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  #1  
Old 05-30-2003, 10:11 PM
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A/c clutch cycling too much please help

What would cause the a/c clutch to cycle on and off causing the vent temp. to not get cold enough?

I have replaced the evaporator temp. sensor.

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  #2  
Old 05-30-2003, 10:38 PM
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In the classic sense,,, you need to check to see if your refrigerant charge is correct. If it is, then you can check the high and low pressure switches in the Reciever/Dryer....If still good through those items then your Thermal Expansion Valve may be bad or have trash in it. You may have moisture in your system causing ice to build up... and excess pressure show at the high pressure switch... and be sure your aux fan is working... if not, then the head pressure builds up too fast to allow the system to cool the inside of the car before the high pressure switch cuts out the electricity to the clutch....There are other things ... but this is sorta an easiest simplest first list...
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Old 05-31-2003, 07:16 AM
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I've been working on this problem for a while.......If it is about 75 deg. out and I hotwire the clutch so it stays on I can get the vent temp down to 33 deg. real quick.

The I've even replaced the evap temp sensor with the same results.
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Old 05-31-2003, 08:02 AM
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Then why don't you tell us what you HAVE done... ?
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Old 05-31-2003, 08:23 AM
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O.k.... Rebuilt the compressor, new condensor (pinhole in it), new dryer, new expansion valve, new switches in dryer, all new o-rings in system, vacuum for 8 hours, 8oz. of mineral oil, 1/4 oz. of u.v. dye, and proper charge of r-12. Run a/c in min. on climate control and vent temp only goes down to 55 deg. Checked mono valve, ok, hotwire clutch goes right down to low 30's. Removed evap. temp. sensor from air box and left it plugged in clutch stays in. got a new one from dealer no difference, clutch still cycles on and off starting at 60 deg. vent temp?

Like I said, the system gets very cold if the compressor stays on, low 30's, but it's got to cycle or it will freeze up for sure.

Thanks for the input!!



I posted this on tech help and got no responce.

I feel this is an electrical problem with the clutch.
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Old 05-31-2003, 08:33 AM
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OK,,, Did you flush the system , and did you put all this new stuff on at one time ?
I think you need to attach guages to your system, as this is really the only way to tell what is going on inside... all else is just circumstantial... and many symptoms have two or more possible causes....
What you do NOT want to do is overide any system which is designed to save the compressor ( high or low pressure switches,superheat , etc ) innards long enough to trash it...and without guages on it you can not be assured of that.

Why do you think it is electrical / clutch problem based ?
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Old 05-31-2003, 09:03 AM
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Sorry about that.

Yes, I flushed the system and put all the components on at once.

High side 250 psi low 22 psi.

I figure if the system has the capability to go down to 34 deg vent temp in a short time with the clutch engaged that it is related to something controling the clutch. If it would stay on a little bit longer it would cool great.
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Old 05-31-2003, 09:20 AM
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On the surface that certainly seems right... but the problem is that you are ( potentially ) looking at very few variables... and if you miss one that is important to compressor life... then the cost is very high...
So it may not be something trying to protect the compressor which is causing it to cycle too soon....but it seems very prudent to be SURE that is not the case, given the risk.
"that it is related to something controling the clutch"... I think that is a very safe statement... the question is whether or not it is Legitimately controlling it in that fashion, AS PER ITS DESIGN SPECS... ( ie safety or longevity ).... or there is some malfunction...
Have you compared those guage figures with what is listed for your car ?
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Old 05-31-2003, 09:42 AM
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I have checked the pressures in the manual and they are right on the money.

If you start out with a warm interior how long should it take the a/c to cool down?

I feel like I am about to give up.

On our 84' sd the a/c would cool very well and cool the interior down rapidly on a hot day.

I don't feel like it sould take a 1/2 hour to cool it down on our 87'.
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Old 05-31-2003, 10:15 AM
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Something is certainly wrong if you are not feeling cooler , even after it sitting in the sunshine, after 10 minutes if you have air moving across the condenser..... 15 perhaps if you have black car/ black interior...
The problem is that these systems are very complicated... not the generic ac parts you have replaced or that we have been talking about... but the overall sensors / vents and stuff...
The " pressures in the manual " you refer to... do you have the MB Air conditioning service manual for YOUR car ?
THAT is the final word on figuring out your car's system.... I only have the one for my 123 chassis.... That is what I suggest you find... the actual hard copy ac service manual for your car....
Don't give up... it will probably be something very small that is affecting this....
And the manual will give you step by step instructions... and specs...
For instance, it will tell you the exact points that the high and low cutout switches are supposed to function... so you hook up the guages, run the ac and watch the pressures at which things happen....
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Old 05-31-2003, 04:03 PM
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You have done a great job on component replacement and your pressures look "by the book". When you say "hotwire" the clutch, did you apply voltage directly to it? or short the leads to the pressure switch (I'm assuming this is what you did)? If you shorted the leads and got 33F at the evaporator, you can pretty much rule out the ETR and some of the other management electrics as the cause and I am then highly suspicious of the switch at the R/D .

What RPM are your pressures being read?

When you increase the RPMs (from idle while watching the gauges) and the low side pressure starts to decrease, does this corelate to the clutch disengaging? How low does the low side go when you rev it a bit?
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Old 05-31-2003, 06:32 PM
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Leathermang... I am using the service cd. but to be honest with you, the cd doesnt hold a candle to the factory chassis manual that I have.

I plan on getting a Hard MB hvac manual for our car but they a hard to find.

jbaj007...In my cd it says you can directly "hotwire" the clutch @ pin 5 and 7 of the compressor control module on the firewall so thats what I did.

Readings are @ 2000 RPM. The lowest I have seen the low side is about 15 PSI @ 3000 RPM.

I'm not sure about the RPM increase I'll check it and get back with you.

Maybe the comp. module is kicking the clucth out because the RPM is too high.
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Old 05-31-2003, 07:13 PM
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I'm sure you already looked at this, but what the hey:http://www.peachparts.com/diy_stevebfl_0.htm

You pulled the ETR bulb out of the air box and it didn't cycle? The clutch stayed engaged and the temp. dropped to ~33F.? I missed that on my first read. Sorry. I would have fallen for the "old replace the ETR trick" also. MB says not to adjust the ETR, but there is a screw to do that. I lowered my cut out point to just above freezing, but I can't remember if it's clockwise or CC on the adjustment.
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Last edited by jbaj007; 05-31-2003 at 07:21 PM.
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  #14  
Old 05-31-2003, 07:24 PM
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Steve's site is great...

BJ300SDL, I have emailed you TOP SECRET information....
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  #15  
Old 06-01-2003, 08:19 AM
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Leathermang, I did not get your E-mail please send it to bargyn@msn.com. Thank You!!!

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