1994 Audi S4FACTORY OEM SERVICE PROCEDURE

A/C Auxiliary Signal Checking: Testing

  1. Start engine and let idle.
  2. Select "READ MEASURING VALUE BLOCK" function 08 using (VAG 1551) Scan Tool and input display group number 05. See READING/MEASURING VALUE BLOCK (FUNCTION 08) under OUTPUT DIAGNOSTIC TEST MODES (DTM) in SELF-DIAGNOSTICS article.
  3. Observe display values in channel 4.
  4. Display position 1 indicates if A/C compressor is ON: Must read 0.
  5. Display position 2 indicates if A/C system is ON: Must read 0.
  6. Display position 3: Must remain blank.
  7. Switch ON A/C, selecting lowest temperature and highest blower speed.
  8. Observe display values. Position 1 must change from 0 to 1. Position 2 must change from 0 to 1.
  9. If positions do not change as described, connect (VAG 1598) test box, using (VAG 1598/5) adapter cable to Engine Control Module (ECM). See CONNECTING VAG 1598 TEST BOX .
  10. Connect (US 1115) LED tester between terminal No. 19 (ground) and terminal No. 41.
  11. Allow engine to idle.
  12. Switch ON A/C, selecting lowest temperature and highest blower speed.
  13. LED tester must light up.
  14. Idle speed must increase.
  15. If LED tester does NOT light up: Switch ignition OFF (stop engine). Wait at least 30 seconds before disconnecting adapter cable from ECM.
  16. Check wiring for breaks/opens between ECM terminal No. 41 (terminal 41 of test box), and A/C control head -E87-. See Figure. Also, see WIRING DIAGRAMS .
  17. If wiring is Okay, check function of A/C. If A/C function is Okay, replace ECM. After replacement of ECM, see ECM IDENTIFICATION and ECM CODING under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS article.
Factory Service Tools

Use the Manual With the Right Hardware

Pair factory procedures with proven DIY tools so the instructions are easier to execute.

Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.

Shop Manual-Ready ToolsCompare Repair QuoteQuote Shield Pro

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.