1984 Mercedes Benz 300CDFACTORY OEM SERVICE PROCEDURE

Automatic Altitude Compensating Device

The governor is equipped with an altitude compensating device to control emissions at various altitudes. The device is connected to the intake manifold to sense boost pressure variations.

As altitude increases, air pressure decreases, the push rod in the compensator moves downward and pushes the main rack, slightly decreasing the amount of fuel injected (leans out the mixture).

When the turbo is operating under boost, the rack is pushed in the other direction and more fuel is injected. Injection pump and governor operation are affected by the compensator when the vehicle is operating near sea level or idle RPM.

Fig 1: Altitude Compensation & Vacuum Units
G34657
Factory Service Tools

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • β€’ You smell fuel, burning insulation, or see smoke.
  • β€’ Brakes feel soft, pull hard to one side, or make grinding noises.
  • β€’ The engine overheats, stalls repeatedly, or misfires under load.
  • β€’ You are missing required tools, torque specs, or safe lifting equipment.
  • β€’ You are not confident in the next step or safety outcome.