Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSAUDI1998A4 AVANT QUATTRO, AUTOMATICREPAIR AND DIAGNOSISHEATING, VENTILATION & A/C (HVAC)HVAC CONTROL SYSTEMSA/C-HEATER SYSTEMTESTINGA/C COMPRESSOR SPEED SENSOR, PRESSURE SWITCHES & INTERIOR TEMPERATURE SENSOR FAN
1998 Audi A4 Avant Quattro, Automatic
A/C Compressor Speed Sensor, Pressure Switches & Interior Temperature Sensor Fan
1998 Audi A4 Avant Quattro, AutomaticSECTION A/C Compressor Speed Sensor, Pressure Switches & Interior Temperature Sensor Fan
- Put DVOM in 20-amp range. Check amperage between test box terminal No. 44 and ground. Turn ignition on. If reading is not less than one amp or interior temperature sensor fan does not run, check for open or short in Brown wire between interior temperature sensor fan and A/C-heater control panel. Repair as necessary.
- Connect LED Tester (US 1115) between test box terminals No. 12 and 14. Start engine. If LED in tester comes on, check for short in circuits between A/C compressor, A/C-heater control panel, and ground. See WIRING DIAGRAMS . Check for A/C compressor switch-off conditions caused by ECM or TCM. See A/C COMPRESSOR SWITCH-OFF CONDITIONS under SELF-DIAGNOSTICS. Repair or replace as necessary.
- Connect LED tester between ground and test box terminal No. 51. Start engine. If LED in tester comes on, check for short in circuits between A/C compressor, A/C-heater control panel, and ground. See WIRING DIAGRAMS . Check for A/C compressor switch-off conditions caused by instrument cluster. See A/C COMPRESSOR SWITCH-OFF CONDITIONS under SELF-DIAGNOSTICS. Repair or replace as necessary.
- Set voltmeter to 200 milliamp scale. Connect test leads between ground and test box terminal No. 12. Start engine. With A/C off, less than 5 milliamps should be present. If greater than 5 milliamps are present, repair short to voltage. See WIRING DIAGRAMS . With A/C on, less than 50 milliamps should be present. If greater than 50 milliamps are present and/or compressor engages, replace A/C-Heater assembly. See A/C-HEATER ASSEMBLY under REMOVAL & INSTALLATION. Remove test leads from test box. If compressor does not engage, diagnose A/C compressor cut-off conditions. See TESTS W/CODES article in ENGINE PERFORMANCE.
- Set ohmmeter to 200 ohm scale. Ensure ignition is off. Connect test leads between test box terminals No. 2 and 42. If resistance is not less than 20 ohms, replace A/C pressure switch.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.