For owners of a classic 1989 Mercedes-Benz 300E, understanding and maintaining your vehicle’s systems is crucial. One invaluable tool for DIY mechanics and enthusiasts working on these iconic W124 chassis cars, especially California models, is the 8-pin diagnostic scanner. This guide will walk you through how to use this system to retrieve diagnostic codes and reset your check engine light, ensuring your 300E runs smoothly.
Reading Diagnostic Codes on Your 1989 300E
The 8-pin diagnostic system on your 1989 300E provides a straightforward way to access fault codes from the car’s engine management system. By counting LED blinks, you can pinpoint potential issues and address them effectively. Here’s a step-by-step process:
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Prepare the Vehicle: Ensure your 1989 Mercedes 300E is turned off.
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Ignition Position: Turn the ignition key to the #2 position. This is the position right before you start the engine – accessory power is on, but the engine is not running. Do not start the car.
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Access the Diagnostic Port: Locate the 8-pin diagnostic port in your 300E. (Note: specific location may vary slightly, consult your vehicle’s manual if needed).
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Engage the Switch: Press and hold the switch located at the diagnostic port for approximately one second, then release it.
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Count the LED Blinks: Observe the LED indicator light. It will blink a certain number of times before staying solid or always on. Carefully count the number of blinks. This sequence of blinks represents a diagnostic trouble code.
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Check for Additional Faults: To see if there are more stored fault codes, press the switch again. The system will display the fault codes in descending order. If no further faults are stored, the first fault code will repeat.
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Record the Codes: Write down each fault code (number of blinks) you observe. This record will be essential for diagnosing the problem using the error code list provided below.
1989 Mercedes-Benz 300E (W124) 8-Pin Diagnostic Error Codes
Understanding the error codes is key to interpreting the diagnostic information from your 8-pin scanner. Here is a list of the diagnostic codes for your 1989 Mercedes 300E, represented by the number of LED blinks:
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1 Blink: No Malfunctions Detected – This is good news! It indicates that the system is currently not detecting any faults.
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2 Blinks: Full load contact, throttle valve switch implausible – Indicates a potential issue with the throttle position sensor at full throttle.
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3 Blinks: Coolant temp. read by CIS-E implausible – Signals a problem with the coolant temperature sensor reading by the CIS-E fuel injection system.
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4 Blinks: Potentiometer voltage implausible – Suggests an issue with a potentiometer, possibly related to fuel mixture or air intake.
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5 Blinks: O2-sensor signal implausible – Points to a problem with the oxygen sensor signal, which is critical for fuel efficiency and emissions.
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7 Blinks: TNA-signal (RPM-signal) read by CIS-E implausible – Indicates an issue with the RPM signal being received by the CIS-E system, potentially affecting engine timing and fuel delivery.
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8 Blinks: Altitude pressure signal from EZL control unit implausible – Suggests a problem with the altitude pressure sensor signal from the EZL ignition control unit, important for adjusting engine performance based on altitude.
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9 Blinks: Current to EHA is implausible – Relates to an issue with the Electro-Hydraulic Actuator (EHA) current in the CIS-E system, which fine-tunes fuel mixture.
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10 Blinks: Idle contact, throttle valve switch implausible – Indicates a problem with the throttle position sensor at idle.
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11 Blinks: Air Injection System – Fault in the air injection system, which reduces emissions.
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12 Blinks: Absolute pressure values from EZL is implausible – Problem with the absolute pressure readings by the EZL unit, impacting ignition timing and fuel calculations.
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13 Blinks: Intake air temp. reading is implausible – Issue with the intake air temperature sensor reading, affecting fuel mixture adjustments based on air temperature.
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14 Blinks: Speed signal read by CIS-E is implausible – Problem with the vehicle speed signal input to the CIS-E system.
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16 Blinks: EGR (Exhaust Gas Recirculation) – Fault in the Exhaust Gas Recirculation system, designed to reduce NOx emissions.
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17 Blinks: O2-sensor line shorted positive to ground – Oxygen sensor wiring issue, specifically a short to ground.
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18 Blinks: Current to idle speed air valve is implausible – Problem with the idle speed control valve current, affecting idle stability.
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22 Blinks: O2-sensor heating current implausible – Issue with the oxygen sensor heater circuit, which ensures the sensor reaches operating temperature quickly.
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23 Blinks: Short circuit to positive in purge switchover valve circuit – Short circuit to positive voltage in the purge valve circuit of the evaporative emissions system.
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25 Blinks: Short circuit to positive in start valve circuit – Short circuit to positive in the cold start valve circuit.
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26 Blinks: Short circuit to positive in shift point delay circuit – Short circuit in the shift point delay circuit (likely related to automatic transmission models, though less relevant for engine diagnostics).
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27 Blinks: Data exchange between CIS-E unit & EZL unit interrupted – Communication problem between the CIS-E fuel injection computer and the EZL ignition control unit.
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28 Blinks: Intermittent contact in coolant temp. sensor circuit – Intermittent wiring or sensor issue with the coolant temperature sensor circuit.
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29 Blinks: Difference in coolant temp. read between CIS-E unit & EZL unit – Discrepancy in coolant temperature readings between the CIS-E and EZL units, suggesting sensor or wiring issues.
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31 Blinks: Intermittent contact in intake air temp. sensor – Intermittent wiring or sensor issue with the intake air temperature sensor circuit.
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34 Blinks: Coolant temp. from EZL unit is implausible – Coolant temperature reading from the EZL unit is not plausible, indicating a sensor or circuit problem specific to the EZL system.
Resetting the Check Engine Light on Your 1989 300E
After diagnosing and correcting the issue indicated by the fault code, you’ll want to reset the check engine light. Here’s how to do it using the 8-pin diagnostic system:
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Immediately After Code Reading: Within 20 seconds of the LED going solid/always on after reading codes, proceed with the reset process.
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Depress and Hold: Press and hold the switch at the diagnostic port for 6 to 10 seconds.
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Release the Switch: Release the switch.
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Observe the LED: The LED should blink once and then go to solid/always on. This indicates the reset command has been received.
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Turn Off Ignition: Turn the ignition switch off completely.
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Restart and Verify: Restart your 1989 Mercedes-Benz 300E and check if the Check Engine Light is now off. Important: Ensure you have actually corrected the underlying problem that triggered the fault code. Resetting the light without fixing the issue will only result in it illuminating again.
Using the 8-pin diagnostic scanner on your 1989 Mercedes-Benz 300E W124 California car is a valuable skill for any owner. By understanding how to read and interpret these codes, you can maintain your classic Mercedes and keep it running in top condition for years to come. Remember to always consult your vehicle’s repair manual for detailed procedures and safety information.
