High Altitude Compensation System: Operation: Diesel

At high altitudes (above 4000 ft.), the solenoid valve closes or engages and fuel pressure builds up between it and the pressure control valve. As soon as the pressure acting on both sides of the valve equalizes, the spring force closes the valve slightly. This causes less fuel to be returned to the pump inlet. Pump pressure increases which moves the injection timer piston and the roller cage turns in the opposite direction to the cam disc, causing the disc to move up onto the cage earlier than usual so that fuel injection takes place earlier.

At low altitudes (below 4000 ft.), the solenoid valve is open or disengaged. Fuel is returned to the pump inlet by means of 2 feedback channels; the control valve and solenoid valve, and a channel beneath the control valve.