Need some experienced A/C diagnosis - PeachParts Mercedes-Benz Forum

ps2cho · #1 06-12-2016, 02:28 PM

So I am having a problem with my 300TE's A/C...
I think there is a blockage in the system somewhere, but I need help diagnosing this first.

I evacuate the system, and vacuum down to ~29 hg. The problem is that with only 12oz of refrigerant, I am getting 45/320 psi lo/hi. (25/380 at 1500rpm).
That is only a 1/3rd of the recommended capacity for r134a (the recommended is ~28oz). Why would my pressures be so high after only ONE can? The only thing I can think of is that there is a blockage?

Could there be anything else that could be causing this? I'm purging the lines of air before I begin and vacuum held fine for 30mins after I shut off the valves.
I need some help diagnosing here its driving me crazy!!!
Is there a specific test I can perform so I don't have to waste hours and hours swapping parts unnecessarily?

Mike D · #2 06-12-2016, 03:25 PM

Using R134a you need to calculate by the ambient air temp.

At 90F, using R134a, you should be seeing 45-55 psi on the low side and 250-270 psi on the high side.

It appears you could have a orifice tube/expansion valve problem.

Evacuate, replace, flush, vacuum and re-fill.

Ain't automotive A/C fun!

jcyuhn · #3 06-12-2016, 05:27 PM

Feel the high side tubing with the system running. It should be quite hot to the touch all the way to the txv. If you find a spot where it turns cold or cool, there is your blockage.

dkveuro · #4 06-12-2016, 06:49 PM

Possible the drier desiccant bag contents have plugged up the orifice in the drier.

Remove drier and blow through with air line. Clear ? Then blow through rest of system isolating the hi side and lo side as two separate systems.

Possible TXV has jammed closed, nearly closed or blocked..

Ran mine with Duracool up to 200,000 miles. Duracool may not be legal where you are.

FYI : Holding a vacuum is no indication it will hold pressure.

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lorainfurniture · #5 06-12-2016, 07:32 PM

If there was a restriction, you should be able to find it easily. Normally it will produce an ice ball, and you will have super high high side, and or super low low side.

My experience with mb Ac systems is that you should just charge it by weight and don't look at the gauges. My vague recollection of my 300e was that the pressures were high after about half the r12 was in. I just stuck to the scale and I had perfect Ac after.

lsmalley · #6 06-12-2016, 10:00 PM

Quote:

Originally Posted by lorainfurniture

My experience with mb Ac systems is that you should just charge it by weight and don't look at the gauges. My vague recollection of my 300e was that the pressures were high after about half the r12 was in. I just stuck to the scale and I had perfect Ac after.

I was experiencing the same thing, except with propane. Then I said forget the propane something isn't right. So we put the r134 back in, but we were going based on the gauges and pressures were still high. My friend decided that we should stop because the pressures were too high on the high side. When I left his place the temp was about 110°F, so I ended up going to Wal-Mart after picking up my son and just decided to purchase 2 of the 505 gram cans of 134 and I just charged the system until it got cold. With the outside temp the way it was I pretty much had nothing to lose and damaging a compressor was the least of my worries as I would just buy a new one. Sure enough after emptying the first can, there was no change, then half way through the second can the temps dropped at the vent. I added nearly the entire second can, save for about 2 - 3 grams and now my ac is better than I can remember. My vent temps are well below 60°F, I'd say maybe 45°-50° at the vents. Of course this isn't nearly as cold as what I was getting with the propane when then system was working properly, but I will not revisit messing with my ac system until the fall. So, I am in agreement with Lorrainefurniture, ignore the gauges, matter of fact, ignore the weight to a certain extent and fill the system until you notice a change. Obviously the original system was designed for about 1000 grams of refrigerant so you don't want to go past that and not have any change, but I'd say give it a shot, especially since I'm sure the AZ heat is just as intense there as it is here. Good luck and let us know.

benzzy · #7 06-15-2016, 04:51 AM

Quote:

Originally Posted by lorainfurniture

If there was a restriction, you should be able to find it easily. Normally it will produce an ice ball, and you will have super high high side, and or super low low side.

My experience with mb Ac systems is that you should just charge it by weight and don't look at the gauges. My vague recollection of my 300e was that the pressures were high after about half the r12 was in. I just stuck to the scale and I had perfect Ac after.

I agree Charging a system by other means than a recommended required Weight

is only charging to the systems limitations. Inefficient condenser , TX valve not fully opening. sensor out of speck. Vacuum??? if you don't use a vacu stat YOU ARE ONLY GUESSING Yes the gauge says you have a good vacuum , But in reality without a vac u stat you would be surprised at how much you could be off a SAFE vacuum.
Remember what the vacuum is doing . Not sucking crap out the system . Only lowering atmospheric pressure to allow moisture to BOIL off and out the system. If you want to blow the lines NITROGEN not your air compressor. AIR is Moisture it just doesn't mix.
Pressure test a home A/C with air . You will have a hole in the ground where it once used to be.

dkveuro · #8 06-15-2016, 09:21 AM

Quote:

Originally Posted by benzzy

......................Pressure test a home A/C with air . You will have a hole in the ground where it once used to be.

