geniushanbiao

Dear all,

I recently bought a used 2012 ML350 Bluetec with 88k miles and I'm experiencing a weird intermittent A/C problem. Sometimes when I start the vehicle the A/C only blows warm air. I checked the refrigerant charge it looks fine and there's no leak in the system. The recent time it was doing this, I hooked up my Xentry and read the AC module status. There's no error code. The compressor magnetic clutch shows closed, drawing about 800-900mA, but the evaporator temperature is showing close to 30C. Then I used the Xentry program to cycle the magnetic clutch once and then the evaporator temp dropped rapidly to about 4C and it started blowing ice cold air. At the same time I noticed that the current draw from the compressor magnetic clutch dropped to about 500mA. So every time it happens, I simply press the A/C button to turn the compressor off, wait a minute or so and then switch it back on it would get back to work. Sometimes I have to cycle it 2 or 3 times but most of time 1 cycle would make it work.

I'm thinking there are a few possibilities. First the clutch is on its way out and it needs a new compressor. Second the battery is kind of weak, since this only happens when I start the engine(cranking takes amps). The battery is the original battery from 2012 so it's 7 years old but doesn't show any sign of weakness. Third the control module is bad(this is not very likely to me though).

Any thoughts?

geniushanbiao

Update: I tested with this habit. Every time before I stop the engine I turn the A/C compressor off(not the whole A/C unit but only the compressor). Then next time after starting the engine I wait a few seconds before switching the compressor back on. So far I did about 10 stop-start cycles all work well. Looks to me like the low voltage during cranking caused the issue and if I switch it on when voltage is stable it works fine, which points to a possible weak battery. I still don't get why the compressor clutch is closed but not cooling.

peter2772000

If you're seeing 8-900 mA when not cooling, and 500 mA when it is cooling, my money's on the clutch. A solenoid pulls more power before actually actuating whatever it's trying to actuate.

geniushanbiao

I think I solved the issue. It appears to be the compressor clutch. MB implemented the new type of compressor from 2012. This compressor has both variable displacement and conventional electromagnetic clutch equipped. I guess they put the clutch back on because 1 when the compressor is seized we can still disengage the clutch and drive the car without A/C, while without the clutch a seized compressor will lead to a tow and repair immediately, 2 to bump the gas mileage a little bit.

Anyway, I looked at the hub when A/C is not working and appears the clutch is not engaging. The coil is drawing current and I can see the hub move back/forth so I was first suspecting the air gap is too large. However the feeler gauge measurement didn't agree. So I went out to source the parts. Unfortunately MB doesn't list the EM clutch as a replaceable part, nor does any manufacturer make the EM clutch for this vehicle(If anyone knows where to find a matching one, please let me know). So I went on researching and found that this compressor is a Denso 6SBU16C and it's also used on Lexus GS350 and Toyota Camry, while on the GS350 it has almost the same setup than the MB vehicles, only except that the connector is different. So I went ahead bought a EM clutch for the GS350(it's hard to find as well but I found a "refurbished" EM clutch on ebay, hopefully it's not a crappy one).

Took the old clutch hub and pulley off today and compared them to the one I bought. They are not 100% the same, but I'm pretty sure they are interchangeable. The bearings are the same, pulley diameter are the same, also 7 grooves and when installed they line up with the belt line very well. The difference is that the pulley on my MB is a little thicker than the Lexus one, but I installed the Lexus one without any issue. I didn't swap the coil since the resistance measurement showed the Lexus one only has 4Ohm, so it's going to exceed the 1000mA current limit of the MB. So I just swapped the pulley and hub. It's essentially a half an hour job. There's a snap ring but it only needs a screwdriver and a needle nose plier to pop out and put back on. I had to put both the 2 shims that came with the clutch on to make a proper air gap. Tested a few minutes so far so good. I will drive it tomorrow see how it works.

For the old part that came off, the clutch is definitely worn. I was surprised to find that there's a band of stone just about 1/4" wide on the pulley that grips the hub when engaged, while all other surface are metal. The old clutch lost the grip completely. I hold the hub and pulley together and applied as much pressure as possible with my hand but I can still turn the hub with the pulley standing, while on the new clutch I applied a little force and can't turn it at all.

