High Altitude Compensation System Dodge Ram I

CARBURETED

All vehicles using feedback carburetors are equipped with a high altitude compensation system. The system consists of a High Altitude Compensator (HAC), vacuum switching valve, distributor equipped with high altitude advance system, passages for additional bleed and jet air, and a check valve. As altitude increases, air pressure decreases and the air/fuel mixture becomes richer. The HAC system supplies additional air to the primary main well.

FUEL INJECTED

Fuel injected models are equipped with an atmospheric pressure sensor which acts as the high altitude compensator. The atmospheric pressure sensor senses pressure and converts it to a voltage which is sent to the ECU. The ECU computes altitude and corrects air/fuel ratio and ignition timing. For complete information on this system, see ELECTRONIC CONTROL INJECTION (ECI) SYSTEM article.

At high altitudes, the control valve is opened by the HAC aneroid bellows. This allows air to bleed through the valve into the primary main well. At low altitudes, the control valve remains closed and no additional bleed air flows to main well.

California

1. Engine coolant temperature should be 185-205°F (85-95°C). Remove air filter. Disconnect Black vacuum hose from carburetor primary emulsion well bleed nipple. Plug nipple at carburetor. Connect vacuum pump to vacuum hose and apply vacuum. On low altitude models, 3900 ft. (1200 m) or less, vacuum should hold while running engine at idle. On high altitude models, vacuum should leak.
2. Reconnect hose to carburetor. Run engine at idle and check ignition timing. See «IGNITION TIMING SPECIFICATIONS»(/dodge/ram/i-1981-1993/remont/testing-diagnostics/#high-altitude-compensation-system) . On low altitude models, with engine at idle, disconnect White-striped vacuum hose from HAC. There should be vacuum.

Federal

1. Engine coolant temperature should be 185-205°F (85-95°C). Remove air filter. Disconnect both Black vacuum hoses and Red-striped vacuum hose from carburetor primary and secondary emulsion well bleed and jet air nipples. Plug nipples at carburetor. Connect a vacuum pump to vacuum hoses one at a time, and check while running engine at idle. On low altitude models, 3900 ft. (1200 m) or less, system is okay if vacuum holds. On high altitude models, system is okay if vacuum leaks.
2. Reconnect hoses to carburetor. Run engine at idle and check ignition timing. See «IGNITION TIMING SPECIFICATIONS»(/dodge/ram/i-1981-1993/remont/testing-diagnostics/#high-altitude-compensation-system) . On low altitude models, with engine at idle, disconnect Yellow-striped vacuum hose from HAC. There should be vacuum.

IGNITION TIMING SPECIFICATIONS

1. Disconnect White-striped vacuum hose from HAC. Connect vacuum pump to HAC. At low altitude, 3900 ft. (1200 m) or less, when vacuum is applied, vacuum should leak. At high altitude, vacuum should hold.
2. On low altitude models, disconnect Black hose from HAC and connect vacuum pump. Vacuum should hold when applied. On high altitude models, disconnect Black hose from HAC and connect another vacuum pump (HAC now has 2 vacuum pumps connected to it). Hold vacuum with first pump, and apply vacuum with second pump. Vacuum should leak when applied.

1. Disconnect Yellow-striped vacuum hose from HAC. Connect vacuum pump to HAC. At low altitude, 3900 ft. (1200 m) or less, when vacuum is applied, vacuum should leak. At high altitude, vacuum should hold.
2. On low altitude models, disconnect Red-striped hose from HAC and connect vacuum pump. Vacuum should hold when applied. On high altitude models, disconnect Red-striped hose from HAC and connect another vacuum pump (HAC now has 2 vacuum pumps connected to it). Hold vacuum with first pump, and apply vacuum with second pump. Vacuum should leak when applied.

VACUUM SWITCHING VALVE

Remove valve and connect vacuum pump to Black nipple. Apply 15.7 in. Hg of vacuum to valve and check that it holds. With 6.0 in. Hg of vacuum applied, air should not blow through valve from carburetor air bleed side nipple. With 10.0 in. Hg of vacuum applied, air should blow through.

CHECK VALVE

Remove check valve and connect vacuum pump to Dark Blue nipple. When vacuum is applied, vacuum should leak. Connect vacuum pump to White nipple. When vacuum is applied, vacuum should hold.

Scheme 14

Scheme 15

See also:
• IGNITION TIMING SPECIFICATIONS