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.
Engine Control Module (ECM) power supply, checking
Special tools and equipment
- VAG1526 A
- VAG1594 A
- VAG1598/31
Test requirements
- Fuses for engine electronics OK
Refer to Electrical Wiring Diagrams, Troubleshooting & Component Locations
- Battery voltage at least 12.7 V
- Alternator OK
Test sequence
- Connect test box VAG1598/31 to wiring harness for engine control module; do not connect engine control module. Refer to Wiring and component check with test box VAG 1598/31 .
- Switch ignition on.
- The positive voltage supply to the engine control module is provided via terminal 3 (terminal 15) and terminal 62 (terminal 30).
- The ground connection to the engine control module is provided via terminal 1 and terminal 2.
- Connect multimeter as follows to measure voltage:
| Test box VAG1598/31 Socket |
Measure to |
| 1 | Battery positive |
| 2 | Battery positive |
| 62 | Engine ground |
- Specified value: approx. battery voltage
- Connect multimeter as follows to measure voltage:
| Test box VAG1598/31 Socket |
Measure to |
| 3 | Engine ground |
- Switch ignition on.
- Specified value: approx. battery voltage
If the specified values are not obtained:
- Check wiring connections.
Refer to Electrical Wiring Diagrams, Troubleshooting & Component Locations
- Connect multimeter as follows to measure voltage:
| Test box VAG1598/31 Socket |
Measure to |
| 121 1) | Engine ground |
1) The Motronic Engine Control Module (ECM) power supply relay -J271- carries voltage on terminal 121 of engine control modules after switching off the ignition for up to 15 minutes.
- Switch ignition on.
- Specified value: approx. battery voltage
If specified value is not attained:
- Check wiring connections.
Refer to Electrical Wiring Diagrams, Troubleshooting & Component Locations
- Check Motronic Engine Control Module (ECM) power supply relay -J271-. Refer to Motronic Engine Control Module (ECM) power supply relay -J271-, checking .
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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.