Hopefully this will work. If anyone interested or have similar issue, let me know I can post some pictures.

geniushanbiao

Oops I guess I was too optimistic. The problem came back. Really have no clue now and I'm suspecting problem with the control unit. I've read somewhere that some ML/GL owners experienced delayed A/C start upon starting the engine and that looks exactly what I'm facing. They got it fixed with a software update though. I did some research and found that the current control unit (1669001804) has been replaced with the newest one 1669008821 which is for the newest GLE models and it's also the replacement of many other part numbers that spreads out the W166 platform. Searched on ebay and found a used one for a 2018 GLE with part number 1669003317 according to MB it's interchangeable so I'm planning to give it a try since it's only $80... Fingers crossed~

peter2772000

Good luck and report back

geniushanbiao

Actually the problem is much simpler than I thought. Yesterday I tried swapping the controller and hooked up my Xentry and performed a "compressor break-in" and then the A/C almost completely quitted working. Swapped back to the original control module but didn't help. So today I evacuated the system and recharged it with correct amount of R-134a(actually the evacuation shows the charge was OK). Now when the A/C is stopped both high and low side show about 90psi with high side slightly higher. When I turn the max A/C on the high side shows about 150psi up and down and the low side shows about 70psi or so. So my diagnosis is that either the compressor is weak or the expansion valve is stuck open. I rev the engine up and held at ~3000rpm for like 20 seconds and saw the high side went up to about 170psi or so and the air blows a little cool so my guess is that the compressor is still good only the expansion valve is stuck open. I ordered the expansion valve and receiver/dryer but am wondering if I should get the compressor replaced as well? I did a quick search and found that this compressor is obsolete and MB is not selling it any more. Most of the compressors I can find are either used or remanufactured. Any idea?


This hypothesis can also explain the intermittent issue I had before. The expansion valve was on its last leg and was sticky somehow so sometimes it works sometimes it doesn't. Now it's completely stuck...

sfestiva

I am having similar issue.
This seems a common problem with Denso compressors. The compressor is variable flow controlled by solenoid valve. If the request for compressor to engage but you are getting warm air, then probably the solenoid valve is not engaging.
I have tried to rev the engine and all of sudden the compressor kicks in. So instead of replacing the ac compressor, I am thinking just replace the solenoid valve - common problem on denso compressors used in vw, toyota and now mercedes.

geniushanbiao

So I finally solved the issue and it's been a few weeks since then and the A/C has been working very stable. The issue was simply an insufficient charge of R-134a. This type of variable displacement compressor is very sensitive about refrigerant charge because the solenoid valve need refrigerant pressure in the compressor to change the piston stroke. If you lose 10% of refrigerant charge the compressor can give up, although the pressure on gauge seems perfectly fine. I did a very thorough leak test and found that the low pressure port has a very tiny leak so I decided to change the valve stems and do a complete evacuation/refill. The evacuation showed there was less than 900g refrigerant left in the system, while the spec says 1050+/- 50. I recharged the system with about 1070g refrigerant with 2oz PAG-46 oil using a scale. Then the problem was gone. Maybe you can give it a try. Make sure to charge the systme using scale not by estimating or pressure. The pressure in the high and low port stayed pretty consistent after I charged in about 600g refrigerant.

peter2772000

Incredible! I think that the MB engineers stay up nights trying to make MB products more complicated.

geniushanbiao

I agree with your statement but this specific matter has nothing to do with MB engineers actually. The compressor is manufacturered by DENSO and it's used on various makes and models including Toyota Camry, Lexus GS350, VW, MB, etc. The added complexity of the variable displacement compressor was all driven by the gov orders to increase gas mileage so manufacturers have to exhaust their ideas to achieve the goal otherwise they wouldn't be allowed to sell the cars on the market.

peter2772000

Good point!

juanmor40

Thank you all in advance,

@geniushanbiao wonder if you could summarize your fix?:
1 - Changed clutch hub
2 - Changed TXV, and receiver/dryer
3 - Changed the Schrader valves at the ports

I ask because I am having a similar problem with my sister's 2012 ML350 4MATIC GAS which is a kind of cooling (54F at the central vent) when set at MAXCOOL, and 1500 RPM, and I am trying to have a plan where I only need to evacuate/recharge only once (none if possible of course).

Besides measuring temperatures at the central vent, I connected XEntry, and the pressure gauges, and here are my observations:
1 - Static pressure is the same before starting the vehicle @85+psi @70F ( a bit higher than I would have expected @70psi)
2 - Dynamic pressure jumps towards 60/150 psi on the low/high ports once engine+AC are started at MAXCOOL+1500RPM
3 - XEntry report clutch CLOSED, and compressor usage @850/900, not 1000
4 - After a few minutes, High pressure drops to @120psi for a while, climbs back to 150psi for a moment, and a period of the same continue if patient enough to wait.
5 - With XEntry I actuated the radiator fan up 80% (sounds like an airplane), reviewed pressure gauges, and barely notice any differences
6 - If the AC is disconnected, the pressure gauges jump toward the original static pressure with an initial difference of 15 psi, i.e. 70/95 psi. Similar if I stop the engine, and if wait about 10 min 80/90 psi, and then equalizes if wait a bit longer.7 - Like you, I have also noticed a little noise on the low-pressure side once I disconnect the hose.Based on the "low side = Higher than NORMAL" and "high side = About NORMAL and definitely not higher", I was debating between the TXV being stuck open or the Solenoid/ECV (electronic control valve) not working properly. Step 6 above has me thinking about the ECV (which I have not been able to source yet).