Eh ?

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lorainfurniture · #9 06-15-2016, 10:37 PM

Quote:

Originally Posted by dkveuro

Eh ?

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Basically moisture from compressed air will destroy a central air system in short order. I personally have never seen one leave a crater, I shire would like to see it.

Mobile systems are more tolerant to moisture, but sweeping the system with nitrogen helps blow out any moisture that was boiled out during vacuum. You really wouldn't think its there, but with a vacuum gauge you can really tell.

benzzy · #10 06-16-2016, 04:57 AM

Quote:

Originally Posted by dkveuro

Eh ?

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Sorry I didn't finish explaining .
I was only referring to what a bad mix AIR is with A/C systems .
An example is AIR/Compressed air being used for a leak check (PREASURISING A SYSTEM) OR flushing a line if that air meets and mixes with the compressor oil . Boom can occur. Always nitrogen, clean safe, inert . added benefit it helps absorb MOISTURE in the system. If a system has been open for a prolonged time or LOW on refrigerant it can run in a Vacuum where AIR can enter , If air enters so does moisture. Only 2 good ways to remove moisture .1 use nitrogen and allow it to stay long enough in the system to absorb, then purge it off. DO NOT run the system with nitrogen inside. 2 A good vacuum . IT Can only be measured with a vac u stat. A good Vacuum is 1000 microns and lower. the ideal vacuum is 500 microns and lower. This unfortunately can not be measured by gauges.
Also remember with car A/C systems that were not designed for 134A from manufacture The condenser just cant do the job properly . Meaning the condenser needs to be a lot larger or the refrigerant charge lower. A poor compromise . TX valves should always be changed if a system changes refrigerant . More flow is required with a LESS yielding refrigerant . High head pressures are MOISTURE in the system, secondly Restrictions. Or a inefficient condenser, Remember also oil migrates around the system with the refrigerant. DO NOT blow it to places in the system where it cant return unaided. Thank you for the link. a good read

I am new to forums and still finding my way. No disrespect meant for anyone.

dkveuro · #11 06-18-2016, 07:19 PM

Quote:

Originally Posted by benzzy

............................
An example is AIR/Compressed air being used for a leak check (PREASURISING A SYSTEM) OR flushing a line if that air meets and mixes with the compressor oil . Boom can occur. .

Are you suggesting that the compressor can auto ignite the oil in an a/c system ? or the gas, should you be using propane?

I think you ought to read this :Fuels and Chemicals - Auto Ignition Temperatures

These temperatures listed only exist inside a diesel engine and to start most diesels it is often only with the aid of glow plugs...ie: An ignition point.

I doubt any refrigerant pump can run at these temps'....and where would the ignition point be ?

The 'hole in the ground' sounds spurious.
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ps2cho · #12 06-12-2016, 10:20 PM

Ignoring the pressures is asking for trouble. If I just willy nilly charged by weight then I'll have the compressor blow a seal as pressures would easily get above 450psi...you MUST use gauges, every time!

Thanks for the line info. I ordered a Infrared Temp Gun and I'll get a readout across all the lines and see what it shows and reply back at that time.

lorainfurniture · #13 06-13-2016, 09:33 PM

Quote:

Originally Posted by ps2cho

Ignoring the pressures is asking for trouble. If I just willy nilly charged by weight then I'll have the compressor blow a seal as pressures would easily get above 450psi...you MUST use gauges, every time!

Thanks for the line info. I ordered a Infrared Temp Gun and I'll get a readout across all the lines and see what it shows and reply back at that time.

Charging by weight is the only truly accurate way of recharging a system. The car should never get pressures that high, there is a cutout switch on the high pressure line for those situations. Also, the fans should be running full speed after 350 psi.

You keep thinking that because you had 350 psi with 12 oz of r12 in , that you will have 3x the pressure with 3x the refrigerant in. This is not true.

The system is already pumping at capacity the moment there is enough refrigerant in there to liquify. Remember, the refrigerant is the stuff that absorbs the heat from the cabin. With only 12 oz in, there is not enough for you to notice a difference. The system is is working at full pressure, just not enough volume to make a difference. Just charge the system 2.2 lbs, or 1 kilo, and be done with it.

ps2cho · #14 06-13-2016, 10:23 PM

Probed with the temp sensor today, no cold areas or anything that seemed odd. All the high side hoses were too hot to touch, and the low side was mediocre cold at best...
Condenser inlet and outlet temps show a 80F drop, so the fans are shedding heat fine and its not a blockage in the condenser...the pressure is just off the charts.

Here's proof the pressures keep going up and up. I added about another 1/2 a can in. I stopped because with whatever is going on, is not following your theory.

This is at 1500rpm with both aux fans going full speed. Ambient temp right now is 98F.
Vent temps were showing 63F.

duxthe1 · #15 06-13-2016, 11:50 PM

The logical explanation is that the condenser is not properly shedding heat. Make sure that the aux fans are turning the correct rotation. For about the last decade the replacement fans come with the wiring backwards in the plug. I've seen plenty of fans spinning the wrong way.