Checking map-controlled engine cooling thermostat: VW Golf IV / Bora (AEH, AKL, ALT, APF)

🧭 Before you start

Fits: VW Golf IV / Bora (1J)

🔧 Engine codes this applies to

📋 Procedure

1. Pull 2-pin connector off map-controlled engine cooling thermostat.
2. Measure resistance of heater element of map-controlled engine cooling thermostat using adapter cables from auxiliary measuring setV.A.G 1594C. Specification at 25 °C: 14...16 Ω.
3. Renew map-controlled engine cooling thermostat F265.
4. Connect fault readerV.A.G 1551 (vehicle system testerV.A.G 1552). Start engine and select "“Address word”" 01 of engine control unit. Connect fault reader and select engine control unit 01.
5. Press keys 0 and 8 for function "“Read measured value block”" and confirm entry with Q key.

6. Press keys 1, 3 and 1 for "“Display group number 131”" and confirm entry with Q key.
7. Connect multimeter to measure voltage at contacts 1 and 2 of 2-pin connector for thermostat (wiring harness side)
8. Pull 4-pin connector (1) off coolant temperature sender G62(2).
9. Allow engine to continue to run at idling speed and observe the left "bit" in display zone 4: if after a couple of seconds a "“1”" is displayed (fault detected) the duty cycle of the thermostat must change to 100% in display zone 3.
10. Now read voltage on multimeter. Specification: approx. alternator voltage.
11. Reconnect 4-pin connector to coolant temperature sender G62 again.
12. Check functions of thermostat .
13. Switch engine off.
14. Connect multimeter to measure voltage to contact 1 of 2-pin connector and earth.
15. Switch on ignition. Specification: at least 11.5 V
16. Check wiring/connections to current supply relay for Simos engine control unit (in protective housing for relays in engine compartment)
17. Switch off ignition.
18. Connect 121-pin adapter cableV.A.G 1598/31 to control unit wiring harness. The engine control unit remains disconnected.
19. Check wiring between test box and 2-pin connector for open circuit referring to current flow diagram. Contact 2 and test box socket 116, wiring resistance: max. 1.5 Ω.
20. Additionally check wire for short circuit to battery positive and earth. Specification: ∞ Ω.

🎬 Video walkthrough

Independent video tutorial by AUTODOC (YouTube) — a helpful visual companion to the steps above.

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