On the Solenoid/ECV topic
====================
Has anyone been able to source the ECV for these DENSO compressors? There are a few variants that MB uses, but I wonder what are the ECV differences between them.
I am aware of the DIode/Non-Diode ones, and wonder if I can use a Diode one for an original Non-Diode application (the reverse is definitely not possible since it can trigger errors)
Also, how different can the ECV can be for the compressor models with clutch, or clutchless? For example, it is a lot easier to find ECV for 7SEU17C/6SEU16C than the 6SBU16C. In theory, the difference between an xSEUyyy and xSBUyyy is ONLY the clutch.

geniushanbiao

Happy to give my comments. The only thing I replaced was the valve stems of both low and high ports (which are not those ordinary A/C valve stems so I needed to order them). I did not bother to touch the compressor although it seemed quite like a solenoid problem. The only thing I did very careful was that when recharging I measured the charge weight very carefully using a small scale (not the type of large one A/C specialists usually use) because I was using the 340g small cans to do it.

The compressor on my vehicle have cluch, but those clutches seldom wear out due to that the compressor is variable stroke. Traditional compressors kick in with full load that causes clutch to wear out, but this one kick in with no load.

I think before you touch the compressor the first thing I would recommend you to do is to find a shop or machine that can measure the capture weight, evacuate the whole system and measure the weight of refrigerant you captured, and compare to the specs (there is a sticker under the hood). If it's outside the spec, just recharge with the correct amount and give it a try. If the refrigerant is within spec and it's still not working properly, you can dive deeper to find the actual cause. There are 2 things that you need to keep in mind: 1. If you ever open the system to service any parts (except for just a evacuation/recharge), get the receiver/dryer replaced. 2. If the compressor needs replacement, get the dryer, condenser also replaced as well as give the evaporator and hoses a VERY GOOD flush, otherwise you will ruin the new compressor in a short time.

Regarding your questions:
1 - Static pressure does not matter at all.
2 - Dynamic pressure also does not matter. The variable stroke compressor makes the system adjustible.
3 - With Xentry saying clutch closed, you can take a look at front see if the compresssor is turning. If it's turning, then clutch is good.
4 - Computer is probably adjusting the compressor load. You can take a look at the load on Xentry.
5 - The fan helps dissipate heat and does not affect A/C system very much.
6 - The TXV should work this way. It's designed with a "bleeder valve" that equalizes pressure slowly after stopped.
7 - Again whenever there's problem in the A/C system, first make sure you have the CORRECT charge, not too much and not too low. The variable system is VERY SENSITIVE to refrigerant charge.

juanmor40

@geniushanbiao Thank you very much for the prompt and detailed response. Forgot to add : 4 - carefully charge to the correct weight.

Could you please point me to where you bought the Schader valves? I may need to buy a couple of them since I care for 3 vehicles and 2 already with "leaking noise" on the low-pressure port.

Which R134a brand did you get? Is anything specific (besides the no-additive ones)?

The 6SBU16C is a magnetic clutched compressor (BU designation by Denso), and it turns when engaged.

On the advice about replacing the receiver/dryer, do you think is a mandatory replacement even if just replacing the Schrader valve? Otherwise, I will buy the tool to "replace in place", so I can do it w/o opening the system.

My plan is morphing into:
1 - Evacuate, and determine the amount left in the system as you suggested
2 - Replace the ports valve immediately, either opening the system or using the "in place" tool.
3 - If I can get away with not replacing the dryer, I would swap ECV immediately, i.e. old out, new in w/o even checking. Assuming I can source the correct ECV ( Leaning towards ).
4 - Vacuum it (and leak test it for a few hours)
5 - Charge with a scale to a specified amount 1050 +/- 10gr

juanmor40

Update
The system was evacuated already, missing @25% of the required refrigerant, @820g inside --> It must have a leak somewhere. I pulled the vacuum for 1hr, it is @-29in Hg, and I am waiting now to see if it holds it.

