Automotive Air Conditioning Information Forum

Good day gentleman,

I have an 01' S430 Benz with 4.3l V8 that is only mildy cooling here in FL. It's extremely hot now, and internal cabin temps when car sits on the sun is like 130f or more so you'd imagine I'd like to solve this problem. I have a decent understanding of a/c systems in cars but what I need help with is how not to parts cannon the vehicle. Essentially I want to do a pin point diagnosis instead of, "yea just throw a compressor, condenser, txv, etc." in it.

Right now the system is only cooling down to 60-65F at the center vents whether it's idling or driving with outside temps of 95F or more. I have an advanced scantool and it's reading the evaporator temps are only 58-60F. The low and high side pressures are 35-40 low and 175-200 psi high. My intuition here is that the high side is kind of low, suggesting the compressor isn't compressing 100 percent, i think. This compressor is also variable displacement with refrigerant valve in it. I have checked the condenser fan and it spins pretty well and blows air through condenser. Condenser does not have debris/dirt in it. Receiver drier has recently been replaced. I added a few more ounces of 134 into it and it did not make a difference in the pressures or outlet temps.

How do I know if this is a compressor issue, expansion valve, or whatever else? I should be getting 40-45F at vents at least! I'll be happy to conduct any tests or diagnostics, just tell me what I should check. Thank you!

event3horizon wrote: ↑Fri Jun 21, 2024 11:06 am The low and high side pressures are 35-40 low and 175-200 psi high.

Measured at how many rpm ?? Typically, must measure these pressures at 1800 to 2000 rpm, post those.

1. We assume that your 2001 uses R134a.
2. We assume that the AC formerly did work OK on this vehicle.
3. Does the larger-diameter AC metal line (the low side) feel cold at the engine firewall?

Cusser wrote: ↑Fri Jun 21, 2024 11:42 am

event3horizon wrote: ↑Fri Jun 21, 2024 11:06 am The low and high side pressures are 35-40 low and 175-200 psi high.

Measured at how many rpm ?? Typically, must measure these pressures at 1800 to 2000 rpm, post those.

1. We assume that your 2001 uses R134a.
2. We assume that the AC formerly did work OK on this vehicle.
3. Does the larger-diameter AC metal line (the low side) feel cold at the engine firewall?

Hi Cusser. Thanks for your reply. Yes, it uses 134a and it did cool better in the past for sure. The low side line is cold to the touch, measured about 68F and sweaty. The pressure readings were read at when at about 1500 rpm. At idle, it is 45 low and 180 high. High side never seems to go above 200. When car idles it has the worst vent temps at around 65. If I drive the car at highway speeds for over 30 mins then vents get eventually to 55f.

I did some quick tests again. Car was 140F inside, sitting on sun. Outside temp shows 100F. Started up, waited 5 mins, rev'ed to 1500-2000 rpm while in park. Hi side pressure was 220-230 PSI. Not sure what low side was as I didn't have gauges at the moment. Vent temps 65F. At Idle: vent temps 70F , high side 188 psi. Recirculate or fresh air setting does not make a large difference. Climate control was set to lowest setting 'Lo'. Blower was Max. When I set the blower to low, it provides a bit cooler vent temps.

As far as you know, has the system ever been recovered, evacuated, and recharged since new and/or the compressor replaced? How much mileage on the clock?

Measured low-side pressures of 35-40 PSI at ~1,800 RPM mean that the evaporator temperatures are going to be ~40-46°F, with the center vent temperatures up to 10°F higher, which won't provide much cooling at an ambient temperature of ~95°F. It might be that the charge amount is "off" (just a few ounces either way can make a big difference), or it could be something else.

JohnHere wrote: ↑Fri Jun 21, 2024 2:09 pm As far as you know, has the system ever been recovered, evacuated, and recharged since new and/or the compressor replaced? How much mileage on the clock?

Measured low-side pressures of 35-40 PSI at ~1,800 RPM mean that the evaporator temperatures are going to be ~40-46°F, with the center vent temperatures up to 10°F higher, which won't provide much cooling at an ambient temperature of ~95°F. It might be that the charge amount is "off" (just a few ounces either way can make a big difference), or it could be something else.

I'll have to measure the low side again when I get time but I usually measure those pressures at idle. Didn't know it's better for diagnostic at higher RPM. Few weeks ago I measured at idle when car was sitting on sun midday and when the compressor started working the initial pressures were 50/200. The compressor has never been replaced per the service records and current mileage is 111K. I did evacuate, vacuum and recharge to weight just about a year ago. At that time, I believe it did cool a bit better. Maybe 5-8F better than what I'm seeing now.

event3horizon wrote: ↑Sat Jun 22, 2024 10:00 am I'll have to measure the low side again when I get time but I usually measure those pressures at idle. Didn't know it's better for diagnostic at higher RPM. Few weeks ago I measured at idle when car was sitting on sun midday and when the compressor started working the initial pressures were 50/200.

