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ItalianJoe1: Thanks for your knowledge of the variable displacement compressor design!
Apparently I'm going to have to do a lot of updating my knowledge to service these systems, whenever it comes time.
BTW-It sounds like that RCV failure mode is similar in symptoms to a failed compressor.(IE: high side too low + low side too high)
Can you easily change just that valve?
Yes, the valve is removable and much cheaper than a compressor. You will still have to discharge and re-charge the system to replace it, and in some applications it faces the engine when installed so you have to remove the compressor for access, some can be changed in place. It just has a snap ring retaining it, and will pull straight out once that is removed. Some are quite stuck in there from age, but they do come out with a little persuasion. Be sure you source the correct valve for your compressor, last I saw MB does not sell them, so you need to get it from an A/C repair place, but the valve should be around $50, far cheaper than a compressor.
ItalianJoe1: This is great info!
How can someone in OP's situation accurately distinguish between bad compressor -vs- bad RCV? (Ohm reading on the RCV leads perhaps?)
ItalianJoe1: This is great info!
How can someone in OP's situation accurately distinguish between bad compressor -vs- bad RCV? (Ohm reading on the RCV leads perhaps?)
The RCV is driven by a PWM current from the HVAC controller, would be almost impossible to diagnose with a meter without having a similar vehicle to compare to for "known good" values.
The OEM way to diagnose is with a scan tool, watching evap temp versus compressor demand. The system should run high compressor demand until evap temp is below about 5C, almost freezing, then it will back down the compressor current to maintain. The two failure modes, stuck 'off' (current increases but temp does not decrease), or stuck 'on' (current drops but evap cools to 0 or below, very rare), would be readily apparent watching the live data, but would require probing both the evap temp voltage and compressor amperage, and knowing good values, without a proper scanner.
It's more telling to put a simple gauge set on it, if the high side is not increasing and the low side is not decreasing, but the system is charged and the compressor is engaged, the valve is stuck 'off'.
Hey guys, I had a set of gages a friend of mine let me use, but it does not support 134a. I need to get a set of gages that have the 134a display numbers.
Also the only port that I have to hookup to is the low pressure. there is no high pressure port to connect to. But i think that is for a safety reason.
Theoretically pressure reading should be the same even if these guages are not meant for 134a. But, why don't you stop by autozone or O'Reilly, and ask them that you want to loan AC guages?
what are your low pressure reading with AC off and AC on at full blast?
The high pressure port is on the top left corner of radiator (in relation to driver seat)
Theoretically pressure reading should be the same even if these guages are not meant for 134a. But, why don't you stop by autozone or O'Reilly, and ask them that you want to loan AC guages?
what are your low pressure reading with AC off and AC on at full blast?
The high pressure port is on the top left corner of radiator (in relation to driver seat)
Found The High pressure port!! Thanks man..
Here are the pictures. Top picture is with car running and AC Off
You are welcome
It seems like bad compressor to me.
Compressor is definitely engaging as there is mild decrease in low side pressure and mild increase in high side pressure. But compressor doesn't have enough compression (umph) to make a huge difference in low side and high side. Seems like your system does have good amount of freon.
You are welcome
It seems like bad compressor to me.
Compressor is definitely engaging as there is mild decrease in low side pressure and mild increase in high side pressure. But compressor doesn't have enough compression (umph) to make a huge difference in low side and high side. Seems like your system does have good amount of freon.
I am thinking it is bad also. And the Low pressure line never gets cold and sweaty. I hooked up a can of Freon with a cheap pressure gage that it came with. It says I have a full system of Freon. The red arrow is what it sits at when connected to the car with AC running.
Your low is not low enough, high is not high enough, meaning very little compressor activity. Considering there is no change between ac on/off, I'd wager your RCV is stuck in an almost closed position. With the gauges on, give it a couple sharp revs, see if it suddenly starts pumping (low drops to 35ish, high goes to 180ish), this is 100% indicative of a sticking valve. The pressures should be equal when "off", and yours aren't, looks like 70/120, which is a little pumping action but nowhere near enough to cool down the system.
