Why does the car shake intermittently when the air conditioner is turned on?
3 Answers
Intermittent shaking of the car when the air conditioner is turned on is caused by insufficient power after turning on the air conditioner. In other words, the idle speed is too low. The following situations can cause this fault: 1. Insufficient air intake, check the idle speed motor or throttle valve. 2. Insufficient fuel supply, check the fuel pump or fuel filter. 3. Poor ignition, check the spark plugs or cylinder wires. 4. Problems with the belt. Over time, the belt may become loose, hardened, or entangled, especially entanglement can cause "intermittent" load changes. A sudden increase in load can cause the idle speed to drop instantly, resulting in shaking; check the condition of the belt.
This is a pretty common issue. I encountered several similar cases last time I was repairing a car. When the AC compressor kicks in, it puts an additional load on the engine. If the engine itself isn't in good condition, it's prone to shaking. The most common cause is severe carbon buildup in the engine, especially around the throttle body and fuel injectors. Thick carbon deposits can lead to insufficient air intake and inadequate power. Also, worn-out spark plugs or faulty ignition coils can cause incomplete combustion of the air-fuel mixture, resulting in misfires and shaking. Additionally, check the engine mount bushings. If they're aged or cracked, they won't provide proper cushioning, and the compressor's vibrations will directly transfer to the car body. Insufficient belt tension or worn bearings inside the compressor can also cause rhythmic shaking. It's recommended to first use an OBD scanner to read the trouble codes, prioritize cleaning the intake system and replacing the spark plugs, and if the engine mounts are soft or collapsed, replace them outright.
My old car was just like that—when I turned on the AC at idle, the body would vibrate like a phone ringing. Later, I figured out that the main issue was the compressor switching on and suddenly putting a heavy load on the engine. If the battery was low on charge or the alternator output voltage was unstable, the insufficient power supply could easily cause jerking. Abnormal AC system pressure can also cause trouble, like overcharged refrigerant or blocked lines, leading to the compressor cycling on and off repeatedly. An experienced tire shop mechanic taught me to regularly check the belt for cracks—if it’s loose, the compressor will drag the engine and make it shudder. A dirty air filter shouldn’t be overlooked either, as it can suffocate the engine. A simple check you can do is to listen to the compressor sound after starting the car—if there’s a metal grinding noise, get it fixed immediately. The easiest method is to turn off the engine and press down on the engine mounts—if it wobbles too much, it’s time to replace them.