At idle, the compressor isn't pumping much of anything, and likewise, pressures measured at idle tell us very little. It's best to test the system at an engine speed of about 1,800 RPM, set to Max A/C, blower set to Medium, airflow direction set to Dash Vents, and doors/windows open to maximize the heat load on the system. Let the system run like that for a couple of minutes to stabilize, and then proceed with testing the pressures and vent temperatures.

event3horizon wrote: ↑Sat Jun 22, 2024 10:00 am The compressor has never been replaced per the service records and current mileage is 111K. I did evacuate, vacuum and recharge to weight just about a year ago. At that time, I believe it did cool a bit better. Maybe 5-8F better than what I'm seeing now.

I think you meant recovered, evacuated, and recharged to weight.

Do you recall how much refrigerant and oil were recovered at that time, and does the car have rear A/C?

The specifications I have (front A/C only) are as follows: 33.5 ounces net weight of R-134a, and 6.0 fluid ounces of PAG-46.

event3horizon wrote: ↑Fri Jun 21, 2024 11:06 am Right now the system is only cooling down to 60-65F at the center vents whether it's idling or driving with outside temps of 95F or more. I have an advanced scantool and it's reading the evaporator temps are only 58-60F. The low and high side pressures are 35-40 low and 175-200 psi high. My intuition here is that the high side is kind of low, suggesting the compressor isn't compressing 100 percent, i think. This compressor is also variable displacement with refrigerant valve in it. I have checked the condenser fan and it spins pretty well and blows air through condenser. Condenser does not have debris/dirt in it. Receiver drier has recently been replaced. I added a few more ounces of 134 into it and it did not make a difference in the pressures or outlet temps.
How do I know if this is a compressor issue, expansion valve, or whatever else? I should be getting 40-45F at vents at least! I'll be happy to conduct any tests or diagnostics, just tell me what I should check.

The control valve, governed by the ECM/PCM, should keep the pressures of the variable-displacement compressor in line according to the heat load on the system and other factors. Therefore, you should be seeing a rather steady 30 PSI on the low side and about 225 PSI on the high side at ~95°F, with far lower center-vent temperatures than 60-65°F.

From a distance, and without any hands-on, my guess is threefold: The refrigerant charge is "off," the control valve is malfunctioning, and/or the compressor is a bit "tired."

Thank you for your helpful information.

JohnHere wrote: ↑Sun Jun 23, 2024 8:55 pm Do you recall how much refrigerant and oil were recovered at that time, and does the car have rear A/C?

I do not, sorry. The shop just recovered it and did not tell me how much of anything was pulled. The car does not have rear a/c.

JohnHere wrote: ↑Sun Jun 23, 2024 8:55 pm The specifications I have (front A/C only) are as follows: 33.5 ounces net weight of R-134a, and 6.0 fluid ounces of PAG-46.

Yes that's correct. That's how much I charged the last time. Almost 3 of the 12 oz 134 cans (no additives). I did not add any pag oil.

JohnHere wrote: ↑Sun Jun 23, 2024 8:55 pm The control valve, governed by the ECM/PCM, should keep the pressures of the variable-displacement compressor in line according to the heat load on the system and other factors. Therefore, you should be seeing a rather steady 30 PSI on the low side and about 225 PSI on the high side at ~95°F, with far lower center-vent temperatures than 60-65°F.

From a distance, and without any hands-on, my guess is threefold: The refrigerant charge is "off," the control valve is malfunctioning, and/or the compressor is a bit "tired."

Now that I own my own recovery tank and recovery machine, I'll be removing the refrigerant and re-adding as precisely as I can to 33.5 oz. I can't find any clear evidence that the compressor or the control valve is at fault. I found more electronic data and it shows the compressor torque value (requested or actual, don't know) is 24 newton meters, 70 percent duty cycle (which is maximum from what I read) and 700 mA current.

What I'm going to do next is recover the refrigerant and replace the expansion valve. I spoke with some other Benz owners of this platform (w220) and they've said if I haven't replaced the txv in the last 24 years, it may be time to replace as even a small restriction can affect cooling performance. It's worth it to try even though there aren't any obvious signs of txv problems and it's a cheap part.

I'm going to agree, if the expansion valve and recharge doesn't fix this, the compressor may be 'tired' as you say. My 2015 Audi A5 that uses 134 shows the exact pressures you mention in the same weather conditions (30-35 low, 225 high) and this is at IDLE. The car blows ice cubes. I can never get 225 on this benz at idle and i get 215 hi side at 1500-2K rpm only.

Can you get at the TXV without disassembling the dash? If so, then the job will be much easier.

You can replace only the control valve, but not without first recovering the refrigerant to attain zero system pressure. If you have to remove the compressor anyway to gain access to the CV, you might as well replace the entire compressor given it's age and the mileage factor.

The control valve seems to be the weak link with some variable-displacement compressors. That, plus the wonky pressures and vent temperatures you're seeing, suggest that the CV might be failing.

Diagnosing and repairing MVAC systems is often very tricky, time consuming, and costly, as you've probably noticed.