DESCRIPTION & OPERATION
Note. The High Altitude Compensation (HAC) system is used on Federal vehicles with high altitude option.
As altitude increases, air pressure decreases and air/fuel mixture becomes richer. High Altitude Compensation (HAC) system supplies additional air to primary low and high speed circuits on all models, and secondary high speed circuits on some models. This helps reduce emissions and improve driveability. Ignition timing is advanced for better driveability.
HAC system is designed to be functional at all altitudes. When altitude is above 3930 feet, HAC valve bellows expand and supply additional air. Components vary between the different models. (Scheme 58)- (Scheme 63).
Scheme 58
Scheme 59
Scheme 60
Scheme 61
Scheme 62
Scheme 63
HAC Valve (Carbureted)
Visually check and clean HAC valve filter. Start engine and set at idle. Blow air into any of the 3 ports on top of HAC valve. If air exits filter, valve is open and in high altitude position. If air does not exit filter, valve is in low altitude position. High altitude is greater than 3930 feet and low altitude is less than 2570 feet. If valve functions properly, proceed to TIMING CHECK.
Timing Check (Carbureted)
Warm engine and set at idle. Disconnect hose from distributor sub-diaphragm and plug hose. Check timing. Base timing should be noted. Reconnect hose to distributor. Timing should advance to approximately 12 degrees BTDC. If not, inspect check valve and distributor vacuum advance.
Check Valve (Carbureted)
Disconnect vacuum hose at check valve and plug vacuum hose. Check valve must stay connected to distributor. Ignition timing should remains stationary for more than one minute. Remove check valve and blow air through one end. Air should pass freely one way only. If not, check individual components.
HAC System Test (Fuel Injected)
- On Pickup and 4Runner models, disconnect HAC valve electrical connector. Connect an ohmmeter between White wire terminal and Green/Blue wire terminal of valve connector. Resistance should be 3000-5000 ohms. Check resistance between White wire terminal and Brown/Black wire terminal. Resistance should be 2000-4000 ohms. Check resistance between Green/Black wire terminal and Brown/Black wire terminal. Resistance should be 400-1200 ohms. If not within specifications, replace HAC valve.
- On Supra models, disconnect HAC valve electrical connector. HAC is located behind glove box. Connect an ohmmeter between Blue/Red wire terminal and Red/Black wire terminal of valve connector. Resistance should be 700-860 ohms. Check resistance between Blue/Red wire terminal and Brown wire terminal. Resistance should be 2200-3300 ohms. Check resistance between Red/Black wire terminal and Brown wire terminal. Resistance should be 2900-4200 ohms. If not within specifications, replace HAC valve.