Experiencing air conditioning issues in your Mercedes 1999 Sl can be frustrating, especially when the weather gets hot. If you’re noticing problems like inconsistent cooling, or a complete lack of cold air despite hearing the fan running, the culprit might be a faulty evaporator temperature sensor. Before you invest in expensive repairs based on guesswork, diagnosing this sensor is a straightforward process you can undertake yourself.
Symptoms of a Faulty Evaporator Temperature Sensor
The original poster described experiencing AC failure in their Mercedes 1999 SL, particularly in warmer conditions. The key symptoms included:
- No cold air output: Despite the AC being set to a low temperature and the fan running, no air was blowing from the vents.
- “AUTO” mode failure: The problem occurred when the AC was in “AUTO” mode.
- Temporary fix with “EC” mode: Switching to “EC” (Economy) mode temporarily resolved the issue, but it would return after about ten minutes of switching back to regular AC.
- Hot car interior: The car would become excessively hot, especially when driving with the roof up in warm weather.
These symptoms strongly suggest an issue with the evaporator temperature sensor, which is responsible for regulating the AC system’s cooling output.
Diagnosing the Evaporator Temperature Sensor
A simple self-diagnostic routine using the climate control panel can help determine if the evaporator temperature sensor is malfunctioning. This test can be performed without starting the engine, making it a convenient first step in your troubleshooting process.
Here’s how to check the evaporator temperature sensor on your Mercedes 1999 SL:
- Turn the ignition to the “ON” position, but do not start the engine.
- Set the climate control temperature to 72°F (22°C).
- Press and hold the “REST” button on the climate control panel for more than 6 seconds.
- The left display will begin to cycle between “01” (the test number) and the cabin interior temperature.
- Press the fan speed “+” button repeatedly to advance through the test numbers until you reach “05”.
- Test number “05” displays the evaporator temperature.
A functioning evaporator temperature sensor should display a reading consistent with the ambient temperature, especially if the car has been sitting overnight. A logically impossible reading, such as a high temperature like 40°C (104°F) when the car has been cool, strongly indicates a faulty sensor.
The Solution: Replacing the Evaporator Temperature Sensor
Replacing the evaporator temperature sensor is a relatively quick and easy repair. It’s often located in an accessible area, making it a DIY-friendly task for many Mercedes 1999 SL owners. Before considering more expensive repairs, replacing this sensor is a prudent first step, especially given the diagnostic test suggests it’s faulty. In the original case, replacing this sensor resolved the described AC issues completely, saving the owner from potentially unnecessary and costly dealer “guesses.”
Exploring Other Diagnostic Tests
While the evaporator temperature sensor is a common issue, the Mercedes 1999 SL climate control system offers further diagnostic tests that can be accessed through the same “REST” button procedure. These tests can provide readings for other sensors and system parameters, including:
- Test 01: In-car temperature sensor
- Test 02: Outside temperature sensor
- Test 03: Heater core temperature sensor
- Test 06: Engine Coolant Temperature (ECT) sensor
- Test 07: Refrigerant pressure (in BAR)
- Test 08: Refrigerant temperature sensor
- Test 21: Engine speed
- Test 22: Vehicle speed
By exploring these tests, you can gain a more comprehensive understanding of your Mercedes 1999 SL’s climate control system and potentially pinpoint other issues.
Conclusion
If your Mercedes 1999 SL is experiencing AC problems characterized by intermittent cooling or a lack of cold air, don’t immediately assume the most expensive component is at fault. Utilize the built-in diagnostic test to check your evaporator temperature sensor. A simple sensor replacement might be all it takes to restore your AC to its optimal performance, ensuring comfortable drives in your classic Mercedes. This proactive step can save you time, money, and unnecessary frustration in resolving your 1999 SL’s air conditioning woes.
