is lifted allowing both pipes to be connected via a larger air bleed restrictor in the switch port that leads to the choke chamber, causing a large amount of air to mix with the fuel around the choke valve thus weakening the mixture. Then when accelerating manifold pressure is high (pale blue) the diaphragm is pushed by spring pressure closing off the port X in the switch, when the switch is closed only a small amount of air can bleed into the rotary choke through the small restrictor in the switch, causing the fuel mixture to become richer. Once the choke is off then no fuel passes through the rotary choke hence the air bleed through the vacuum switch to the choke no longer has an effect on fuel mixture.

    On later models with ERIC (electronically controlled ignition and carburetion). The vacuum switch was not required because the acceleration enrichment whilst the choke was operating was provided by the ecu indexing additional stepper motor steps in response to high manifold pressure.
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