It's that time of year, AC ? on late W124 - PeachParts Mercedes-Benz Forum

wolf_walker · #1 06-03-2017, 01:24 PM

I bought this pretty decent 94 E320 this past winter, I think it'd been sitting for a year or so when the PO got too old to drive. AC has worked, but not super well. I noticed on a genuinely warm (high 80'sF) day last week the AC was useless in town, and about ten minutes on the interstate had it working "ok".
I didn't have a thermometer with me, and it's my Very Significant Others daily, she's used to crappy AC the poor thing and didn't mention it.

Anyway, the PO had the compressor and some other stuff replaced not too long ago at an inde shop, I have the receipt somewhere. I verified the electric fans were not coming on with the AC, verified they did in fact work, took a wild guess and put a little R134 in it from one of those generic cans from the McParts store I had laying around, and the fans started to cycle.
For what little it's worth, it was in the low to mid 70's by then and by the little gauge on the bottle on the low side I finally got it up to where it was cycling between 26/36psi, fans were cycling, but it never managed a center vent temp on full cold, recirc, high blower, of lower than 62. I intend to try a 2nd thermostat as I swear it felt colder than that but I know "feel" isn't a good way to judge things.

So, the system seems to work, wasn't so low after a year or two that it wouldn't function on the highway, has a newish compressor(which I assume is from autozone or such), the compressor never cycles, the climate control seem to function correctly(after having the AC on a while and turning it off I don't detect any warmer than ambient air as if a blend door was leaking or such), it has had at least one of the dash pods serviced in the past and also the evaporator earlier in life by a dealer, the condenser is clean. My thought it to evacuate the system, vacuum it a while, and recharge it with the stated 2.2lbs or whatever it is and see what that gets me. My Father has a vacuum pump and manifold gauges and has a lot of experience with older R12 systems and will assist. Thoughts?

nulu · #2 06-03-2017, 03:05 PM

Put gauges on it and then diagnose, may be a lot of things, likely a door not shutting, don't forget you have a filter in there too under wiper

mercron · #3 06-03-2017, 04:21 PM

Since the system still works, I'm guessing it's either under or overfilled with refrigerant. You shouldn't second guess the amount here.

I'd get it to an AC shop and have them professionally evacuated, vacuumed, and recharged with the exact amount of refrigerant and PAG oil. This shouldn't be too expensive as there's no parts to replace.

Sent from my iPhone using Tapatalk

Jorn · #4 06-03-2017, 05:06 PM

Check the electric fans. My AC (300SE) had the same symptoms, crappy or no AC in the city and decent on the highway. The fans were not coming on, the wires under the fuse box were fried. Fixed it and cold AC now.

wolf_walker · #5 06-03-2017, 10:03 PM

Quote:

Originally Posted by Jorn

Check the electric fans. My AC (300SE) had the same symptoms, crappy or no AC in the city and decent on the highway. The fans were not coming on, the wires under the fuse box were fried. Fixed it and cold AC now.

Working now after adding some refrigerant the other day, pressure was below cut-in on the red pressure switch I assume. Fans cycle now.

Ambient 76F today, muggy, AC was doing jack crap driving around town at 30-40mph, maybe a 12-15F drop over ambient. I'll try to stick gauges on it next week. Am I good using a generic temp/pressure chart like this?
R-134a System Pressure Chart | AC Pro

I don't think I've ever had a car with a factory r134 AC system lol...

Hirnbeiss · #6 06-04-2017, 09:04 AM

R134 conversions always perform worse than the R12's. If everything is checked and in spec / working, the next best place to look for improvement is in the condenser (bigger, parallel flow type, more air flow, ...). The quick and dirty test is to measure duct temps while hosing down the condenser - if you get a big drop, then you know you have a lot of potential there.

Diseasel300 · #7 06-04-2017, 09:48 AM

1994 should have been factory R134a.

You really need to know what's going on with the high side as well. Without knowing what both high and low are doing, you're just shooting in the dark and guessing.