Your low is not low enough, high is not high enough, meaning very little compressor activity. Considering there is no change between ac on/off, I'd wager your RCV is stuck in an almost closed position. With the gauges on, give it a couple sharp revs, see if it suddenly starts pumping (low drops to 35ish, high goes to 180ish), this is 100% indicative of a sticking valve. The pressures should be equal when "off", and yours aren't, looks like 70/120, which is a little pumping action but nowhere near enough to cool down the system.
Thanks for that info.. I will be installing the new Comp/Dryer on Tuesday. Ordered a new set of 134a gauges for the job when it comes time to pull a vacuum and recharge the system. I will let everyone know how it goes afterward. I really appreciate all of the great help from everyone. Its nice when people can come together and help one another!
So is there an AC Relay that I should look at first to make sure the relay is not bad? I just now thought of that. I am at work right now and am google searching for info, but not having allot of luck finding any info on the location of the compressor relay.
Typically, if a clutch relay wasn't working then you'd see pressure equalization (same PSI) across the High & Low sides. (Thats different than what your pics show above)
It sounds like ItalianJoe1has your issue likely zeroed in.
The only thing I'd try before I did all that work is a little test I've been seeing where they check to make sure the RCV is getting a signal (power) to actuate. Because if that isn't happening, then your problem may be upstream of the mechanical parts.
You could do a search on YouTube but I've been seeing videos of people using a small cheap automotive lightbulb plugged into the connector that attaches to the RCV. When they have the AC turned off inside the car, light is off. When they adjust controls to cool and call for AC compressing, the light turns on. I'm sure that test isn't perfect, but it seems many DIY mechanics are using it to help decide if the problem is likely the RCV itself or something electrical.
Typically, if a clutch relay wasn't working then you'd see pressure equalization (same PSI) across the High & Low sides. (Thats different than what your pics show above)
It sounds like ItalianJoe1has your issue likely zeroed in.
The only thing I'd try before I did all that work is a little test I've been seeing where they check to make sure the RCV is getting a signal (power) to actuate. Because if that isn't happening, then your problem may be upstream of the mechanical parts.
You could do a search on YouTube but I've been seeing videos of people using a small cheap automotive lightbulb plugged into the connector that attaches to the RCV. When they have the AC turned off inside the car, light is off. When they adjust controls to cool and call for AC compressing, the light turns on. I'm sure that test isn't perfect, but it seems many DIY mechanics are using it to help decide if the problem is likely the RCV itself or something electrical.
Is the RCV located on the back side of the compressor?
Yes to your earlier question about RCV location. Your pic above looks correct.
Now that you have changed the Compressor (and therefore a new RCV) how's the system working??????
Yes to your earlier question about RCV location. Your pic above looks correct.
Now that you have changed the Compressor (and therefore a new RCV) how's the system working??????
Blowing cold.. Need to double check what the pressures should be reading on the high and low side for that car. with the outside temperatures at 98 to 100. Its been hot here in Texas this last week. It took 3 cans. about 970 grams of Freon.. The low pressure line is finally getting cold, but not sweating. The high pressure line gets hot like it should. It was not doing that before. I think I will let my wife drive it for the rest of the week and then try and adding a little more to see it I can get that low line to sweat. But I dont want to over charge the system either.
I forgot to take a picture of the gages right before I finished up. but the low was at 45lbs & high was at 130lbs. that was using my new 134a gages. Out side was about 98 degrees.
Yes, please do yourself a favor, and don't keep adding more without careful monitoring.
FWIW- I hear many some people say that more "freon" really refrigerant equals colder air. This is most definitely not true at every fill level, and I would bet that many ppl make their situation worse by doing this haphazardly. (I'm guilty of this myself, so no finger pointing...) It gets into a pretty big conversation that is fun for nerds but not for most ppl.
