A common issue in older Mercedes-Benz vehicles, particularly the SLK R170 (1996-2004), is intermittent starting problems. Understanding the motor OBDII drive cycle for vehicles manufactured between 1996 and 2018 can be crucial in diagnosing these issues. While this article focuses on a specific anecdote involving a K40 relay module, the underlying principles regarding drive cycles and their importance in setting readiness monitors remain relevant for various makes and models within this timeframe.
The K40 Relay Module and Starting Problems
One potential culprit for starting issues in the R170 is the K40 relay module. A forum user described a scenario where their 1998 SLK230 exhibited intermittent starting problems. After replacing the K40 relay module multiple times with used parts, they found success with a module bearing a different part number (“A 170…”) compared to the original (“A 210…”).
The user observed a significant difference in the circuitry of the “A 170…” module. Specifically, the resistors near the third relay measured the expected 69-71 ohms, while the corresponding resistors in the original “A 210…” module measured only half that value (approximately 34 ohms). This discrepancy suggested a potential parallel connection of the resistors in the faulty module, leading to increased current and possible soldering issues due to overheating.
The Importance of the OBDII Drive Cycle
While the user’s experience points to a hardware issue, completing a proper OBDII drive cycle is essential for accurate diagnostics. A drive cycle allows the vehicle’s onboard diagnostic system to run various tests and set readiness monitors. These monitors track the performance of different emissions-related components. If a monitor doesn’t set, it can indicate a problem, prevent passing an emissions test, and even hinder accurate troubleshooting.
For 1996-2018 vehicles, specific drive cycles vary by manufacturer and model. Consult your vehicle’s repair manual for the correct procedure. Generally, a drive cycle involves a sequence of driving conditions, including:
- Cold Start: Starting the engine from a cold soak (at least six hours without running).
- Idle: Letting the engine idle for a specific duration.
- Acceleration: Accelerating to a certain speed.
- Cruising: Maintaining a constant speed.
- Deceleration: Slowing down without braking heavily.
Completing a full drive cycle after a repair, such as replacing the K40 relay, ensures that the OBDII system can accurately assess the repair’s effectiveness and set the appropriate readiness monitors.
Conclusion
Troubleshooting starting problems in a Mercedes SLK R170, or any vehicle from the 1996-2018 era, requires a thorough understanding of potential hardware issues like the K40 relay module and the importance of the OBDII drive cycle. Performing the correct drive cycle allows the vehicle’s diagnostic system to accurately assess repairs and set readiness monitors, enabling proper diagnosis and ensuring the vehicle runs smoothly. Always consult your vehicle’s repair manual for specific drive cycle instructions and troubleshooting guidance.
