Air conditioning intermittently not blowing cold air

JAVIER GOVEA

MEMBER
1999 MERCEDES BENZ CLK430
V8
2WD
AUTOMATIC
120,000 MILES

My AC doesn't blow cold at the first start of the car, but will the second or third time the car has been shut off and started after sitting for a little. Per example, yesterday after school I started my car and it was blowing hot (as expected because it's been sitting in the sun). I then drove to the bank, shut it off and turned it on about five minutes later when I was done and it still blew hot. Then I went to my girlfriends house and shut it off, it sat for about thirty minutes but still blew hot when started up. From there I went home, shut it off and about fifteen minutes later left for work but this time the AC was blowing cold. This occurs throughout the days but the amount of times needed to restart before working fluctuates. When the AC starts working again, it will work just fine all day, but the next day the problem happens again. It's not the refrigerant as I had that filled by the auto shop, plus the AC does blow cold when it does work. Sometimes I can get it to work when driving by turning the AC entirely off (no AC or air coming through the vents) and turning it back on, but this does not always work.

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

Hi Javier,

This sounds like the blend air door is not moving to block the air from go over the heater core which is what heats the air before it comes into the passenger compartment. In most all vehicles, the air always goes over the evaporator (this cools the air and dehumidifies it) but only goes over the heater core if the driver commands the heat on. So if the system appears to work at times, the only thing that makes sense is that the air is getting heated when it should not.

If this is the case, it could be a couple things. It could be the door itself but this would make more sense if it blew hot all the time. The fact that sometimes it works properly points to more of a control module issue or a sensor issue.

For a control module to make sense you would most likely need to have auto temperature control. Meaning you just set a temperature and allow the vehicle to regulate or determine if the heat or AC is needed. A manual temperature system usually has dials and the temperature only changes if you change the setting.

If the control module is malfunctioning then it could think that it is colder then it is and it commands the heat on when it should not. So this means the blend door is just doing what it is told.

The last thing that I suspect it could be a sensor issue that is giving the control module bad information. Usually, when the ambient temp sensor or another fails it goes to reading -40 degrees and if the control module sees this, it will think it is very cold outside so it is going to blow the heat.

One way to test the door operation is to move the temperature setting all the way up to the max hot setting and then move it all the way to the cold setting and see it you start getting cold air. Moving the temp up and down like this will force the module to move the door.

In order to test the sensor or module we would need a scan tool in order to look at the sensors. Do you have access to one of these?