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Old 06-26-2008, 11:00 PM
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300SDL - A/C Compressor Runs Continuously (won't cycle)

I don't know when it started, but I noticed today that my A/C compressor is running continuously (doesn't cycle). Aux fan is also running all the time. System is fully charged with 134a and blows nice cold air in the cabin. If I switch the CCU to a non-AC setting, the compressor does turn off (as it should).

I've read a number of the posts where people can't get their A/C compressor to turn ON (often related to the KLIMA), but I haven't found any posts where it won't turn OFF.

Anyone have any suggestions or experience?

The only thing I can suspect is the Overload Protection Relay was recently replaced by the dealer. If I pull it the compressor does stop running, but I can't figure out if that really means anything.
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Old 06-26-2008, 11:15 PM
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What are your pressure readings?
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Old 06-26-2008, 11:43 PM
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Location: Blue Point, NY
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If the ambient temperature is quite high, the system can't keep up with the demands of the evaporator and the compressor will run continuously.

It's not uncommon with 134 in very warm climates. That's the fundamental problem with 134..........system cannot get evaporator cold enough to cool the cabin adequately.

If the evaporator is not freezing up.........the system is working as designed.
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Old 06-27-2008, 08:57 AM
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Location: Tuscaloosa, Alabama
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Prepare for a broken serpentine belt. Carry a spare.

Put a new one on now. Find out why it is not cycling. I had a Conti belt break and it swiveled and swirled up like spaghetti. I now have a Gates, but it was made in Mexico and I'm still a little bit uncomfortable with it. On the Conti belt, my engine ran hot as I was looking at the temp gauge fly up to the red mark indicating overheat. The only good thing was I had driven only about 14 miles in a 55 minute time period. I have a #14 head, but I had recently installed the head with a new head gasket and so far no crack has developed. The crack issue to me now is basic, not like it was five years ago. Also, no pressure build up in the radiator hose in the mornings is really a great feeling.

Anyway, carry a spare belt and watch that temp gauge if you are going to run with the ac compressor on all of the time. It isn’t like the 617, where when the belt breaks all you lose is cold air. But with the 603 belt you risk losing a head and all of your transmission fluid when the broken belt cuts your transmission lines in half if you don't have guards on your transmission lines.

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Old 06-27-2008, 10:43 AM
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If you have a variable-displacement compressor and a TXV and everything is working, your compressor is not supposed to cycle.
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Old 06-27-2008, 03:00 PM
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Matt - interesting points... i've found it's always dangerous when I assume something is "broken", and it might be working just like it's supposed to (and i've just not noticed how it works).

- how do you figure out if you have a variable displacement compressor? mine is the stock one
- what is TXV?

I did some additional diagnostics that led me to believe I had actually OVERCHARGED the system. I added 134a early in the spring since the sight glass appeared to have bubbles... and used one of those one time use refrigerant/oil mixes from walmart (had worked before). I added to the upper allowable range of low side gauge (45psi).

At this point my hypothesis is that was OK until the weather recently got hot... at which point the internal PSI exceeded some allowable threshhold.

I bled out some refridgerant from the low side valve today (down to ~35 PSI) and system appears to be running normally again. the compressor still runs all the time, BUT the aux fan now cycles instead of running continuously. air temp out the vent is in the low 40sF.

Anyone else who has a SDL that can tell me whether their A/C compressor runs continuously when AC is selected on the CCU (or cycles on/off)?
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Old 06-27-2008, 03:17 PM
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I am almost 100% sure that your original compressor is variable-displacement. TXV is Thermal eXpansion Valve, and meters the refrigerant into the evaporator depending on the refrigerant outlet temperature.

The TXV should prevent your evaporator from freezing under normal conditions. Freezing of the evaporator is the only reason that a variable-displacement/TXV system will ever shut down the compressor (unless you turn it off). There is an evaporator temperature sensor to prevent freezing, and to also prevent the compressor from running if it is below freezing outside.

Most American cars use fixed-orifice-tube designs with fixed-displacement compressors. Unlike in the TXV system, the quantity of refrigerant entering the evaporator is only dependent upon the difference in pressure between the two sides of the system. If the system is not under heavy load, the low-side pressure drops due to lower refrigerant evaporation. These have a switch on the low side, usually called a "cycling switch," to shut off the compressor when the low side drops below a certain figure. These cycle continuously while in use.

You have no switch on your low side line at all for compressor cycling. Only freezing the evaporator will shut it off, and freezing the evaporator means that something else is wrong (or it's winter).
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Old 06-27-2008, 03:28 PM
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The AC compressor on my SDL runs all the time, at least at idle, in all modes except economy. It even runs when calling for max heat. I noticed this when I converted it back to R12 and thought something was wrong, but I guess it's okay. It's been working w/o a problem for 3 years. I'm in FL so temps never go much below 45. My original thread with a simliar question.