@geniushanbiao I wonder if you recall where you got your stems valve? MB dealership wants to sell the line instead , $139

However, I tried to take the valve core (stem) out, and I was afraid of breaking something. Are those valves torque down that much? Were they easy to take out for you? Start wondering if a previous mechanic just tightened them hard to prevent the leak.

geniushanbiao

I got them from Advanceautoparts for about $20. You can take a look if any store near you has stock. Here's the product link.
https://shop.advanceautoparts.com/p/...779/18893430-P

Amazon also has tons of those for a very low cost. I'm putting one here but you can easily find others.

Make sure you have the correct tools for those Schrader valve cores. They are different from the small valve cores you can find in other cars.Doing it without the proper tool can easily damage the thread inside the valve and then you will run into a bigger problem.

For others:
Yes the 6SBU16C does have magnetic clutch, but it is also a variable stroke. It's strange though but it is designed to work like that. I was surprised at the beginning but latter confirmed it.
The R134a was a generic brand bought long time ago. I bought a case of 340g cans and am happy about them. I don't think the brand matter,s but one thing to note is that those fancy products from parts stores claim they have oil in it, I would avoid them. The WIS calls for adding 2oz oil (total amount in the system is 8oz) whenever system is opened and you need to make sure you GET THE CORRECT OIL! Failure to use the correct oil can get the compressor damaged easily. I would suggest that only buy plain R-134a (with no additive) and manually add the correct type and amount of oil when you charge the system.
Receiver/dryer does not need to be replaced if you are only replacing the schrader valve. But keep in mind don't leave the system open (with the valve core removed) for too long -- that will ruin the receiver/dryer. Receiver/dryer has to be replaced if you are replacing any of the compressor, condenser, evaporator and the hoses.

Another thing you can try is to get a UV flashlight and glasses and try to pinpoint the leakage. I heard the factory refrigerant is charged with UV dye and you can find the leak easily by a UV flashlight and glasses. They are quite cheap.

juanmor40

@geniushanbiao Thank you. None of the 5 nearest AdvanceAutoParts carries it in stock, neither AutoZones . At least found it in a NAPA AutoParts close by.

It took a bit of care, but the old valve finally came out. I had to add leverage to the tool (pliers), and carefully loosen/tighten it and it came out. It was stuck, definitely way above 5 inch-lb recommended torque for AC valves.

After an hour, the system held a vacuum w/o dropping not even a mark in the gauges. So I assume the leak is through the valves (likely the low side I heard noises from the other day).

geniushanbiao

Yeah the Shrader valve is a common leak spot in the system. You might have exactly the same problem as I had back 4 years ago. Even the recovered refrigerant amount is quite close. I agree with you that the extra tight valve core likely resulted from a technician overtightening it due to leakage, given the fact that this type of valve core is not very common and a lot of small shops don't have it in stock. So instead of ordering a new one and have to wait a few days, the technician simply overtighten it and check it out to make the customer happy.

juanmor40

Update:
1 - Replaced valve cores for both low and high side, using M8 (low side), and M10 (high side).


I found them in Amazon; however, since I wanted to finish the job yesterday I bought them locally. They were in stock in

• NAPA

• AdvanceAutoParts

• Amazon

Replaced the previously stuck valve cores, pulled vacuum for an hour, and let sit overnight: same pressure in the morning --> No leak.

Since I had already decided to switch/bought the ECV due to the 60->70 low side, and 130->150psi high side fluctuations. I used the following ECV for the 6SBU16C Denso compressor




I had some exchanges with RKX-Tech, and they recommended their RCV-019

https://www.rkxtech.com/collections/...32043986026565 or

in Amazon:
I went ahead and replace it as well. A bit of a pain since I had to unbolt the compressor from the crankcase, and replace it in place w/o disconnecting the hoses.

Unfortunately, there was some kind of oily substance within the harness to the compressor connector, see pictures below, and cleaned using an electronic cleaner. I could not figure out where that oil came from. The compressor was clean all around. Used electronic cleaner to clean the connection to the ECV as well.




Took the vehicle for recharging to a total of 1050g = 2.312 lbs. Requested 2.32 lbs = 1053.28 g.

Once filled to specs, started the engine and AC @MAXCOOL and central vent temperature @42F, evaporator temperature 41F, pressure high side @150psi, max @155psi during testing. Used Xentry, and monitored several temperatures, all within expectations. Activated ECV at different %load and observed how cooling changed.

Summary: the system seems to be cooling great. Now I have to wait to see if the new valve do not leak, and the 3rd party ECV holds for a few more years.

Next: repeat effort on the W211's AC (separate thread)

geniushanbiao

Great job man! Your accomplishment is a good example showing that the job can be done great as long as the tech pays attention to and takes care of all the fine details of the procedure.