AC diagnosis - PeachParts Mercedes-Benz Forum

zeke · #1 07-04-2013, 04:15 PM

Just switched to a sanden compressor with ROLLGUY's kit.

Before the switch, the AC worked great until the R4 compressor welds on the face of the clutch broke. Result was uneven clutch engagement and a horrible sound. But, when the compressor would spin, colling was excelletnt.

Before I knew that it was the broken welds, I added a bit (2oz of oil) thinking the compressor had lost oil and was squeaking.

I mention this b/c there may be too much oil in the system right now.

I swapped in the Sanden, and added 2oz of oil per ROLLGUY's instructions. Dropped in the new dryer he supplied.

Pulled vacuum (27-28mm), and it held overnight, so I charged with 36 oz of R134a.

With out compressor engaged, high and low sides are at ~110 psi.

When I turn on the compressor, the low side pressure falls to -10 psi (vacuum), and the high side rises to just under 150 psi.

The compressor is engaging and disengaging properly.

There is some cooling happening, but not enough to say it is functioning even poorly.

When I disengage the compressor, pressures return to ~110.

My thinking is that this is a stuck TxV? (closed)

But shouldn't the high side show much higher pressure in that case?

I plan to recover the 134a and use compressed air to blow out the oil, then refill with the proper amount, and then recharge. But I want to make sure I fix the actual problem first. Is this likely to be solved with a new expansion valve?

mach4 · #2 07-04-2013, 05:02 PM

Quote:

Originally Posted by zeke

My thinking is that this is a stuck TxV? (closed)

But shouldn't the high side show much higher pressure in that case?

I believe your problem is the Expansion Valve, but it's stuck open, not closed. No, high side would lower than expected as the refrigerant is just flowing through relatively freely.

I had a similar issue when I first installed my Sanden. Replacing the Expansion Valve solved the problem. It was a bear on my SL, it's not nearly as bad on the 123s and 126s.

Brian Carlton · #3 07-04-2013, 05:28 PM

Quote:

Originally Posted by mach4

I believe your problem is the Expansion Valve, but it's stuck open............

Can't get a vacuum on the inlet side of the compressor if the TXV is stuck fully open...............unless the system is completely clogged between the TXV and the compressor............unlikely.

I also think it's low on refrigerant.

kmaysob · #4 07-04-2013, 05:33 PM

he is showing vacuum on the low side. i would bet on a txv sticking shut

Brian Carlton · #5 07-04-2013, 05:35 PM

Quote:

Originally Posted by kmaysob

he is showing vacuum on the low side. i would bet on a txv sticking shut

Agreed...........or very close to fully closed.

It would be interesting if the compressor is so strong that it can pull a vacuum against a TXV that is just open fractionally...........seems unlikely.

fdanielson · #6 07-05-2013, 08:09 AM

The symptoms sound like low refrigerant to me. You can test the expansion valve to make sure it is working properly. This web page has a good description. The Case of the Plugged A/C Expansion Valve | Red Dot Corporation | Mobile HVAC

vstech · #7 07-05-2013, 09:01 AM

I wanna know HOW the compressor is staying engaged if it's truly pulling into a vacuum...
the low pressure cut out should drop out the compressor when it hits 20.
verify your gauges are correct, as -10 and 150 are waaay off for 36oz what is the displacement on the compressor you have? too large a compressor would draw a vacuum through the txv.

zeke · #8 07-05-2013, 10:53 AM

Thanks all.
VSTech: The compressor was one provided by ROLLGUY, so I assume it is one that has been used successfully on other w123s.

I can also tell you that if I pull one of the low pressure cutoff leads on the dryer, the compressor disengages. That seems reasonable since the pressure on the high side remains 100-150 psi at all times. AFAIK low pressure on the high side is the only way the compressor would shutdown in the case of low pressure.

As far as I can tell the gages are working properly. My vacuum pump pulls 26-27mm vacuum, and that is exactly what the gage reads. Both high and low gages read similar values when the compressor is off.

The only way I could imagine low refrigerant would be if my 134 retrofitted port leaked during the charge. But all three cans were completely spent, and the low pressure hard line frosted over during the charge, so every indication is that the entire 36 oz was introduced into the system.

