When you turn on the air conditioning (AC) in your car, it adds extra load to the engine. The AC compressor, responsible for cooling the air, requires power to operate. The power comes from the engine, which has to work harder to keep everything running smoothly.
If there are any underlying issues with the engine or related components, this additional load can cause the car to shake.
The shaking can be more pronounced when the car is idling because the engine is running at a lower speed and may struggle to handle the extra load.
When driving, the engine speed is higher, which can mask the symptoms. However, the underlying issues remain and need to be addressed to prevent further damage.
Common Symptoms
Identifying the symptoms can help narrow down the cause of the problem. Here are some common signs that your car may exhibit when the AC is on:
- Vibration at Idle: The car may shake more noticeably when idling, which is often the first sign that something is wrong.
- RPM Fluctuations: The engine’s RPM may fluctuate when the AC is on. You might notice the RPM needle moving up and down on the dashboard.
- Reduced Cooling Efficiency: The AC may not cool as effectively as it used to. It can be a sign that the AC compressor is struggling or that there is another issue affecting the system.
Common Causes of Car Shaking When AC is On
Faulty Motor Mounts
Motor mounts are responsible for holding the engine in place and absorbing vibrations. If the motor mounts are worn or broken, they can’t absorb the extra vibrations caused by the AC compressor, leading to noticeable shaking. Motor mounts are made of rubber and metal, and over time, the rubber can deteriorate, reducing their effectiveness.
How to Identify Faulty Motor Mounts
Visual inspection can reveal cracks or damage in the motor mounts. Gently rocking the engine back and forth can also indicate worn motor mounts if there is excessive movement.
Additionally, listen for clunking or banging noises when accelerating or decelerating, as these sounds can be a sign of faulty motor mounts.
Issues with the AC Compressor
The AC compressor itself can be a source of the problem. If the compressor is failing or has a bad clutch, it can create additional drag on the engine, causing it to shake.
The compressor is a critical component of the AC system, and any issues with it can affect the entire system’s performance.
Signs of a Failing AC Compressor
Unusual noises such as grinding or squealing when the AC is on can indicate a failing compressor. Reduced cooling efficiency and visible damage like leaks can also be signs of compressor issues.
Idle Air Control Valve Problems
The idle air control (IAC) valve helps regulate the engine’s idle speed. If the IAC valve is dirty or malfunctioning, it can cause the engine to idle roughly, especially when additional loads like the AC are applied. The IAC valve adjusts the amount of air that bypasses the throttle plate to maintain a stable idle speed.
Symptoms of a Faulty IAC Valve
A rough idle, fluctuating RPM, and the check engine light coming on are common symptoms of a faulty IAC valve. Cleaning the valve can often restore proper function, but replacement may be necessary if cleaning doesn’t help.
Clogged or Dirty Air Filters
Air filters play a crucial role in ensuring the engine gets the right amount of air. If the air filters are clogged or dirty, the engine may struggle to get enough air, leading to rough idling and shaking when the AC is on. A clean air filter ensures optimal air flow, which is essential for efficient combustion.
How to Check Air Filters
Remove the air filter and hold it up to the light. If you can’t see light through it, it’s time to replace it. Follow the manufacturer’s recommended interval for air filter replacement, usually every 12,000 to 15,000 miles.
Worn Spark Plugs
Spark plugs are essential for igniting the fuel in the engine. If the spark plugs are worn or dirty, they can cause misfires, leading to engine shaking, especially under additional load from the AC. Spark plugs need to be in good condition to ensure efficient combustion.
Signs of Worn Spark Plugs
Engine misfires, poor fuel economy, and difficulty starting the engine are common signs of worn spark plugs. Replacing them can improve engine performance and reduce shaking.
Faulty Throttle Position Sensor
The throttle position sensor (TPS) helps the engine control unit (ECU) manage the air-fuel mixture. If the TPS is faulty, it can cause the engine to run inefficiently, thus leading to shaking when the AC is on. The TPS provides information about the position of the throttle to the ECU, which adjusts the fuel injection accordingly.
Symptoms of a Faulty TPS
Erratic idle, poor acceleration, and the check engine light being illuminated are common symptoms of a faulty TPS. Replacing the sensor can restore proper engine function.
Cooling System Issues
Problems with the cooling system, such as a failing radiator fan or a clogged condenser, can cause the engine to overheat. An overheating engine can shake, especially when the AC is on. The cooling system is vital for maintaining the engine’s temperature, and any issues can lead to serious problems.
Identifying Cooling System Issues
Overheating, visible coolant leaks, and poor AC performance are signs of cooling system issues. Addressing these problems promptly can prevent further damage to the engine.
If the cooling system is causing the problem, it may be necessary to replace the radiator fan, flush the cooling system, or clean the condenser.