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I always run my A/C throughout the year to keep the seals properly conditioned. Modern compressors no longer put a drag on the engine like they did 40-50 years ago. In the winter, having the A/C on helps keep the inside dry (from fogging the windows). In the summer, well...to stay cool.
I should had done the same, oh well, looks like I destroyed the a/c compressor because it got used to the lazy lifestyle and lost its ability to function or forgot how to work from me constantly telling it to have a day off.
I think it is the seals in the PWM solenoid valve that are guaranteed to hold back the plunger. That sets the compress plate near neutral pumping.
If it was indeed the compressor, and the solenoid, can I salvage it? and... ugh always the solenoid eh, first the oil pump solenoid now this solenoid, I officially hate solenoids now...
The hissing sound you are hearing could be from the TXV.
TXV is what controls liquid R134A to become gas state R134A.
Low R134A fill level can cause hissing sound too, big leak can cause that sound too.
R134A has a smell, its like hospital smell . Sorry I can't attach a SMELL here "faint, sweetish, ethereal odor. Some sources describe it as having a "sweet chloroform" smell."
If indeed there is a leak at the EVAP ( common ), you would probably smell the R134A.
Depending on where the leak is at the EVAP, if indeed leaking, you probably won't see any trace of oil as the leak out is R134A is gas state with very little compressor oil.
EVAP leak is expensive affair. The whole dashboard has to come out.
Go to a good HVAC workshop, which has refrigerant sensor ( for contamination ) , do RRR first to get the weight of the R134A fill correct.
Later use their analog pressure gauge HP side and compare it to MB own pressure gauge which is HP side, which would read different only 2-3 PSI.
LP = Low Pressure, blue color connector, small fitting on the car.
HP = red color connector, High pressure.Big fitting on the car.
Better, if they have and use the inspection sight glass to see compressor oil + liquid R134A color and state.
R134A is colorless. The ND-8 Denso PAG compressor oil is near colorless. There is no UV dye on new car. So you get like above, super clear liquid R134A with also clear compressor oil.
This is compressor oil starting to turn yellowish due to heat and age. The greenish hue is not UV dye, there is no UV dye. This is my E400 in April 2023.
.
.
Below, compressor oil color
Compressor oil
If the color is not clear and rather dark going to black, good bye compressor and the entire HVAC system of your car...yes 100% gone.
All hoses, hose with built-in heat exchanger, condenser, TXV and evap and compressor everything must be replaced new. You can't fush the microchannel type heat exchanger we use today.
I hope yours is only the compressor control valve
.
Last edited by S-Prihadi; May 9, 2025 at 12:39 AM.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Wow! That would really suck. But the point of getting it to an HVAC specialist is dead on. Something like this is not your normal DIY. (Yes, @S-Prihadi , you're not normal.)
Whether it is a bad valve core or the whole things needs to come out, right now we are just guessing. The colder air right after turning off the A/C is still a mystery.
The hissing sound you are hearing could be from the TXV.
TXV is what controls liquid R134A to become gas state R134A.
Low R134A fill level can cause hissing sound too, big leak can cause that sound too.
R134A has a smell, its like hospital smell . Sorry I can't attach a SMELL here "faint, sweetish, ethereal odor. Some sources describe it as having a "sweet chloroform" smell."
If indeed there is a leak at the EVAP ( common ), you would probably smell the R134A.
Depending on where the leak is at the EVAP, if indeed leaking, you probably won't see any trace of oil as the leak out is R134A is gas state with very little compressor oil.
EVAP leak is expensive affair. The whole dashboard has to come out.
Go to a good HVAC workshop, which has refrigerant sensor ( for contamination ) , do RRR first to get the weight of the R134A fill correct.
Later use their analog pressure gauge HP side and compare it to MB own pressure gauge which is HP side, which would read different only 2-3 PSI.
LP = Low Pressure, blue color connector, small fitting on the car.
HP = red color connector, High pressure.Big fitting on the car.
Better, if they have and use the inspection sight glass to see compressor oil + liquid R134A color and state. https://cimg0.ibsrv.net/gimg/www.mbw...203d4183c6.jpg
R134A is colorless. The ND-8 Denso PAG compressor oil is near colorless. There is no UV dye on new car. So you get like above, super clear liquid R134A with also clear compressor oil.
If the color is not clear and rather dark going to black, good bye compressor and the entire HVAC system of your car...yes 100% gone.
