CARBURETED MODELS
To meet Federal emission standards at all altitudes, all vehicles using feedback carburetors are equipped with a high altitude compensation system.
The system consists of a High Altitude Compensator (HAC), vacuum switching valve, distributor equipped with high altitude advance system, passages for additional bleed and jet air, and 2 reed switches. The system may also include dual catalytic converters. As altitude increases, air pressure decreases and the air/fuel mixture becomes richer. The HAC system supplies additional air to the primary main well.
FUEL INJECTED MODELS
In a similar manner to carbureted models, fuel injection models are equipped with a pressure sensor and solenoid valve which acts as the high altitude compensator.
At high altitudes, the control valve is opened by the HAC aneroid bellows. This allows air to bleed through the valve into the primary main well. At low altitudes, the control valve remains closed and no additional bleed air flows to main well.
When ignition switch is turned to "ON" position, a solenoid valve is energized by the ECU for a specified time. This allows the pressure sensor to detect the ambient barometric pressure. Fuel delivery time is determined by the ECU in response to ambient barometric pressure, so that air/fuel ratios at high altitudes can be maintained to the same degree as those at sea level.
Low Altitude
- Disconnect high altitude compensator hose harness at carburetor. Plug hose connections at carburetor and test drive vehicle. If HAC opens below 3900 ft. (1200 m) it causes a very lean mixture which can be felt as hesitation, surge or lack of power. HAC is temporarily eliminated from system by plugging connections.
- If performance improvement is noticed, inspect hose harness for proper routing and/or leakage. If there is no performance improvement, reconnect hose harness to carburetor. HAC is not cause of problem.
- If hose harness is not leaking and routing is correct, HAC must be replaced.
Disconnect high altitude compensator hose harness at compensator. Connect hand vacuum pump to one of compensator hose connections. Attempt to create vacuum. If vacuum can be created, replace compensator. If vacuum is not created, compensator is working properly. Reconnect hose harness to compensator.
SYSTEM TESTING
Note. Testing information is not available.