Is there a way to test an txv?

I just put in a new condensor (because of a slow leak), rebuilt the comp., all new o-rings, new txv, and new dryer.

Low side 25psi, high side 200. Ambient 70 deg., 50% hum.
Vent temp., only 50 deg. No mositure on the suction line just a little cool. discharge line warm not hot.

Had someone on an A/C forum tell me it is possible to have a bad txv and act like that.

I also charged the system watching the sight glass so I can't tell you if it has the proper amount of r-12. Any ideas?

It is an 87' sdl.

Thanks Barry!!

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87' 300sdl

Barry, did you pull a vacuum prior to adding refrigerant?

If not, I wonder if you might have possibly have some air in the system.
Does the compressor stay on?

dave

Barry,
Is the auxillary fan running while the AC is on?

Tom

WHo is advising you on this job ? That is not the way to determine the amount of refrigerant to put in the system... it needs to be weighed...

Your problem may be that you have too much in the system.

Once I replaced the components I evacuated it for about 1 hour.

The compressor seems to cycle at a normal rate. On for 10-12 seconds then off for 5-6 soconds. the aux. fan has not come on as of yet- i also replaced both switches in the dryer when the system was apart.

I guess I have always had good luck charging r-12 units with the sight glass. I ASSumed that if you had liquid at the dryer that you were close to a nominal charge......But you know what they say about ASSsuming........

If It was overcharged would'nt the pressures be high and the discharge line real hot?

Thanks for the feedback I need the help.

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87' 300sdl

All I know is that a hair undercharged is better than any over... I would have to consult the books to find about pressures...

We are just trying to find the parameters to see if something more basic could be your problem.. very unusual for a TXV valve to be bad out of the box...so these other things are easier to deal with... it can be checked , but I think on the car you have to go with general things like air temp... ac performance .. to test the actual valve you would need specialized equipment and have it off the car....( I could be wrong, only quote me annonamously on that...LOL )

If , since this is a combination valve,,,, ie, TXV plus throttle suctioning valve... you are getting too cold air out of the evaporator... then it is the valve.... but isolating on the upper side..I do not know how to do except by interpolating pressures... which I can not help on... due to lack of experience....

PS, I , myself, and me do not think one hour is enough to evaucuate the system.. it is not just the total inches of mercury of vacuum that you pull , you have to allow time for any moisture to get out...I know the old guys , with the old pumps which would not overheat if left overnight, preferred to leave it on overnight....You don't want ANY moisture left in there....

I checked the 124 climate manual for the expected performance values. It may not be exactly correct, but the 124 has a very similar a/c system as the 126.

Under your conditions the discharge air should be 5C - about 41F. High side pressure should be about 250, your low side is fine.

The one thing which strikes me as odd is that your compressor is cycling. There's no reason it should be doing so. Compressor cycling on these cars is controlled by an evaporator temperature sensor which feeds the pushbutton control unit. The PBU then turns the compressor off/on to prevent the evaporator from falling below the minimum temperature selected by those wizards in Stuttgart. Mercedes is very conservative and switches off the compressor when the evap reaches ~38 degrees. Perhaps you have an ETR or PBU problem?

How certain are you about the temperature and relative humidity? Humidity has a tremendous affect on discharge air temperature. According to the charts, the expected output air temp for our conditions in Dallas this A.M. (75F and 90% humidity) is 12C - 51F.

- JimY

Sounds like a good place to start would be to reclaim, evacuate for a good long time, recharge with the proper amount and retest?

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87' 300sdl

When I did the test it was 70 deg and low- 40% humidity.

Could there be heat leaving the heater because of a faulty monovalve and how could I test this?

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87' 300sdl

Although weighing the refrigerant or using a graduated cylinder is much more accurate, charging to a clear sight glass is not an uncommon method for those who have no scale or cylinder.

The pressures don't indicate a bad TXV. When they fail they usually fail closed which causes the low side to go to a vacuum and the high side to come way down.

Indeed you should evacuate the system thoroughly to remove moisture, but as long as it was pulled down to 28" or so, it should not effect cooling. The overnight evacuation removes moisture to prevent acid formation so it is not to be considered unnecessary but I don't think it has to do with this particular problem.

I think you should check for the introduction of heat to the system from either the monovalve or a blend door.

Good luck,

I know that overcharging is bad... but I have never seen anything mentioned as to how much over specified charge the effects start being noticed....

On a system which needed 32oz of refrigerant... would 3 oz, 5 oz, 7 oz, ? start making a difference .... and where would it make a difference ? Would it just be on really hot days where the 'reserve' cooling capacity was exceeded ?... or would it be that the compressor had to work overtime at higher load and thus was at risk ?

I don't know what would be the amount of overcharge in ounces. The problem with overcharge is excessive high side pressure, so that is what should be watched.

As far as overcharging it would be worth mentioning here that overcharge is not too much of a problem in an accumulator system. It is however a problem in our benzes with a filter drier system.

When charging by sightglass, you should charge the system a little at a time with engine speed at 2,000 and preferably a fan in front of the car to simulate ram air. Charge it a little at a time until you almost see a clear sight glass with an occasional bubble. This is how you keep from overcharge. As leathermang says its better to be undercharged than overcharged.

BTW, in a converted system, the sightglass means nothing. It is only useful if the original refrigerant that the system was designed for is being used.

Good luck,

Larry- If I have to much to much oil in the system would it affect the vent temp?

When I ran the test the heater hoses were hot so I used a hose tool and pinched off the supply hose to the heater with no drop in vent temp. should they be hot? Maybe pinching off one hose is ineffective. Does our car blend air with a door or by cycling the monovalve?

How do you tell if the monovalve is working?

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87' 300sdl

Too much oil will DEFINITELY decrease cooling ability.

I'm not sure about the blend door. I've been fortunate enough to never have problems with a Benz in this area.

Good luck,

I don't think the 126 has a blend door. It cycles the monovalve to control the heater core temperature. The airflow always goes over the heater core; during cooling it just isn't supposed to add any heat to the airstream.

You know, your car may have a temperature sensor for the heater core. If so, you could tap into it with an inexpensive digital multi-meter to measure the resistance. That would give a good idea as to whether the monovalve was functioning correctly.

- JimY