High Altitude Compensation System Toyota 4Runner I

DESCRIPTION

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. The High Altitude Compensation (HAC) system supplies additional air to the primary low and high speed circuits on all models, and secondary high speed circuits on some models. This helps to reduce emissions and improve driveability. Ignition timing is also advanced for better driveability.

The system is designed to be functional at all altitudes by changing the amount of air supplied to carburetor circuits. The system includes a high altitude compensation valve, vacuum advance sub-diaphragm, and check valve.

OPERATION

At altitudes over 4000 feet, the HAC valve opens and allows air to bleed through the valve into the carburetor. When vehicle is at an altitude below 2600 feet, the valve will be closed and air cannot pass through. The valve is to be partially open between altitudes of 2600 and 4000 feet.

HAC VALVE

Remove hoses or caps from top of HAC valve. Blow into valve upper ports. If air passes through, valve is in high altitude position. If not, valve is in the low altitude position. Replace valve if position does not correspond to altitude.

Scheme 30

1. Warm up engine and check ignition timing at idle. Disconnect hose from distributor sub-diaphragm, and plug hose end.
2. Check ignition timing. Timing should be 0° BTDC. Reconnect hose to sub-diaphragm. Verify that timing advances to about 12° BTDC. (Scheme 30): HAC System
3. Disconnect vacuum hose between check valve and vacuum pipe at vacuum pipe side. Plug pipe end. Ignition timing should remain stationary for more than one minute. Stop engine and reconnect hoses to proper locations.
4. Remove 3 hoses from ports on top of HAC valve. Blow air into each hose and check that air flows into carburetor. Reconnect hoses to proper locations.

LOW ALTITUDE

1. Warm up engine and disconnect hose from distributor sub-diaphragm, and plug hose end.
2. Check ignition timing. Timing should be 0° BTDC. Reconnect hose to sub-diaphragm. Check that ignition timing advances to about 12° BTDC.
3. Disconnect vacuum hose from lower port of HAC valve, and plug hose end. Disconnect vacuum hose between check valve and vacuum pipe at pipe side, and plug pipe end.
4. Check that ignition timing remains stationary for more than one minute. If timing changes, replace check valve. Stop engine and reconnect hose to vacuum pipe and HAC valve.
5. Disconnect 3 hoses from pipes on top of HAC valve. Blow air into each hose and check that air flows into carburetor. Reconnect hoses to proper locations.