‘87 SDL. AC Rebuild Not Satisfactory - PeachParts Mercedes-Benz Forum

oilslick · #1 06-14-2018, 06:19 PM

What are these gauges telling me about my AC system?

https://youtu.be/DlfmR1iB7Xg

New compressor
New expansion valve
New receiver/drier
New o-rings
System thoroughly flushed, numerous times
PAG oil and r1234a installed
Suction line is nice and cold, but air temp at the center vent is 79*
Ambient temp is 93*

I’ve rebuilt 5 automobile AC systems over the past three years. Two of those were Mercedes. All 5 are ice cold and working excellent. This one, not so much!

Diseasel300 · #2 06-14-2018, 06:26 PM

Sounds like the monovalve is stuck open. With pressures like that you should be able to blow <60˚ in those ambient conditions with all the doors to the car open. The cold return line is your clue here, if the monovalve isn't sealing off, you're trying to cool the heater core.

Dubyagee · #3 06-14-2018, 06:27 PM

Have you verified the evaporator is getting full airflow? No stuck flaps? Heater core flow blocked?

oilslick · #4 06-14-2018, 06:35 PM

Quote:

Originally Posted by Diseasel300

Sounds like the monovalve is stuck open. With pressures like that you should be able to blow <60˚ in those ambient conditions with all the doors to the car open. The cold return line is your clue here, if the monovalve isn't sealing off, you're trying to cool the heater core.

Brand new Bosch Monovalve installed back during the winter. It HAS been working correctly during the winter. Not saying it’s not goofed up, but I sure hope not. I bought it a few days before the price skyrocketed.

As far as flaps go, which flaps do what in this w126? I’ve been wondering if it could be a flap issue since sometimes it takes a bit for the center vents to open up. It starts out as defrost then goes to center vents.

Diseasel300 · #5 06-14-2018, 06:51 PM

Defrost is the default position. When you switch on the climate control, it has to open the flaps to get the center vents to work and close the flap to the defrost. If you just started the car, it can take a few seconds to develop adequate vacuum. If the center vents open and stay open, you're good there.

The heater core is always exposed to the blower stream, there are no flaps to control the temperature of the heater core, its temperature is controlled by the monovalve being open or closed. If the monovalve is not closing (check voltage at the plug) or is leaking by, you'll be competing for heating and cooling. Test the theory, hit the "EC" button and see if your temperature rises above ambient. If it does, you know for certain that the monovalve is open or leaking by.

oilslick · #6 06-14-2018, 06:58 PM

I clamped off flow to the Monovalve. No change in vent temps.

Diseasel300 · #7 06-14-2018, 07:14 PM

How much R134a do you have in the system? Your video description says you charged with 2.9 pounds? If you did, you're WAY overcharged. 2.9 pounds is the original fill of R12. R134a should be 80-85% of the R12 charge by weight. I found 80% (~37 oz) to be slightly undercharged in my car, I'm running 40 oz and find that to be just about right for my climate.

If you're overcharged, you'll flood back to the compressor. The return line will be nice, cold, and sweaty, but the evaporator won't be.

oilslick · #8 06-14-2018, 07:31 PM

I’ll check that.

jay_bob · #9 06-14-2018, 08:11 PM

Paging Dr. Vstech...

oilslick · #10 06-14-2018, 09:41 PM

Anything is possible, I know, but could a bad CCU cause this?

Diseasel300 · #11 06-14-2018, 10:12 PM

Only if it fails to engage the compressor close the monovalve. If the compressor is engaged and the monovalve is closed, the CCU is doing what its supposed to do. Even if every vacuum pod failed, you'd still be getting cold air out of the defrost nozzle.

oilslick · #12 06-14-2018, 11:20 PM

I used my laser thermometer to get a few numbers. Not sure if they can tell me anything.

Condenser Temps:
In - 142
Out - 123

Expansion Valve Temps:
In - 93
Out - 81

All of these were measured at their connection points.

The evaporator drains are yielding a nice steady stream from both sides.
The suction line back to the compressor is cold and sweaty.
Air temperature from the vents is still cool, but not cold.
Auxiliary fan kicks in around 270 with ambient temp of 80 degrees
High side is 230 at 80 degrees (which is 20 psi too high according to the pressure chart).
Low side is 47 at 80 degrees (normal according to the pressure chart).
After the AC is shut down the pressures equalize to 105 in less than 5 minutes.

I’ve shut off water flow to the Monovalve with no change. I’m stumped.

Diseasel300 · #13 06-15-2018, 12:21 AM

After you pinched off the monovalve line, how long did you wait before you checked temp? If it's leaking by or failing to close, the heater core will be HOT and it'll take a minute or two to cool it down.

Your pressures do suggest you're overcharged at the ambient temperature. 47PSI suction at 80˚ seems high.

Check out the generic R134a charging charts that are out there:

You want to be on the lower side of the acceptable range on the suction side. Your high side will probably be in the middle of the range if you tweak the charge to keep the suction on the lower end of the scale.

If you have too much refrigerant in the system you'll wind up with a flooded evaporator and your cooling will suffer. It's counterintuitive, but that's life. If you have the ability, evacuate the charge and weigh in 37 ounces as a starting point. If you find it's still undercharge, you can add another ounce or two of refrigerant and see how things go.

With the temp wheel to Max cold, fan switch on Max, and all 4 doors open, you should be able to get high 50s low 60s at the center vent when the charge is correct. With the windows and doors closed, you should be able to get mid-upper 40s after 5 mins or so, depending on ambient conditions and temp in the cabin.

oilslick · #14 06-15-2018, 07:02 AM

This IS the chart I’m using. I evacuated the system and recharged it and never made it to 36 ounces and got these pressures at 80degrees. . How many ounces of oil does the w126 system take? I was under the impression it is 6 oz.

Maxbumpo · #15 06-15-2018, 07:39 AM

I'm pretty sure that pressure chart and the FSM require the engine speed to be ~2000 RPM, not at idle. Are you increasing the engine speed when reading the gauge? You also need to hold that speed for a minute or two or three, so that the system stabilizes, in order to get valid readings.

Is your expansion valve meant for r12 or r134?