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Warm A/C R129

Ron MB on Mon June 16, 2008 11:05 AM User is offline

Year: 1991
Make: Mercedes
Model: 500SL
Engine Size: 5.0
Refrigerant Type: r134
Ambient Temp: 90
Pressure Low: 40
Pressure High: 250

While driving and under hard acceleration then letting off the gas a squeal is heard coming from under the hood
lasting only 5 seconds or so, then the A/C blows warm air for approx. 45 sec. then returns to the 70 degrees.
This problem does not occur all the time. Not sure what is happening.

Checks I’ve made.

1.Performed the A/C pushbutton Control module (Self Test). Code 01
2.Next test, all potentiometer and feedback potentiometer read normal.
3.Replaced receiver dryer, vacuumed down the system for 45 min. no leaks. Did not replace the expansion valve it’s a little difficult to get at.
4.Re-charged the system R-134a, approx. 29 oz. And replaced the oil PAG 4oz.
5.Ambient Temperature (Fahrenheit) 90, Low side 40, and high side 250.
6.Checked the Mono Valve default (voltage applied valve closed, also clamped the hose).
7.No difference. With blower high speed Center vent Temp. approx. 69 to 72 degrees F.

All the above checks seem to be normal???? With exception of the warm vent temp.

This is the only symptom that I know I have. Any help or inputs would greatly be appreciated.

Thanks In advance, Ron

TRB on Mon June 16, 2008 3:36 PM User is offlineView users profile

What are the pressures at high RPM.

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Ron MB on Tue June 17, 2008 5:30 AM User is offline

Tim thank you very much for interest in my situation. I'll re-check the pressures as soon as possible.
The pressures above were at idle speed.

Thanks, Ron

Ron MB on Tue June 17, 2008 2:01 PM User is offline

Pressures @ Idle L 43 H 210

Pressures @ 1500 rpm L 25 H 260

Static pressure L 82 H 115

Temp. 89 deg., Humidity 66%

bohica2xo on Tue June 17, 2008 2:14 PM User is offline

Time to check the drive belt / tensioner / idler / pulleys carefully.

Sounds like you have a bad tensioner assembly. Under deceleration, the compressor RPM does not match the engine RPM. System sees this as a fault, and drops the clutch out.

The belt squeal is not normal. Clean all of the pulleys with something like simple green & water. Degrease the belt the same way. Check the idler pulley for smooth rotation. If it rocks or rumbles, replace it. Look at the tensioner assembly. It should move smoothly, with plenty of force. Check to see that you have the right belt, one that is too long will allow the tensioner to bottom out at high RPM.

The tensioner assembly is a 150 buck item. They do wear out, they are constantly in motion. Check the idlers first, and clean everything. If it still happens, then the tensioner is probably the culprit.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Ron MB on Tue June 17, 2008 2:45 PM User is offline

I have let the car idle for approx. 1 hr the clutch never dropped out, as I was watching the sight glass, slight bubbles. You would think at idle 89 deg. in the garage I would get better than 60 deg. out of the vent. I was wrong in my first post in saying 70 deg vent temp. it's 60 deg. As far as the tensioner it may be going bad if it does squeal it only happens once every two weeks or so. All of the pulleys are very clean new belt. I understand what your saying the speed sensor and engine RPM.

Thanks for you time and input,
Ron

bohica2xo on Tue June 17, 2008 8:05 PM User is offline

1991 Model year, 134a? Is this a conversion? If so, there is some of your poor performance.

The other obvious checks to make:

1) Fan Clutch. Poor idle performance is directly related to airflow.

2) Condensor cleanliness. Not just the face, but deep in the fins.

3) Condensor seals. Be sure all of the radiator fan air is directed through the condensor.

4) Re-Heating of the cooled air. Air door seals, actuators, adjustments etc. can have a big impact. That MB has a complex air handling system.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

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