All hoses, hose with built-in heat exchanger, condenser, TXV and evap and compressor everything must be replaced new. You can't fush the microchannel type heat exchanger we use today.
I hope yours is only the compressor control valve
.
Uh oh, that is much more on my plate than I anticipated.
Wow! That would really suck. But the point of getting it to an HVAC specialist is dead on. Something like this is not your normal DIY. (Yes, @S-Prihadi , you're not normal.)
Whether it is a bad valve core or the whole things needs to come out, right now we are just guessing. The colder air right after turning off the A/C is still a mystery.
Ya this is a great May gift for me isn't it? Thanks car : ) ... May day.. mayday? may day! mayday?.
Yes this is very strange, probably another valve is also messed up and it is opening when it meant to be closing.
Last edited by W205C43PFL; May 9, 2025 at 09:53 AM.
Reason: May day! Mayday?
So I have a hissing sound coming from the vents only when a/c is on. Fair to say I have a leak and have low refrigerant at the same time?
I forgot if it was a hissing or a whistling noise, I won't be driving until Wednesday thank you all for all the information, will revisit this on Wednesday. Love this community!
I forgot to ask, is short cycling an issue with car a/c compressors? I understand it is for home a/c units, as the thermostat usually won't let it start again until 5 minutes later.
My AC is last condition ( operational status) before engine OFF memory.
But it is my habit I kill the AC compressor first , that A/C button, set to maximum blower speed to dry the evap from condensation, and then I press OFF.
So next operation when I turned ON AC using blower speed flat button, I must press A/C ( compressor ) again to activate compressor too.
This is my compressor oil and R134A after first RRR of April 2023.
But my RRR is not Recover-Recycle-Recharge, it is Recover-Replace R134a with virgin one as the recharge. No such thing as RECYCLE.
After recover it is always the vacuuming, and I do a long 12+ hours to really dry the system.
Here is much clearer R134A in liquid form, because of the deep vacuuming process, the compressor oil is still yellowish.
Compressor oil
Left, 21st april is before 1st time RRR. This was MB Indonesia crappy work in 2014 when they assembled the car. Right side is after first RRR of 21st April and I inspected it on 6th June.
7th June, I tried to "clean" the oil using filter drier and I took out some compressor oil too.
Result
8th June, I recover/suck out my 3 months old R134A of first RRR.
9th June I added new compressor oil, matching the same amount of oil I sucked out. Only 10 grams of oil. Total oil in system is supposedly 120 ml.
This oil is like 1 gram per 1 milliliter, heavy like water.
This is 10 grams or milliliters of ND-8 Denso
.
.
I vacuum dry the compressor oil first. See the value 60 microns and the high peak ( moisture explosion ) 100+ microns.
1 Atmosphere at sea level is 14.7 PSI or 29.9213 inch of mercury or 760 millimeters or 760,000 microns.
So 60 microns is super dry and you can't even us analog gauge to measure 1,000 microns which is crazy wet for HVAC system.
1 inch hg is 25,400 microns
0.001 inch hg is 25.4 microns.
0.010 inch hg is 250.4 microns
0.050 inch hg is 1,202 microns
0.100 inch hg us 2,504 microns <<< you can't even see analog gauge 0.1 inch hg resolution
Clean clean clean.....I am so happy.
No more yellowish oil and now super clear R134A.
Soonest next year I will do RRR again, and replace refrigerant drier and 3 HVAC hoses and all o-rings at EVAP ( 4pcs ) and o-rings at compressor which is already
at the new hoses. The only hose set I do not replace is the one with internal heat exchanger IHX. The IHX hose set ( LP and HP ) is not at HOT ZONE true engine bay, it is behind fake firewall.
3 hoses new and refrigerant drier will allow me to replace approx 60 milliliters of compressor oil, out of the 120 ml.
Thus I have the chance to renew my compressor oil too.
Doing this I hope to never ever have to replace my EVAP dues to compressor black death. I do not ever want my entire dashboard be removed !!!
I can track per 2 years my compressor wear and tear and can replace it when it is "weak" but not total loss yet.
My AC is last condition ( operational status) before engine OFF memory.
But it is my habit I kill the AC compressor first , that A/C button, set to maximum blower speed to dry the evap from condensation, and then I press OFF.
So next operation when I turned ON AC using blower speed flat button, I must press A/C ( compressor ) again to activate compressor too.
