High Altitude Compensation System Toyota Land Cruiser 60

DESCRIPTION

Note. The High Altitude Compensation (HAC) system is used only on Federal vehicles with high altitude option.

As altitude increases, air pressure decreases and the 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.

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

OPERATION

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

HAC VALVE

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

Scheme 16

1. Warm up engine and check ignition timing at idle. Disconnect vacuum hose from the distributor sub-diaphragm and plug hose end.
2. Timing must remain stationary for more than 1 minute. If timing changes, replace check valve. Stop engine and reconnect hose to check valve. (Scheme 16): HAC System
3. Disconnect 2 hoses 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 vacuum hose between HAC valve and distributor at HAC valve side, and plug hose end. Disconnect vacuum hose from distributor sub-diaphragm, and plug hose end.
2. Check ignition timing. Timing should be 5° BTDC. Reconnect hose to distributor sub-diaphragm. Check ignition timing. Timing should be about 13° BTDC.
3. Disconnect vacuum hose from check valve at Black side and plug hose end. Check that ignition timing remains stationary for more than 1 minute. If timing changes, replace check valve. Stop engine and reconnect hoses to proper location.
4. Disconnect 2 hoses on top of HAC valve. Blow air into each hose and verify that air flows into carburetor. Reconnect hoses to proper locations.