Answering a South African customer's questions about the blower motor resistor.
To test the interior heater fan resistor (often referred to as a "blower motor resistor") in a Mercedes W203, follow these steps:
1. Tools You'll Need:
Multimeter
Screwdriver or trim removal tool (to access the resistor)
Service manual or guide for W203 (optional, for precise resistor location)
2. Symptoms of a Faulty Resistor:
Before testing, confirm the need to check the resistor:
Blower works only on certain speeds.
No airflow regardless of settings.
Blower operates intermittently.
3. Locate the Blower Resistor:
The blower resistor is typically located near the blower motor, under the passenger side dashboard.
Remove the glovebox or access panel to expose the blower motor and resistor.
4. Inspect for Physical Damage:
Check for visible damage like burnt spots, corrosion, or broken connectors on the resistor pack.
5. Test the Resistor with a Multimeter:
Set the multimeter to ohms (Ω) for resistance testing.
Disconnect the resistor from its wiring harness.
Use the multimeter probes to measure resistance across the terminals (refer to a wiring diagram for exact pin layout).
Each resistor in the pack should show continuity (a measurable resistance value).
Open circuits (infinite resistance) indicate a fault in that segment.
6. Voltage Test (Optional):
If the resistor physically appears fine:
Reconnect the resistor to the wiring harness but leave it accessible.
Turn on the ignition and adjust the fan speeds.
Use the multimeter set to volts (V) to measure voltage output from the resistor.
Different speeds should show varying voltage levels. Lack of variation or no voltage indicates a fault.
7. Check Related Components:
Ensure the blower motor itself is functional. A seized motor can damage the resistor.
Check the wiring and connectors for damage or loose connections.
8. Replace If Necessary:
If the resistor is faulty, replace it with a new one compatible with the W203 model. Ensure the replacement part meets OEM specifications.
