Automotive Air Conditioning Information Forum (Archives)

pdrayton on Mon August 16, 2010 4:04 PM

Year: 1992
Make: Mercedes
Model: 300D
Engine Size: 2995
Refrigerant Type: R134a
Ambient Temp: 88
Pressure Low: 30
Pressure High: 350
Country of Origin: United States

As you can see, the high side is high - about 350 psi at 2000rpm. The low side flutters a little from 30 - 33 psi - does these movements in about .5 seconds every 2 seconds or so. Humidity is high, about 80% (north florida). high side gets really hot (no surprise at 350 psi right?). There are bubbles in the sight glass.

History: I recently replaced the drier and vacuumed and recharged with only about 2/3 of the recommended charge of R134a (600 g of 1000g) and 20cm3 of PAG46 oil.

Cooling is not great - probably a 20F drop standing still, does OK on the highway, but I'm scared to use it with those high side readings.

Does the 350psi high side with the system not even fully charged yet indicate there is too much oil in the system? A bad TXV? Something else?

Stumped. Thanks for any suggestions.

-------------------------
1983 Mercedes 240D on R12 and WVO!
1192 Mercedes 300D 2.5 R134a
1993 Toyota Pickup converted to R134a

TRB on Mon August 16, 2010 4:22 PM

Sounds more little one is using R134a in a system designed for R12.

-------------------------

pdrayton on Mon August 16, 2010 5:30 PM

Really? What gives that appearance? it has the official conversion sticker on the compressor and the R134a sticker on the frame.

I just did a misting the condenser test, and the pressures drop - 275 psi hi, 30 low (does move around from 30-34). Cooling was poor - dropped about 10deg from ambient of 85.

When I took the gauges off, I collected the oil from the pressure in the high side hoses, and there was quite a bit of oil, perhaps 1/8 teaspoon in a 4 ft hose.

-------------------------
1983 Mercedes 240D on R12 and WVO!
1192 Mercedes 300D 2.5 R134a
1993 Toyota Pickup converted to R134a

TRB on Mon August 16, 2010 6:14 PM

Because a T & F condenser is not going to cut it with R134a! Unless it is 4 feet x 6 feet in dimensions. MB also have small evap cores which does not help the issue!

-------------------------

pdrayton on Mon August 16, 2010 10:04 PM

Thanks for the reply.

I don't know what a T & F condenser is.

All I know is my wife has the exact same car - same year and same model, and it blows really cold on R134a - had to turn the vents away with 88 ambient (sorry, no offical measurements, but I know it is better than mine by a long way). It has 30psi low, 275psi high at 2000rpm, just like the FSM says. It's WAAAAY better than mine.

-------------------------
1983 Mercedes 240D on R12 and WVO!
1192 Mercedes 300D 2.5 R134a
1993 Toyota Pickup converted to R134a

Tennessee_Timmy on Mon August 16, 2010 10:12 PM

T&F - tube and fin

-------------------------
Good Luck 2 ya,
Timmy

1stbscout on Mon August 16, 2010 11:26 PM

Couple of questions....
Why add oil?
How much oil did you remove?
Why did you evac and charge to begin with?

Without accurate temperatures on the different parts of the system it is very difficult to troubleshoot your problem. It is the temperature of the different parts that is most important. The pressures are only indicators of what is happening. The comparison of pressure to temperature is what makes it possible to determine if you have a problem and what is causing the problem.

pdrayton on Tue August 17, 2010 8:22 AM

Thanks guys.

The reason I recharged it was because the sight glass had what looked like rust in it. But when I took the drier off and looked at it, it was fine.

I blew out the lines and there was no sign of rust or contamination that I could see. There is the neon dye though, and some oil came out when I blew the lines out (didn't measure it, but probably 5-10cm3).

So I put a new drier in, added 20cm3 of oil because the FSM said 10cm3 for the new drier and 10cm3 for a line. I vacuumed it for an hour to -25mmHG (probably not quite enough, but unlikely to cause the problems?)

1stbscout - are you talking about the temps at the start and end of the condenser? I can get those. If there are other temps I should get, please let me know which. Thanks.

Does anyone know the symptoms of too much oil? I really have a feeling there is too much oil. When I've recharged other cars, there doesn't seem to be so much oil in the lines when working on the ac.

This car is missing the radiator cowling. Sorry to mention it so late - I felt the air flow behind the radiator at idle and there was good air flow all the way to edges of the back of the radiator, so I dismissed it as a possible cause of problems. Also, when misting the condenser and with pressures of 30/275 cooling was still very poor - 70F vents. I cleaned the condenser too, but no improvement in cooling.

-------------------------
1983 Mercedes 240D on R12 and WVO!
1192 Mercedes 300D 2.5 R134a
1993 Toyota Pickup converted to R134a

Edited: Tue August 17, 2010 at 9:40 AM by pdrayton

1stbscout on Wed August 18, 2010 1:51 AM

The most important temperatures or the outlet from the condenser and the outlet from the evaporator. Inlet temps will help with the diagnosis but it is the outlet temps that tell most of the story.
I apologize .... I do not have time tonight to get into all of it. The pressures are only part of the story.... For some systems it is perfectly acceptable to have high side in the 300 range and lows in the 50 range. For others it is not. The pressure only tells you part of the story. The temperature tells the rest. Without both and a very clear understanding of how they "interact" it becomes very difficult to figure out what the real issues are.
Too much oil will cause the High side to be higher than it should be and the cooling to be limited ...... to a point. In a perfect world you would have no oil mixed with the refrigerant. In the same perfect world ANYTHING other than refrigerant in the system is a contaminant and causes a loss in performance.

mk378 on Wed August 18, 2010 8:37 AM

You've undercharged it trying to keep the pressure down. An undercharged system can never cool. You need to put all the airflow parts on, poor condenser cooling is the root of the problem. Too much oil works much the same as poor airflow, condenser performance is impaired because the oil just gets in the way. It may not have been converted properly, with a bunch of mineral oil left in it.

Edited: Wed August 18, 2010 at 8:40 AM by mk378

pdrayton on Wed August 18, 2010 9:31 AM

MK378 - the cooling performance didn't improve when I misted the condenser (pressures dropped 350 -> 275), so would you say that points more at too much oil versus poor condenser air flow?

Yes, I'm lazy... and not crazy about flushing ac systems.....

1stbscout - reading into what you said - high condenser outlet temp would be poor condenser performance (too much oil, or poor airflow). high evap outlet would be? What's high?

-------------------------
1983 Mercedes 240D on R12 and WVO!
1192 Mercedes 300D 2.5 R134a
1993 Toyota Pickup converted to R134a

bohica2xo on Wed August 18, 2010 11:45 AM

Eurotrash cars of that era have crappy A/C to begin with, and converting them is a bad idea.

Without a flush to bare metal, a proper oil charge, a full charge of the design refrigerant & all of the OEM parts in place - all you will ever have is less than the OEM marginal performance it was shipped with.

If you are too lazy to even fix part of that, then I suggest pulling the A/C fuse, and rolling down the windows. Takes the least amount of effort.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

pdrayton on Thu August 19, 2010 9:16 AM

Wow, not much love for the 90s Mercs....

My wifes ac works fine for humid, hot north florida on R134a (same model and year car), so I'll flush and recharge and see what I get.

Thanks for the help guys

-------------------------
1983 Mercedes 240D on R12 and WVO!
1192 Mercedes 300D 2.5 R134a
1993 Toyota Pickup converted to R134a