Here's the handy thing: Get that digital meat thermometer, stick it in the center left vent (closest to the steering wheel) and record the temps. It will be handy to know what its getting down to when you are driving around and at idle. My 08 GL450 absolutely freezes us out and we live in Tucson AZ where its commonly 113F outside. I will be happy to give you the details of what temps I achieve if you'd like to use that info as a reference.
At any rate, IF and WHEN you adjust your refrigerant fill level, carefully record the new temperatures (performance) and compare that to where you are now. You will likely see a small amount of refrigerant makes a real difference in system performance. (At least all of this holds true with the conventional fixed displacement compressor systems)
You can add a little, or remove a little until you achieve the lowest temps (ie: best performance) for your system. Then you can be confident you have the charge right. Easy peazy. But a bit time consuming...
Yes, please do yourself a favor, and don't keep adding more without careful monitoring.
FWIW- I hear many some people say that more "freon" really refrigerant equals colder air. This is most definitely not true at every fill level, and I would bet that many ppl make their situation worse by doing this haphazardly. (I'm guilty of this myself, so no finger pointing...) It gets into a pretty big conversation that is fun for nerds but not for most ppl.
Here's the handy thing: Get that digital meat thermometer, stick it in the center left vent (closest to the steering wheel) and record the temps. It will be handy to know what its getting down to when you are driving around and at idle. My 08 GL450 absolutely freezes us out and we live in Tucson AZ where its commonly 113F outside. I will be happy to give you the details of what temps I achieve if you'd like to use that info as a reference.
At any rate, IF and WHEN you adjust your refrigerant fill level, carefully record the new temperatures (performance) and compare that to where you are now. You will likely see a small amount of refrigerant makes a real difference in system performance. (At least all of this holds true with the conventional fixed displacement compressor systems)
You can add a little, or remove a little until you achieve the lowest temps (ie: best performance) for your system. Then you can be confident you have the charge right. Easy peazy. But a bit time consuming...
I will do that. What temps should I be seeing at the vent? And do I do this with the doors open and recycle air off? Or doors closed with recycle on? And should my low pressure line start sweating? Because right now its not sweating. Its cool when u touch it but not COLD. But the high pressure line is hot like it should be.
My Honda Civic and my Jeep, both low pressure lines get cold and sweaty. Maybe the Mercedes AC just works differently?
Yes, should be sweating. I pull the vacuum overnight to ensure it will operate as designed.
I pulled a vacuum on it already, no leaks. I let the vacuum pump run on it for 4 hours, then let it sit for another hour to see if it lost any vacuum. It didn't.
I pulled a vacuum on it already, no leaks. I let the vacuum pump run on it for 4 hours, then let it sit for another hour to see if it lost any vacuum. It didn't.
Usually sweating on the line is the humidity condensing on the cold line. That said, houston area is very humid and it should have sweating on it.
As far as I remember the capacity was 32 Oz (correct me if this is wrong). How much did you put in?
if you put 3 cans of 12oz, it might mean 36 Oz which might have overfilled the system, which in turn will affect the performance.
Last edited by TX07GL450; Jul 28, 2021 at 10:55 PM.
I pulled a vacuum on it already, no leaks. I let the vacuum pump run on it for 4 hours, then let it sit for another hour to see if it lost any vacuum. It didn't.
I did that too and it turned out not to be long enough. At least, not for the pump I have. Pulled the vacuum overnight and it worked as it should. Just sharing a data point.
Hi Slack007, After dinner this evening I'll go outside and do a test drive taking some measurements and pics and I'll give you the details. It should help you decide if you are in the ballpark.
A good vacuum pump for a/c systems will pull almost complete vacuum, 29.9in/hg or lower. With this, 30-40min is sufficient, I often set the machine for an hour just to be safe. It will fully boil off any moisture at that pressure at ambient temp, so the system will be fully dry, which is what you are hoping to accomplish.
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