What causes the shaking of a Mercedes-Benz at idle with the air conditioning on?
3 Answers
Mercedes-Benz shaking at idle with the air conditioning on is caused by the air conditioning not speeding up and excessive compressor pressure. Air conditioning not speeding up: Generally, a car's idle speed is maintained at around 800 rpm. If the air conditioning is turned on, the idle speed should normally increase accordingly, usually maintaining above 1000 rpm to avoid shaking at idle. This speed increase is mainly controlled by the computer via the throttle. If the idle speed does not increase when the air conditioning is turned on, the throttle should be cleaned and then recalibrated. Excessive compressor pressure: Excessive air conditioning pressure increases the engine load, causing shaking. Generally, poor condenser cooling, excessive refrigerant, or dirty air conditioning pipes can lead to excessive pressure, indirectly causing shaking.
My Mercedes had the same issue before. When idling with the AC on, the car body shook noticeably, especially at red lights. Later, I asked a mechanic who said common causes include: 1) aging or loose engine mounts—Mercedes' rubber support components tend to harden and fail after years, causing vibrations when the compressor starts under load. 2) Faulty spark plugs or ignition coils leading to unstable ignition and RPM fluctuations. 3) Fuel system issues like clogged injectors or insufficient fuel pump pressure. 4) AC compressor seizing or belt slippage, increasing engine strain. Such shaking isn't just annoying but may accelerate part wear—get mounts and ignition checked promptly to prevent worsening.
After driving my own Mercedes for a long time, I noticed that it vibrates when idling with the AC on. This is usually because the engine load suddenly increases. When the AC compressor starts, it draws power from the engine. Since the idle speed is already low, if the supporting components like engine mounts are damaged or their cushioning effect has degraded, vibrations can easily occur. Another possible cause is an issue with the electrical system, such as an unresponsive idle control valve or a malfunctioning ECU adjustment, leading to unstable RPM. This is more common during hot summer months due to more frequent AC usage. It's best to start with simple checks—look for cracks or looseness in the mounts and test whether the voltage is stable. Don’t ignore these minor symptoms; addressing them early can save you from major repairs later.