I'm going through the same thing in my SDL right now (different refrigerant). Center vent won't get blow ~65 in my case. Was working perfectly for a period of time, blowing in the upper 30s. I've tracked the issue in mine down to a stuck expansion valve. Stuck open in my case. Compressor pulled in, good charge of refrigerant, but low side is high and high side is low. Bang on the expansion valve and it'll work briefly until it moves again and gets stuck. You'd never know this was happening unless you had gauges hooked up to both high and low. You can watch it hunt a few times then "stick".

puzzler · #8 06-04-2017, 03:35 PM

My system was similar, old neglected AC (95E320) and with a fresh charge it began cooling but then I overcharged and it shut off. I evacuated the system including tons of excessive oil from K-mart refrigerant. Recharged the system with the proper amount and it worked great until the fan fuses began blowing, first the fan relay fuse and then fuse D on the main panel began melting out with continual activation. Cycling but it still overheated the fuse and melted the fuse block. Very good to keep the AC working on this car and if the AC works you need the fans too or the engine will overheat..Great system! Good luck!

wolf_walker · #9 06-04-2017, 08:20 PM

1500RPM

Idle

Just off after other readings.

We charged it a hair more and got it up to what looks to us like reasonable working pressures. 78F and a bit muggy in the shop. Vent temps with above were crap. I can't find any fault in the operation of the CCU in general, vents all switch, temps change rapidly, fans working, etc, etc.
It should be cooling.

Soooo... I plan to buy an expansion valve, receiver/dryer and some o-rings, replace, vacuum, make sure it holds, and recharge. I'm not sure if I should add anymore oil or not? I know, a pro machine separates and measures the oil, but this is DIY land.

Diseasel300 · #10 06-04-2017, 08:23 PM

I think you're a little overcharged. It's also worth checking that the heater is turning off. If it's stuck open for some reason, it can easily overpower the A/C.

wolf_walker · #11 06-04-2017, 09:59 PM

Quote:

Originally Posted by Diseasel300

I think you're a little overcharged. It's also worth checking that the heater is turning off. If it's stuck open for some reason, it can easily overpower the A/C.

Well it wasn't working any better before I added some freon the other day, or before/after Pop added some this afternoon. It might well be a little over based on my understanding of pressure readings we saw but not like 65F vent temps on an 80 degree day over I don't think.

I did ponder the heat thing, looking at the diagrams of the HVAC box, it looks like they run the air through the evaporator, then through the heater core, and then direct it as required with the vacuum pods. The monovalve is as far as I can tell a PWM coolant flow valve, and it seems to work based on rolling the temp wheel from hot to cold with a thermometer in the center and outer vents. Near as I can tell they precisely control the coolant flow to the heater core, and rely on the evap to cool off the air and the heater core rapidly when changing from hot to cold? I didn't really see what is generally called a blend door in the system that directs air through one or the other. I guess if you have the capacity and control over the coolant, ac and airflow it would work blowing through them both. Any other functional test I can do I'm all ears, I have observed that with the ac off on the highway, going from hot to cold gives ambient air vent temps pretty quickly, if there was a blend door failure introducing hot air(or whatever the Mercedes equivalent is) I'd expect to see warmer than usual air once the dash and heater box internal temp was normalized after being on the road for awhile. I'm sure it's not all perfectly sealed at it's age, but still.

I'm not sure if it's relevant at all, but the low side line is cooler than ambient and sweating, and just on the evap side of the expansion valve the low pressure pipe temp with an IR thermometer is within a few degrees of what the vent temps are(65-75 depending).

hs_300e · #12 06-06-2017, 09:23 AM

You might have a monovalve malfunction. The Monovalve is normally open. When powered, it shuts off the flow of coolant to ensure cold air. If it fails, or is partially open, then it will let hot coolant flow resulting in warm air out of the vents.

When the engine is cold, i.e does the a/c blow cold? Your window might be very small for this test.

wolf_walker · #13 06-06-2017, 01:10 PM

Quote:

Originally Posted by hs_300e

You might have a monovalve malfunction. The Monovalve is normally open. When powered, it shuts off the flow of coolant to ensure cold air. If it fails, or is partially open, then it will let hot coolant flow resulting in warm air out of the vents.

When the engine is cold, i.e does the a/c blow cold? Your window might be very small for this test.

Small window indeed, but a good suggestion. I've observed how quickly vent temps return to ambient but that isn't as conclusive as it could be. Trouble this time of year is not only does the car warm up very quickly (good engineering to be sure) but the dash is generally very heatsoaked when the car has been sitting. Maybe I can catch one more coolish morning and try it.