ROLLGUY expressed concern that I had overcharged the system since I was well over 80% of the R12 capacity. This was before I had the pressure readings.

fdanielson: I had come across that web page. One thing I will note is that the system's behavior is similar when the interior temp. of the car is ~72* (early morning) and when it is 120* (afternoon). Certainly don't think I could warm the body any more than that with my hand and I am a bit reluctant to aim a heat gun at my plasticized dashboard and its guts!

Doktor Bert · #9 07-05-2013, 10:54 AM

Agreed....

I have replaced many a stuck/obstructed expansion valve on MBZ's over the years....

vstech · #10 07-05-2013, 11:05 AM

as to why the high pressure is not climbing, it's because no additional refrigerant is being added. obstruction limits flow of refrigerant, and lack of heat into the charge keeps the pressure low.
pull the dryer, and the TXV, and replace, evacuate, and recharge.

however, you said your pump only pulls down to 27? if that's the case, you could easily still have moisture in the system, and THAT could stop up the TXV...

fdanielson · #11 07-05-2013, 11:27 AM

Quote:

Originally Posted by vstech

I wanna know HOW the compressor is staying engaged if it's truly pulling into a vacuum...
the low pressure cut out should drop out the compressor when it hits 20.
verify your gauges are correct, as -10 and 150 are waaay off for 36oz what is the displacement on the compressor you have? too large a compressor would draw a vacuum through the txv.

On the W123's (and possibly others) the low pressure cutout is on the high side of the drier. It cuts out at 2 bar (29 psi) if I remember correctly which explains why it didn't cut out when the low side was pulling a vacuum because the high side still had pressure.

Doktor Bert · #12 07-05-2013, 12:31 PM

Quote:

Originally Posted by vstech

as to why the high pressure is not climbing, it's because no additional refrigerant is being added. obstruction limits flow of refrigerant, and lack of heat into the charge keeps the pressure low....

Precisely....

Another indication of an obstruction is a system that is slow to equalize...

ROLLGUY · #13 07-07-2013, 10:58 AM

I will also reiterate that 36 oz of 134a is way too much for this system. The factory specs call for 2.2 pounds for a 123 chassis, and that is with the R4 (larger capacity than a Sanden). I have found that 20-24 oz works best with the Sanden and stock condenser. If running a PF condenser, even less 134a is adequate.

zeke · #14 07-07-2013, 02:08 PM

Update: I opened up the lines and blew them out with compressed air. Oil came out of the low pressure side pretty clean, but there was some crud that came out of the high pressure line (the one from the cabin to the receiver.

So that seems consistent with grit in the constricted side of the Txv.

I drained the oil from the compressor, and about 2-4 oz came out - very discolored.

My plan is to flush all the lines with solvent, replace the dryer, and TxV. I have some Ester Oil left over, so I plan to put that into the compressor, and drain it back out again to help clean it out. Then I will refill with 6oz of PAG100 per the label on the unit.

I saw a video from restaorationair.com that said that 6oz of oil is all a sanden system needs. He said to put the oil in the compressor, turn it over a few times, then install it - no additional oil.
Here is the video.
Changing the oil in a Sanden Style Compressor - YouTube

I will refill with 24oz 134a and see how it goes.

ROLLGUY · #15 07-07-2013, 02:27 PM

Quote:

Originally Posted by zeke

Update: I opened up the lines and blew them out with compressed air. Oil came out of the low pressure side pretty clean, but there was some crud that came out of the high pressure line (the one from the cabin to the receiver.

So that seems consistent with grit in the constricted side of the Txv.

I drained the oil from the compressor, and about 2-4 oz came out - very discolored.

My plan is to flush all the lines with solvent, replace the dryer, and TxV. I have some Ester Oil left over, so I plan to put that into the compressor, and drain it back out again to help clean it out. Then I will refill with 6oz of PAG100 per the label on the unit.

I saw a video from restaorationair.com that said that 6oz of oil is all a sanden system needs. He said to put the oil in the compressor, turn it over a few times, then install it - no additional oil.
Here is the video.
Changing the oil in a Sanden Style Compressor - YouTube

I will refill with 24oz 134a and see how it goes.

Sounds like a great plan. It is hard to do without making a mess, but you should flush the evaporator separately after removing the TXV. I would put a total of 8 oz in the system- 6 in the compressor, and 2 in the high side hose (or add when using a machine). I would also make the 2 oz the oil/dye mix, or mix the entire oil charge with the dye. Either way works, as the oil eventually gets mixed together.