GM Prizm/Nova 1985-1993 Repair Guide

High Altitude Compensation (HAC) System

Print

OPERATION



See Figures 1, 2, 3 and 4

As altitude increases, air density decreases so that the air/fuel mixture becomes richer. (The same amount of fuel is mixing with less air so the percentage of fuel is higher.) The high altitude compensation (HAC) system insures a proper air/fuel mixture by supplying additional air to the primary low and high speed circuits of the carburetor and advancing the ignition timing to improve driveability at altitudes above 3930 feet. Above 3930 feet, the bellows in the high altitude compensation valve is expanded which closes Port A .

With Port A closed, the manifold vacuum is allowed to act on the HAC diaphragm. The diaphragm opens Port B to the atmosphere (through the HAC valve), allowing air to enter the carburetor's primary low and high speed fuel circuits. This same vacuum signal acts on the distributor sub-diaphragm adding 8 degrees of timing advance.

At altitudes below 2,570 feet, the HAC bellows are contracted opening Port A . This vacuum signal with air reduces the vacuum to the distributor's sub-diaphragm (no timing advance). With vacuum strength reduced, port B is closed allowing no air bleed into the carburetor's low and high speed circuits.



Click image to see an enlarged view

Fig. Fig. 1: High altitude compensation valve



Click image to see an enlarged view

Fig. Fig. 2: High altitude compensation system components

TESTING



High Altitude Compensation System Operation
  1. Check the HAC valve as follows:
    1. Visually check and clean the air filter in the HAC valve.
    2.  
    3. At high altitude (above 3930 ft), blow into any 1 of the 2 ports on top of the HAC valve with the engine idling and check that the HAC valve is open to the atmosphere.
    4.  
    5. At low altitude (below 2570 ft), blow into any 1 of the 2 ports on top of the HAC valve with the engine idling and check that the HAC valve is closed.
    6.  

  2.  
  3. Check the ignition timing as follows:
    1. Disconnect the vacuum hose with the check valve from the distributor sub-diaphragm and plug the end of the hose.
    2.  
    3. Check the ignition timing. It should be a maximum of 5° BTDC at 950 rpm.
    4.  
    5. Reconnect the hose to the distributor sub-diaphragm.
    6.  
    7. Check the ignition timing. It should be about 13° BTDC at 950 rpm.
    8.  

  4.  
  5. Disconnect the vacuum hose from the check valve at the back side and plug the end of the hose. Check that the ignition timing remains stable for more than 1 minute.
  6.  
  7. Stop the engine and reconnect the hoses to their proper locations.
  8.  
  9. Disconnect the 2 hoses on the top of the HAC valve. Blow air into each hose and check that the air flows into the carburetor.
  10.  
  11. Reconnect the hoses to their proper locations.
  12.  
  13. Any component not opening or closing properly should be replaced.
  14.  

Check Valve
  1. Check the valve by blowing air into each pipe:
  2.  
  3. Check that air flows from the orange pipe to the black pipe.
  4.  
  5. Check that air does not flow from the black pipe to the orange pipe.
  6.  

Distributor Vacuum Advance

Remove the distributor cap and rotor. Plug 1 port of the sub-diaphragm. Using a hand-held vacuum pump, apply vacuum to the diaphragm, checking that the vacuum advance moves when the vacuum is applied. Reinstall the rotor and distributor cap.



Click image to see an enlarged view

Fig. Fig. 3: High altitude compensation (HAC) system operation



Click image to see an enlarged view

Fig. Fig. 4: Distributor vacuum advance

 
label.common.footer.alt.autozoneLogo