cycling compressor
 

I have never noticed my compressor cycling ever in my 420sel. Today I noticed it would cycle on and off as I was idling. Are they supposed to cycle, or is that an early symptom of worse to come?

Must not be too important then.

They only cycle off when the evaporator is close to freezing up. Normally they do not cycle.

are the vent temps supoosed to be close to the temp of the evaporator?

The more it cycles, the better it is working. When it turns off, it has reached a temp at the evap below which the evap will freeze.

If it is cycling on and off quickly, the charge is probably low.

ive only noticed it do that on a cool day.

It will typically cycle more at highway speeds and less (or none) at idle.

It will be more likely to cycle on a cool day as yours did, because the evaporator has to take away less heat.

BTW, the AC does not add cold to the air, it removes heat from the air, the heat being the energy that must be moved.

so is there a magic number for all a/c systems that the air coming from the vents dont get any colder? Say can a mercedes blow out 38 degree temps and a ford blow out 20 degree temps only because the ford has a better design or would they both just reach maximum cooling at the same temp?

20 degree vent temps do not exist in any vehicle. rapid cycling may also be caused by an excessive charge. too much refrigerant is detected by the high pressure switch and the unit short cycles. i'm not saying this is your problem, just making a point.

go to an auto parts store and buy a vent thermometer. put a given amount of water in a container and place in feeezer. get it near freezing - slushy mixture. take it out and drop the the vent thermometer in the slushy solution and let the reading drop. there's and adjustment on the back of the face. maybe an 8mm or so wrench will loosen it. turn and set to 32 degrees; then drop it back in the slushy mix and ensure it reads 32 degress after it's been in there awhile. this is how some hvac guys calibrate their vent temp thermometers.

drive your car 'til it's cooled down with a vent thermometer installed. record temp. all of this is just to provide you with a vent temp.

you'll also need pressure gauges for your vehicle. different types avail. for r134a and r-12. buy whichever you have, then buy "automotive air conditioning basic service training manual" by mastercool, inc. read every page 3 or 4 times. "learn" the principles of a/c before doing real work.

consult this forums search facility. read any/everything on the a/c subject written by arthur dalton. a member here named "jcyuhn" has done a fair amount of serious a/c work and has written/contributed to many good a/c threads. i would search out his posts as well.

low refrigerant is not always the problem. sometimes over-charge is the culprit. people go to places like autozone and buy those all-in-one kits thinking "more is better" and away they go.

arthur dalton's posts will point out key areas to check before doing anything else.

there's more to it than just shooting refrigerant into a system and i'm not accusing you of holding this belief, just making a point.

Man that is awesome.I have two of those thermometers from autozone and they both read two different temps. one will say as low as 25 degrees while the other one will get to 40 at the best. Im glad to see that they can be calibrated because i dont want to buy an infared thermometer. As for a/c knowledge, and all car and mercedes knowledge, i am like a sponge soaking up info and I have wanted to buy a book on a/c. I recently got my 609 cert so i could buy r-12 and stay away from those r-134a dummy kits. So by the way is just above 32 degrees as cold as an a/c system can get without freezing up? Would that translate to vent temps of 33 degrees the maximum any car can blow out? Sorry for asking so many questions.and thanks for your answers

e150gt - it's my belief the evaporator would start freezing up b4 you ever got 33 degree vent temps.

different refrigerants yield different pressures.

a good set of gauges is your starting point. i'm assuming your fans work as intended and the condensor is free of dirt/debris.

refrigerants as a rule of thumb boil at approx. 20 degrees below zero. i say boil, in other circles the term evaporate is used. evaporation attracts heat and the first principle of a/c is that heat moves toward cold. the hot air in your car is drawn to the evaporator under the dash - magnetic sort of affect. next stop -> compressor where the heated charge is pumped up even more. eventually the juice gets to the condensor where the heat is expelled. there are a few other parts i left out - just doing a high level thing here.

bottom line - the refrigerant and evaporator act as a heat magnet and the condensor acts like a radiator - expelling the heat.

the mastercool book tells the whole story.

cdplayer
 

It is normal for the air compressor to cycle, as you say, at idle to help stablize engine vibration. Helps to eliminate that chug chug chug feeling while you sit waiting at the light.

First I've heard of that.

I thought the engine had an idle control valve to raise the idle speed slightly when the compressor was on.

:bsflag:

??? Cycling of the compressor has nothing to do with stabilization of engine vibration.

Well i calibrated my thermometer and took the car for a test run on the highway. i got 45degrees at 60mph. This is enough to keep on comfy when it's 80degrees outside, but well see at 100 or 95 here in Texas.