Experiencing a high idle in your 1988 Mercedes 560sl can be concerning. One potential culprit, often overlooked, is the throttle position switch. This component, though simple, plays a crucial role in your classic Mercedes’ engine management, particularly idle control. If you’re noticing your 560SL idling higher than usual in park or neutral, a faulty or misadjusted throttle switch could be the reason.
Understanding the Throttle Position Switch in Your 560SL
Located in the engine’s V, at the end of the throttle butterfly shaft, the throttle position switch in your 1988 Mercedes 560SL is essentially a binary switch. It’s designed to signal the engine control system when the throttle is completely closed (idle) and when it’s fully open (full throttle). This switch is not typically meant for adjustment unless it has been replaced or has physically loosened over time. Its primary function is to inform the engine management system about these two extreme throttle positions, which are critical for proper idle and full-power operation. A failure to signal correctly at idle can indeed lead to a high idle condition in your 560SL.
Diagnosing a Faulty Throttle Switch for High Idle
Before suspecting the throttle switch itself, it’s wise to first eliminate any mechanical issues that might prevent the throttle from closing completely. Check the throttle linkage for any binding or obstructions. Ensure that the linkage allows the throttle butterfly to return fully against its stop when your foot is off the accelerator pedal.
Once you’ve confirmed the linkage is operating freely, you can test the throttle position switch electrically. Locate the connector, typically found on the upper left side of the engine near the valve cover. This connector will have three wires leading to the switch. To test, disconnect this connector. Using an ohmmeter, you’ll be testing for continuity.
With the throttle butterfly in the closed (idle) position, check for continuity between the common wire and the wire designated for the idle signal at the connector leading to the switch. You should observe continuity between two of these wires. Next, manually open the throttle butterfly to the wide-open position (simulating full throttle). Now, test for continuity between the common wire and the wire for the full throttle signal. Again, continuity should be present between a different pair of wires than in the idle position.
If you fail to find continuity in either the idle or full throttle position as described, it strongly indicates that the throttle position switch is faulty and likely needs replacement. Given its location in a high-heat area of the engine compartment, it’s not uncommon for these switches to degrade and fail over time on a classic like the 1988 Mercedes 560SL.
By following these diagnostic steps, you can effectively determine if a malfunctioning throttle position switch is the cause of a high idle in your 1988 Mercedes 560SL and take the necessary steps for repair.
