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.

ps2cho · #7 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 · #8 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 · #9 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 · #10 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.

lorainfurniture · #11 06-14-2016, 12:00 AM

What was the pressure at idle?

At 40/ 440, looks like there is a restriction somewhere. The expansion valve is a common place to clog.

lsmalley · #12 06-14-2016, 02:26 AM

I know you said you vacuumed down to -29, but those numbers could be also indicative of air in the system. When you vacuumed down the system did you do it for at least 15 - 30 mins on low and hi side? Also, before you started charging, did you crack the yellow feed hose first to purge the air out and let the r134 fill up the hose before you introduced the refrigerant into the system? I would say vac the system down again and follow the recommended charging protocol. Or, try getting the rpms up to about 2500 and see where you are at with ac vent temp and the high and low side pressure. If it stabilizes at the higher rpm and then goes back up, you could have a problem with the compressor not properly compressing the refrigerant.
Also, check out this attachment. Good luck.

Zulfiqar · #13 06-14-2016, 05:12 PM

at 450 psig with runnign system your liquid line should be extremely hot, try dousing the condenser with running water and see if it helps.

In one instance in a another brand car I was repairing this behaviour was caused by a faulty expansion valve - its high side was reaching 400 psi while the low side was at 30 and going towards 20 when revved.

It maybe that you have a lot of oil in the system.

benzzy · #14 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.