Diagnosing a high idle issue in a classic Mercedes-Benz 380SL, particularly a 1985 model, requires a systematic approach. This guide outlines a step-by-step diagnostic process, drawing from real-world experience to help you pinpoint and resolve the problem. We’ll explore common culprits and delve into specific checks relevant to the 1985 Mercedes 380sl and its CIS-E fuel injection system.
Initial Checks: OVP Relay and Fuse
One of the first points of investigation for a high idle, especially in models equipped with ABS, is the Overload Protection Relay (OVP). A faulty OVP relay can cause a multitude of electrical gremlins, including idle speed issues.
Start by visually inspecting the fuse located on top of the OVP relay. A blown fuse is a clear indicator of a problem, though simply replacing it might not be a permanent fix if the underlying issue persists.
The Overload Protection Relay (OVP) in a Mercedes-Benz, a key component in diagnosing electrical issues like high idle, shown with its protective fuse on top.
To further assess the OVP relay, carefully remove it and examine the pins for any signs of corrosion. Cleaning the pins with a wire brush and reinserting the relay can sometimes resolve intermittent connection problems. While tapping the OVP relay while the engine is idling might reveal a loose connection, it’s not a definitive test of its overall functionality.
Verifying Alternator Voltage
Before assuming the OVP relay is faulty, it’s crucial to check the alternator’s voltage output. An overcharging alternator can also lead to issues with the OVP and other electrical components.
With the engine running at approximately 1000 RPM, use a voltmeter to measure the voltage across the battery terminals. A healthy alternator should output around 14 volts. Readings significantly higher than 14.5 volts suggest a voltage regulator malfunction, which could be contributing to the high idle and potentially damaging the OVP relay.
Vacuum System Integrity
Vacuum leaks are notorious for causing high idle in older vehicles like the 1985 Mercedes 380SL. The engine’s idle speed is significantly influenced by the vacuum system, and even small leaks can disrupt the delicate balance.
Begin by inspecting the vacuum fittings and hoses, particularly those connected to the intake manifold, especially at the rear of the engine near the firewall. These connections are often exposed to heat and oil, causing the rubber components to degrade and crack over time.
While a visual inspection might not always reveal subtle leaks, it’s a good starting point. Consider replacing suspect rubber connectors, even if they appear intact, as a preventative measure. However, remember that vacuum leaks can originate from various sources, including intake manifold risers and the air guide (throttle boot). A thorough inspection of all vacuum lines and components is essential.
Coolant Temperature Sensor (B11/2) and Idle Speed Control
The coolant temperature sensor (B11/2) plays a vital role in the 1985 Mercedes 380SL’s idle speed control system. This sensor provides temperature readings to both the CIS-E control unit and the EZL ignition control unit. These units, in turn, influence the idle speed based on engine temperature.
Testing the TF signal, as outlined in Mercedes-Benz service manual document 07.3-112, can help determine if the coolant temperature signal is being correctly processed by the control units. This test involves disconnecting the fuel pump relay and using a jumper wire in the relay socket. While the procedure might seem complex initially, understanding the service manual’s diagrams and instructions is key to accurate diagnosis.
Incorrect voltage readings at the fuel pump relay connector (specifically pin no. 2) could indicate a problem with the B11/2 sensor or its wiring. However, ensure you are performing the test correctly before concluding a sensor malfunction. It’s important to note the wiring configuration of the B11/2 sensor, as crossed wires can lead to misinterpretations during troubleshooting.
Idle Speed Control Unit (ISCU)
The Idle Speed Control Unit (ISCU) is another critical component in regulating the idle speed of your 1985 Mercedes 380SL. While the ISCU design might have been updated over the years, its function remains the same: to manage idle speed based on various engine parameters.
While physically manipulating or flexing the ISCU circuit board is generally not recommended, it highlights the complexity of this electronic component. If other simpler checks haven’t yielded results, further investigation into the ISCU’s operation might be necessary, potentially involving specialized diagnostic tools or professional expertise.
Previously Addressed Components: TVS and Idle Air Valve
It’s worth noting that in this particular diagnostic journey, the Throttle Valve Switch (TVS) and Idle Air Valve have already been replaced. While these are common culprits for high idle, their replacement in this case hasn’t fully resolved the issue. This underscores the importance of a systematic approach, ruling out potential causes one by one, even those that have been previously addressed.
Conclusion: Systematic Diagnosis is Key
Troubleshooting a high idle on a 1985 Mercedes-Benz 380SL requires patience and a methodical approach. By systematically checking components like the OVP relay, alternator voltage, vacuum system, coolant temperature sensor, and idle speed control unit, you can effectively narrow down the potential causes. Remember to consult your Mercedes-Benz service manual for detailed procedures and specifications, and don’t hesitate to seek professional assistance when needed to ensure accurate diagnosis and repair of your classic 380SL.