This is my compressor oil and R134A after first RRR of April 2023.
But my RRR is not Recover-Recycle-Recharge, it is Recover-Replace R134a with virgin one as the recharge. No such thing as RECYCLE.
After recover it is always the vacuuming, and I do a long 12+ hours to really dry the system.
Left, 21st april is before 1st time RRR. This was MB Indonesia crappy work in 2014 when they assembled the car. Right side is after first RRR of 21st April and I inspected it on 6th June. https://cimg0.ibsrv.net/gimg/www.mbw...6b91802f44.jpg
9th June I added new compressor oil, matching the same amount of oil I sucked out. Only 10 grams of oil. Total oil in system is supposedly 120 ml.
This oil is like 1 gram per 1 milliliter, heavy like water.
1 Atmosphere at sea level is 14.7 PSI or 29.9213 inch of mercury or 760 millimeters or 760,000 microns.
So 60 microns is super dry and you can't even us analog gauge to measure 1,000 microns which is crazy wet for HVAC system.
1 inch hg is 25,400 microns
0.001 inch hg is 25.4 microns.
0.010 inch hg is 250.4 microns
0.050 inch hg is 1,202 microns
0.100 inch hg us 2,504 microns <<< you can't even see analog gauge 0.1 inch hg resolution
Clean clean clean.....I am so happy.
No more yellowish oil and now super clear R134A.
Soonest next year I will do RRR again, and replace refrigerant drier and 3 HVAC hoses and all o-rings at EVAP ( 4pcs ) and o-rings at compressor which is already
at the new hoses. The only hose set I do not replace is the one with internal heat exchanger IHX. The IHX hose set ( LP and HP ) is not at HOT ZONE true engine bay, it is behind fake firewall.
3 hoses new and refrigerant drier will allow me to replace approx 60 milliliters of compressor oil, out of the 120 ml.
Thus I have the chance to renew my compressor oil too.
Doing this I hope to never ever have to replace my EVAP dues to compressor black death. I do not ever want my entire dashboard be removed !!!
I can track per 2 years my compressor wear and tear and can replace it when it is "weak" but not total loss yet.
.
Thanks for the insight, happy to had the opportunity to read about it.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Originally Posted by W205C43PFL
I forgot to ask, is short cycling an issue with car a/c compressors? I understand it is for home a/c units, as the thermostat usually won't let it start again until 5 minutes later.
Check my diagnosing charts above. Specifically, this...
I understand I have that downloaded on my phone already, my question really was is short cycling an a/c compressor in a car an issue, as in general. I am not talking about the problem I am facing but as in in general. Sorry for the confusion.
I understand I have that downloaded on my phone already, my question really was is short cycling an a/c compressor in a car an issue, as in general. I am not talking about the problem I am facing but as in in general. Sorry for the confusion.
As it turn out... no it is not short cycling because the clutch stays closed and pressure is regulated by TXV plus compressor PWM solenoid.
These variable compressors do not "short cycle" on/off, they are constantly variable. This help preserve the serpentine from surges.
Your compressor appears stuck pumping neutral pressure as witness by AAC asking 100% on a near 100psi charge without changes.
Older "non-PWM compressors" did "short cycle" on low charge pressure.
Compressor pumps down suction side until it runs into low pressure them is forced to stop.
Last edited by CaliBenzDriver; May 9, 2025 at 04:21 PM.
As it turn out... no it is not short cycling because the clutch stays closed and pressure is regulated by TXV plus compressor PWM solenoid.
These variable compressors do not "short cycle" on/off, they are constantly variable. This help preserve the serpentine from surges.
Your compressor appears stuck pumping neutral pressure as witness by AAC asking 100% on a near 100psi charge without changes.
Older "non-PWM compressors" did "short cycle" on low charge pressure.
Compressor pumps down suction side until it runs into low pressure them is forced to stop.
I see, I am curious why home a/c units don't use variable compressors?
I guess I need to confirm if it is stuck first but more likely, then this is the best I presume cheapest way as well.
you know... the pass/fail test will be easy:
cold vs. hot.
no thermometer necessary.
Engine off:
1 - You zap the solenoid with specific polarity for 5mn
2 - replug the original connector.
3 - clear fault
4 - idle engine.
Cabin hot/cold?
Repeat zapping but with engine idling to allow easier plate motin.
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