GDawgC220

When I have my A/C on and the car parked (in P position), the idle is rough, like the whole car shakes or rumbles...is this normal with the A/C on?

sph17

Happend to me with a C-220. Dealer said it is common to be rough.

Is it just rough, or does it seem like the engine is pulsating every couple of seconds, changing revs by 2-300 rpm? If it is pulsating, then your A/C may be overpresurized with coolant and the A/C unit may be turning on/off every couple seconds and causing the engine reves to fluctuate. This tends to happen in hot (over 90F) temps if it is over presureized.

GDawgC220

Yea, seems to pulsate, like u said every few seconds. The RPM needle will jump kind of when it starts to pulsate...maybe just normal when it's hot outside and with A/C running for a 4 banger.

sph17

When you start up the car and it pulsates, go look under the hood at your compressor. Check to see if the compressor clutch is clicking on/off every few seconds. If it is, then your A/C may be over presurized.

This happened to me whenever it was very hot out, and my car was driven a while, parked for a few min. and then re-started. This happened to me for 4 months, until almost by luck I noticed this w/ a MB Tech. When he saw that the A/C clutch was cycling on/off he knew that this was caused by over-pressurization.

GDawgC220

This may be a silly question, but what is the compressor clutch? And what is the remedy for overpressurized A/C?

sph17

In the front left (drivers side) of your engine compartment, there is the A/C compresor. The single serpintine belt goes from the main fan blade, toward the drivers side to the A/C compressor, then down around to go to the alternator on the other side of the engine.

The A/C compressor has an electrically operated clutch. When the clutch is "off" the front of the clutch does not go around in circles. When the A/C clutch is electrically activated, you can see the front of the clutch start spinning. You can watch this by opening your hood, look for the A/C compressor on the front drivers side of the engine, and have a second person turn the A/C On, then Off. Once you see what I am describing, then next time your car pulsates, go open the hood, and see if your A/C clutch is turning On/Off every couple of seconds.

The fix for this is to have a qualified A/C service center (dealer, service station, or repair shop) check the pressure of the coolent in your A/C system. In the olden days this would be called checking the Freon level, but newer cars don't use Freon (or other CFC's) because they are environmentaly unfriendly.

Let me know what you find.