See Figures 1 and 2Carbureted Models Only
At high altitudes, air/fuel mixtures become richer, due to the thinner air available. The High Altitude Compensation (HAC) system installed on 1977-88 trucks sold in federally designated areas insures a proper air/fuel mix by supplying additional air to the low and/or high speed circuits at high altitude (above 4000 feet) to minimize HC and CO emissions. The system also advances the ignition timing to improve driveability at high altitudes.
The HAC system consists of an HAC valve, a dual diaphragm distributor, and a check valve.HIGH ALTITUDE OPERATION
Low atmospheric pressure allows the bellows in the HAC valve to expand and close port A. Intake manifold vacuum acts on the diaphragm in the HAC valve through the check valve, opening the passages between the carburetor and the atmosphere via the HAC valve. These open passages allow air to flow into the low and/or high speed circuits in the carburetor. As a result, the air/fuel mixture becomes leaner.
The intake manifold vacuum also acts on the sub-diaphragm in the distributor. This vacuum in maintained by the check valve, except in the following instance: a vacuum in the main diaphragm of the distributor rises above 127mm of mercury (Hg), the vacuum advance will revert to normal.LOW ALTITUDE OPERATION
High atmospheric pressure, entering through the bottom of the HAC valve acts on the bellows and opens port A. Since the intake manifold vacuum does not act on the diaphragm in the HAC valve, the air passage from the carburetor to the atmosphere is closed by the diaphragm. This prevents a lean mixture at lower altitudes.
The intake manifold vacuum also cannot act on the distributor sub-diaphragm, so the distributor advance in normal.
See Figures 3, 4, 5 and 6Inspection
Before checking the HAC system at altitudes near 4000 feet, determine the position of the HAC valve. This can be done by blowing into any one of the three ports on top of the HAC valve when the engine is idling. If the passage is open, the valve is in the high altitude position. If it is closed, the valve is in the low altitude position. When the position is determined, proceed with the appropriate high altitude or low altitude inspection.INSPECTION AT HIGH ALTITUDE
- Check all vacuum hoses for leaks, breaks, kinks, or improper connections.
- Visually inspect the HAC filter and replace it if clogged. It is located at the bottom of the HAC valve.
- Start the engine and check the ignition timing. If it is about 13° BTDC, go on with the procedure. If it is only slightly out of adjustment, adjust the timing. If the hose between the HAC valve and the three way connector is pinched, does the ignition timing become about 13° BTDC- If so, the HAC valve should be replaced. If not, the check valve must be inspected. It should be possible to blow air through it from the HAC side, and not from the intake manifold side. If faulty, replace. If not, the distributor vacuum advance is faulty and must be repaired or replaced.
- If the hose between the white side of the check valve and the three way connector is pinched, the ignition timing should stay at 13° BTDC for a minute or more. If not, replace the HAC valve.
- If the hose is disconnected from the black side of the check valve and the hose end blocked, the ignition timing should stay at 13° BTDC for one minute or more. If not, replace the check valve.
- Disconnect the two HAC hoses from the carburetor. If air is blown into each hose, it should flow into the carburetor. If not, the carburetor air passages are blocked.
- If the air does flow into the carburetor, reconnect the hoses. The HAC system is operating correctly.
- Check all hoses for leaks, breaks, kinks, and improper connections.
- Inspect the HAC filter, located in the bottom of the HAC valve, and replace it if clogged.
- Start the engine and disconnect the two HAC hoses from the carburetor.
- If air is blown into each hose, it should not flow into the HAC valve. If it does, replace the valve.
- Reconnect the two hoses. Check the ignition timing according to the procedures outlined in Engine Performance and Tune-up . The transmission should be in Neutral. If the timing is correct, the HAC system is okay. It should be:
- If the timing is incorrect, disconnect the vacuum hose from the distributor sub-diaphragm. If the timing does not change, adjust the ignition timing. If the timing changes, replace the HAC valve.