300SDL ACC question
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Old 06-27-2008, 04:52 PM
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Matt/Paul - thanks for the replies... very helpful... it's really amazing what the collective wisdom of this forum can deliver!

At this point I'm going to assume that I have a variable displacement compressor and it running all the time is normal. The thing that had alerted my attention to something being out of normal was the aux fan was ALSO running continuously when the AC was on (which it hasn't in the 9 years i've owned the car). Based on searching the other posts, what I've deduced is the aux fan isn't designed for additional engine cooling, it's to increase the condenser effectiveness... and is triggered when the refridgerant high side temperature is above some limit.

So at this point i believe having an overcharged system created a situation where the refridgerant cycle (compress, evaporate, condense) wasn't working effectively... and the result was refridgerant that was always too hot (and the fan always being on). Bleeding the excess 134a out appears to have fix this. The compressor is always on, but the aux fan now cycles at what appears to be a reasonable interval.

I should still probably take it to the shop to do a real service including evacuating the system and starting over with a new 134a, oil and a receiver/dryer... but at least it generally working for now.

Thanks to all... much appreciated.
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Old 06-27-2008, 05:19 PM
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It's not a variable displacement compressor. MB did not adopt those until much later. The SDL uses a Nippondenso 10P15c. It's a very good little compressor, but it has two modes - on and off.

It's normal for the compressor to not cycle on warm days - say anything much above 85 degrees. Does it cycle at night, when there's no heat load from the sun? That's a better test.

- JimY
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Old 06-27-2008, 06:55 PM
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Originally Posted by BenzDiesel View Post
Put a new one on now. . . BenzDiesel
A superb idea. Suggestion: buy a new belt and put it on immediately. Keep the (presumably still good) belt that's on the car now as a spare. This prevents you from ending up in the boonies with a brand-new accidentally wrong size belt that now does you no good at all.

By putting on the new belt now, you can find out whether it fits or not (just in case). By keeping the old belt as the spare, you know it will fit (since you just took it off of the car).


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Old 06-27-2008, 10:35 PM
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The compressor will only cycle when the evaporator gets too cold (35F or so) or when the cabin temp is low enough not to need cooling for dehumidification, or when the outside temp is below 32F. In summer, it pretty much runs all the time. Unless you have a loss of cooling due to evaporator freezeup, I'd not get too excited.

You can have someone check the temperature sensor -- the dealer or a well equipped independent shop will have a tester, or you can use an ohmeter if you have the pin-out for the pushbutton unit connectors.

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Old 06-27-2008, 11:11 PM
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What about during heavy acceleration?

The compressor is supposed to kick out to my understanding and it makes sense to have the compressor cut out during heavy acceleration. The extra pull if the compressor doesn't dis-engage is what help causes the belt to break, I think. Actually, you can feel it when the compressor keeps pumping during full throttle.

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Old 06-27-2008, 11:35 PM
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Originally Posted by BenzDiesel View Post
The compressor is supposed to kick out to my understanding and it makes sense to have the compressor cut out during heavy acceleration. The extra pull if the compressor doesn't dis-engage is what help causes the belt to break, I think. Actually, you can feel it when the compressor keeps pumping during full throttle.

That was a feature added way later....our older models do not have this feature it is part of the computer controls on the newer cars....
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Old 06-28-2008, 09:20 AM
1986 300sdl 1985 380se
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I live in hot, humid NE Louisiana and I drive a 1986 300sdl. My A/C compressor cycles on and off depending on what temp it is outside and what temp I want it inside the car. It will stay on almost all of the time on a 93 degree day with 60 percent humidity outside. My electric cooling fan will cycle from low speed to high speed when I am in the drive thru at McD's when it is really hot out. I can tell when it does this because of the extra electrical load on the car when it goes from low to high speed.
On a cool morning, 72 degrees outside with 70 percent humidity, the compressor cycles on and off. I have converted it to R134A 5 years ago, and have to top off the freon yearly. When I say top it off I mean 3-4 ounces. This is the original compressor on the car. The car now has 110,000 miles on it.
I wonder if you have a dirty expansion valve. My 85 380se acted this way after I put a new compressor and drier and expansion valve on it 3 years ago. The compressor stayed on all of the time and it never really got cold in the car. I had to evacuate the system and clean out the lines. I had trash in the lines from the old compressor seizing, and they got caught in the expansion valve. What a pain it was to get to the expansion valve too. Once the system was cleaned again, no more problems, and that compressor cycles just like it used to.

By the way, at WOT when you press the accelerator all the way to the floor, there is a compressor cut out switch. It is a microswitch that the wide open throttle hits in the engine compartment. Follow your linkage and you will see the microswitch. With the car idling and the A/C on, use your finger to close the microswitch and the compressor will click off if everything else is working correctly on your car.

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