can't feel AC engage anymore... - PeachParts Mercedes-Benz Forum

JHZR2 · #1 12-13-2002, 02:19 PM

Hi,

Usually (i.e. in the warmer weather), I could turn on my car, turn on the 'econo' fan, and then switch to the AC settings, and feel and hear the engine have a little more load. Lately, I have not been able to notice this... I like to run the AC a bit when starting from overnight to increase the load a bit to help it warm up (Im freezing anyway, so it doesnt really matter), but really arent sure that it has been going on.
I know that it is cold and that the AC cycles depending upon desired temperature, however, I keep it on 'min' temperature, which according to my manual ought to keep the AC compressor constantly cycling on and off, regardless of cabin or ambient temperature.
Is there any good ideas of how to diagnose this? It doesnt work for either of the AC settings that are between the econo button and the defrost one.

Second question... when starting cold after sitting overnight or longer, I will occasionally have the car misfire a few times immediately after startup. I glow once, and the car will catch immediately after turning the key to the start position, no need to crank it at all. All the glow plugs appear OK via the resistance test. Would it still be wise to replace the glow plugs? Its odd that one cyllinder must be misfiring, when the engine starts really good, even at 12°

Thanks for the help,

JMH

#2 12-13-2002, 03:29 PM

You are dealing with an automatic climate control system here. It senses very cold temperature so it knows better than to turn on the compressor, even on the Min setting.

Doing a resistance test on the pin type glow plugs only tells you if they are burned completely open. You can test them and they all look good with the meter, but still do not transfer enough heat into the prechamber for the car to start well. Been there, done that!

If it is starting by just cycling twice, I personally would live with it until it gets a little worse, then just screw in all new glow plugs and forget it.

Good luck,

jcyuhn · #3 12-13-2002, 03:50 PM

Two quick points -

The a/c compressor won't engage below ~35F, regardless of which button you push on the control panel. The low pressure cutout switch disables the compressor when pressure inside the a/c system falls below 2 bars. This happens when a) you got a big leak or b) the temperature reaches 35F. So if it's really cold out, the reason you can't feel the a/c engaging is because it isn't.

Lesson number two: The higher the heat load the a/c is removing from the cabin, the higher the pressure inside the a/c system. The higher the pressure, the more energy required to turn the compressor. Hence when it's hot out, you can tell the load the a/c places on the engine. When it's 45F out, there isn't much load - not much heat being removed from the cabin - and you can't feel the loading of the engine.

It's good to run your a/c perhaps 5-10 minutes each week during the winter. It circulates the oil inside the system, helps keep the o-rings from drying out and possibly leaking. Using it to load the engine and help warmup probably has no effect whatsoever.

- Jim