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.
1981 Mercedes Benz 380SLFACTORY OEM SERVICE PROCEDURE
Electronic Idle Speed Control: Testing
NOTE:
Use the following tests if customer complaints include difficult cold starting, idle too high when warm, or engine starts but stalls immediately.
- Warm engine to normal operating temperature. Stop engine and disconnect plug from idle speed adjuster. Measure voltage across the terminals in connector. If battery voltage is present, go to step 4).
- If no voltage is present, remove connector from electronic control unit and measure voltage across terminals 2 (+) and 4 (-). If battery voltage is not present, check wiring and voltage supply relay. If battery voltage is present, check continuity of wiring from electronic control unit to idle speed adjuster. Repair if necessary.
- Connect wiring harness to electronic control unit and measure voltage at idle speed adjuster connector again. If no voltage is present, replace control unit.
- Connector at idle speed adjuster should be unplugged slightly so voltage readings can be measured while the system is operating. Start engine and run at idle. Measure voltage across terminals at idle speed adjuster. If no voltage is present, replace control unit.
- If voltage measured was 4-6 volts, disconnect coolant temperature sensor and place jumper wires across connector terminals. Idle speed should increase to 750 RPM. If so, system is okay. If not, go to next step.
- Disconnect idle speed adjuster. With engine running briefly apply battery voltage (using jumper wires) to idle speed adjuster terminals. If idle speed drops or engine stalls, replace control unit. If idle speed does not drop, replace idle speed adjuster.CAUTION: Do not apply battery voltage for more than 5 seconds to idle speed adjuster terminals.
Factory Service Tools
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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.