Some TCCS equipped engines like the 3F-E, 3VZ-E (Cab and Chassis), and the 7M-GTE incorporate an altitude sensor in the TCCS system to shorten injection duration when the vehicle is operated at higher altitudes.
Because the density of oxygen in the atmosphere is lower at high altitudes, the air volume measured by the air flow meter will not accurately represent actual oxygen entering the engine. This would result in a mixture which is excessively rich, causing emissions and driveability concerns.
The HAC sensor is integrated with the ECU on the 3-FE, 3VZ-E, and 1989 and later 7M-GTE engines. It is remotely mounted behind the glove box on the '87 and '88 7M-GTE Supra. The remotely mounted HAC sensor is wired to the ECU exactly the same as the manifold pressure sensor is wired on D type EFI. In fact, the HAC sensor circuit is electrically the same as a manifold pressure sensor circuit. The HAC sensor simply measures atmospheric pressure rather than intake manifold pressure.
The signal from the HAC circuit in the ECU is used to determine the fuel correction coefficient to be used after basic injection has been calculated. The accompanying graph represents how this correction factor affects final injection duration.
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