1977-79 Models
Governor is equipped with an automatic altitude compensating device to control exhaust emissions at varying altitudes. On 1979 300SD Turbo, this aneroid valve is equipped with an inlet from the intake manifold to sense increased (boosted) manifold pressure.
With increasing altitude, atmospheric pressure is decreased. This causes two aneroid (non-fluid) compensators to expand. At specified atmospheric pressure, internal force of compensators becomes greater than pretension of aneroid compression spring.
Pushrod moves downward, moving linkage. This causes main rack to move in direction "d", allowing less fuel to be injected. See Fig 1. During increased manifold pressures on 300SD Turbo, 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.
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.