The High Altitude Compensator (HAC) switch is only used on carbureted models.
The High Altitude Compensator (HAC) switch is essentially a barometric pressure switch, which turns on (the switch contacts close) when the vehicle is operated above 4,000 ft. (1220m). Whenever the vehicle is operated below that altitude, the switch is open.
The ECM uses the information provided from the HAC switch to compensate the fuel mixture in the feedback carburetor system. At higher altitudes air density declines, which means that the amount of fuel needed by the vehicle drops because there is less air to mix with the fuel. If the feedback carburetor system does not compensate for a difference in altitude, eventually the air/fuel mixture will become too rich for the engine to operate efficiently.
The HAC switch is mounted on the right-hand side of the firewall, in the engine compartment. It is small (about 2.5 inches from top to bottom), cylindrical in shape and secured to the firewall by two mounting bolts.
- Detach the wiring harness connector from the HAC switch, then attach an ohmmeter to the terminals of the switch.
Check the resistance of the switch.
- If your altitude is 4,000 ft. (1220m) or higher, the switch should show 0 ohms (the switch contacts are closed).
- If the altitude is below 4,000 ft. (1220m), the switch should show infinite resistance (no continuity).
- If the switch resistance values were not as specified, or if the switch exhibited any ohm value other than 0 or infinity, the switch is defective and should be replaced with a new one.
REMOVAL & INSTALLATION
- Disconnect the negative battery cable.
- Disengage the wiring harness connector from the sensor.
- Loosen the mounting bolts, and remove the sensor from the firewall.
- Position the sensor on the firewall, then install and tighten the mounting bolt(s) securely.
- Reattach the wiring harness connector to the sensor.
- Connect the negative battery cable.