Automatic Altitude Compensating Device

Governor is equipped with an automatic altitude compensating device to control exhaust emissions at varying altitudes. On the 300SD (turbocharged engine) models, this aneroid is equipped with an inlet from the intake manifold to sense increased (boosted) manifold pressure. See Fig 1 and Fig 3.

With increasing altitude, atmospheric pressure is decreased. This causes two aneroid (non-fluid) compensators to expand.

Fig 1: Automatic Altitude Compensating Device and Vacuum Control Unit

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At specified atmospheric pressure, internal force of compensators becomes greater than pretension of aneroid compression spring. Push rod moves downward, moving linkage. This causes main rack to move in direction "d", allowing less fuel to be injected. During increased manifold pressures on 300SD engines, the pressure in the aneroid cavity is increased causing the compensator and main rack to move in opposite direction, allowing more fuel to be injected while turbocharger boosts engine compression. See Fig 3.

Fig 2: Typical Governor Linkage For Mercedes-Benz (Start Position Illustrated)

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Fig 3: Diagram of Automatic Compensating Device and Linkage

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As adjusting lever moves closer to idle stop, compensating adjustment in partial load range is gradually reduced. At idle it is almost completely eliminated. At low altitudes, governor settings are not affected by compensators.