55 Low Side - 400 High Side Whats Up? - PeachParts Mercedes-Benz Forum

netboy · #1 08-13-2010, 08:10 PM

My daughter's 1985 300d only blows about 80 at the vents with 134. Thayer gets 45 @ the vents and another guy I know with an 83 300d gets ice cold air with 134. And the fan blows very weak on high, I need to go thru the system. The components and refridgerant came with the car 2 years ago and it did blow colder then.

Low side press- 55...... High side press-400 @ 99 degrees ambient

With moderately cold air and weak fan air at the vents makes me believe the evaporator is dirty or clogged but what the deal with the pressures?

Aquaticedge · #2 08-13-2010, 08:57 PM

might have a plugged dryer and the refrigerant isnt going all the way around, I dont know anything about the pressures though... Sorry

tankowner · #3 08-14-2010, 12:01 AM

The following are causes of both high low-side and high-side pressure as provided in the Haynes AC manual.

Check your high-side refrigerant line, if it is hot then the following may be the problem:

- restricted or blocked condenser fins
- refrigerant overcharge
- faulty condenser cooling fans
- engine overheating
- air in the system
- incorrect refrigerant in the system

Alternativley, check the low-side line, if there is moisture or frost on it, then the following may be the problem:

- expansion valve stuck open, allowing excessive amounts of refrigerant through the evaporator.

If none of that works, check to see if there is enough ice in the cooler and the bilge pump is working. Just kidding.

Good luck, let's us know if you figure it out. Mine is actin up and I am wondering if it might be the latter since the AC used to blow litle ice chunks at me through the vents. Although I need to hook up the gauges again and I think the high-side line does get hot. Not sure, will have to investigate when I have time. Something is screwy.

mach0415 · #4 08-14-2010, 03:26 PM

Tankowner has the right ideas. The low and high side numbers seem like they are too high. A weak compressor would have high low numbers and low high numbers. A blockage would have excessively low low side numbers (below15psi) and very high high side numbers (450+ psi) and may even trip a check valve on the compressor to vent off pressure. A faulty expansion valve could cause this, but an overcharge could too. I would suggest evac. system, vacuum and recharge with the specific amount of R134A to recheck performance before tearing into it too far. Original R12 systems (expansion valves specifically, as well as orifice tubes) do not seem to cool as well with 134A unless the orifice is changed. I cannot speak to any expansion valve systems. Does anyone know if R12 and R134a expansion valve orifice characteristics differ?

Yak · #5 08-14-2010, 04:57 PM

What's ambient pressure with compressor off?

High side seems too high and low seems too high, too. Possible overcharge?

http://www.ackits.com/aacf/ptchart.cfm

weird beard · #6 08-14-2010, 09:42 PM

as stated above as a possible cause, check the condensor fan. At 400 psi it should have already started running. When the condensor fan does not come on, excessive heat builds up in the system, and as a result the pressures increases on both low and high sides. It is a simple check and a simple fix if it is the problem.

If the fan is not coming on, jump the fan switch and recheck opperation.

If the a/c worked before, it is not going to be overcharged unless you are making refrigerant some how.

mach0415 · #7 08-15-2010, 12:34 AM

If it was cooling fine 2 years ago, i am curious if your part of Tennessee has seen 99+ weather in the last two years. Overcharge will be more noticeable in hotter than usual weather. So it could have been overcharged before you got it 2 years ago, if hotter this summer. Fan operation is critical, but you should only notice issues at slower speeds or at idle. When moving, the vent temp will stay warm with an overcharge. Just curious...did you check pressures at idle, or with the engine revving?

mach0415 · #8 08-15-2010, 12:39 AM

FYI average temp for July and August for your area is 85-87 degrees, so ambient is 10-15 degrees hotter this year FWIW.

whunter · #9 08-15-2010, 12:54 AM

Quote:

Originally Posted by netboy

My daughter's 1985 300d only blows about 80 at the vents with 134. Thayer gets 45 @ the vents and another guy I know with an 83 300d gets ice cold air with 134. And the fan blows very weak on high, I need to go thru the system. The components and refrigerant came with the car 2 years ago and it did blow colder then.

Low side press- 55...... High side press-400 @ 99 degrees ambient

With moderately cold air and weak fan air at the vents makes me believe the evaporator is dirty or clogged but what the deal with the pressures?

Low side should be 30.

High side should be 250-280. (if the aux fan works)

400 - 600 is where you start blowing a factory NEW hose or seal..

vstech · #10 08-15-2010, 09:24 AM

if it's not overcharged, (2lbs MAX 134 in the system) you must have a very dirty condenser coil. I'd go to the car wash, pop the two large clips holding your coolant radiator to the front of the car, tilt it back, (careful, you don't wanna break your radiator hose connectors!!!) and soak the a/c condenser with engine presoak detergent. let it soak while you clean the rest of your car, then after 20 minutes or so, set the sprayer to hot soap, and spray directly from the back to the front of the condenser, and see what detritus comes out. be careful not to spray at an angle to the fins, you don't wanna bend or destroy them!
even with a bad aux fan, you wouldn't get above 350 unless your coil was dirty, or somebody pumped in too much 134. a clogged txv wouldn't raise the high pressure unless somebody kept pumping in 134 an open txv wouldn't raise the high pressure unless somebody kept pumping in 134... soooo, it's either a totally clogged condenser, or too much 134.

Yak · #11 08-15-2010, 10:20 AM

Quote:

Originally Posted by vstech

if it's not overcharged, (2lbs MAX 134 in the system) you must have a very dirty condenser coil..... it's either a totally clogged condenser, or too much 134.

One of the other threads posted a pic of a "barn car" with a really dirty condenser.

When I bought my '83 300CD the front of the condenser looked okay, but the driver had evidently driven through a few swarms of butterflies (not too uncommon in Texas depending on the time of year). The gap between the condenser and the radiator was probably 30% filled with butterfly parts. I had to do as vstech recommended and unclip the radiator, lean it back and clean it out with a paint stir stick. Straight water pressure would not clear them out.