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hissing A/C

curtis73 on Thu February 12, 2009 1:39 AM User is offlineView users profile

Year: 1999
Make: Mercedes
Model: E300
Engine Size: 3.0TD
Refrigerant Type: 134?
Ambient Temp: 50F
Country of Origin: United States

I'm getting an increasing amount of expansion hiss from the dash. Low on refrigerant?
Does Mercedes use their own refrigerant, or is it just plain 134?

The long story is this: I had problems twice with the fan shroud getting sucked off the radiator which forced the fan blades forward into the radiator tank. I've now repaired the radiator twice and replaced it once, so this time I left the shroud off. If you saw how poorly it was engineered you would have done the same. It did nothing for directing air and just caused problems. Plus there are two 11" electric pushers on the front of the condenser that come on with the A/C or if temps get high... which they never do. Even in city driving with the A/C on the temp gauge never goes above about 90C.

BUT.. the noise started happening about a month after I removed the shroud. Could the small increase in thermal load have caused this issue?

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1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

Chick on Thu February 12, 2009 7:56 AM User is offlineView users profile

First, I would check the fan clutch or water pump if it's a mechanical fan, no way does the fan blades get sucked into the radiator tank unless there is a problem, and a shroud won't do it..If it's an electric fan, then it also needs to be checked.. That said, yes, without proper cooling (the shroud allows this) you could have had a high pressure event causing the pressure relief valve to let refrigerant out.. Or you may just have a leak, either way the first step is to get both high and low pressures along with the outside temp you take them at, we can go from there.. Guessing really doesn't fix anything.. hop this helps..

Oh, no special Mercedes refrigerant....But don't add anything until you know what the system is doing..Pressures first..

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Chick
Email: Chick

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Freedoms just another word for nothing left to lose

Matt L on Fri February 13, 2009 6:17 PM User is offline

Hissing at the center vents of a Mercedes W210 (which you own) is a sure sign of insufficient refrigerant flow through the expansion valve. Low refrigerant is the main cause of this, but a restriction in the system will do it just as surely. Specifically a restriction within the receiver/dryer.

That the auxiliary fans don't start when you use the AC is a huge red flag. They should start after a minute or so if you're stopped and idling with the compressor on. They will start on very low speed.

Perform a fan test first. With the key in position 2 (engine running or not), press and hold both AUTO buttons on the climate-control pushbutton unit for >10 seconds, until the fan starts. It should come to life at full speed with a roar. If not, well, that's probably the trouble. To cancel the fan test, either turn the key to position 0 or press and hold both AUTO buttons again until the fan stops.

If the fan works with manual demand, the next step is to see if you should be getting automatic demand but are not. A plugged dryer will cause this, because the high-side temperature (and pressure, btw) is sampled AFTER the dryer. A restriction that causes partial boiling of refrigerant will cool the sensor and the computer will think that there isn't enough heat to disperse. It will also be fooled into thinking that the refrigerant pressure is within specs, when in reality the head pressure at the compressor is sky-high and could release refrigerant through the pop-off valve.

curtis73 on Fri March 06, 2009 2:37 PM User is offlineView users profile

Quote
Originally posted by: Chick
First, I would check the fan clutch or water pump if it's a mechanical fan, no way does the fan blades get sucked into the radiator tank unless there is a problem, and a shroud won't do it..That said, yes, without proper cooling (the shroud allows this) you could have had a high pressure event causing the pressure relief valve to let refrigerant out.. Or you may just have a leak, either way the first step is to get both high and low pressures along with the outside temp you take them at, we can go from there.. Guessing really doesn't fix anything.. hop this helps..

It does help. I wish I could show you the shroud. The clearance from the plastic mechanical fan blades to the radiator is about 3/4", and the fan is offset way to the passenger side. The right-most 4" of the fan is behind the passenger side tank, not the tubes. The shroud therefore tries to offset the flow (in that 3/4" of travel) and instead its a flimsy piece of plastic that has a 2" gap on the driver's side and only covers about 1/2 of the radiator. What happens is that the shroud (which is held in place with a single metal spring clip) starts pulling away from the tank with the suction of the fan at about 4000 rpms. Once it contacts the blade, the blade forces the shroud back and wedges one blade forward. Since it only has 3/4" to go before it contacts the tank, that's what it does. The fan blade tip punctures the inside of the tank right behind where the tubes are sweated in. Trust me, when it comes to mechanical diagnosis, I know what I'm doing

its happened 3 times now and its a known W210 diesel issue. Its all kosher... that's how Mercedes did it. I fixed the issue by "relocating" the lower radiator mount. I had to grind a little clearance for the intercooler, but I was able to move the radiator forward about another 3/4". The issue is not whether or not the fan is contacting the tank (which is proven by the fan blade that broke off while sticking into the tank like a knife), I mentioned it as a point of information. The shroud itself is a waste of space. It covers half the radiator, has a 2" gap the entire length of the driver's side, and does little or nothing to keep the car cool. The mechanical fan clutch doesn't kick in any more frequently with it gone, and the coolant temperatures have remained the same. But I thought I'd mention it in case it was an issue.

Matt, when I said "that come on with the A/C or if temps get high... which they never do" I didn't really mean they never come on at all... I meant they never have to come on because the coolant is hot. They don't need to supplement the mechanical fan to cool the engine. They come on just fine with the A/C and with the "auto" test.

Quote
Oh, no special Mercedes refrigerant....But don't add anything until you know what the system is doing..Pressures first..

Ah... great. That is why I was waiting to get highs and lows. I didn't want to hook up my 134 gauges and screw something up. All of the stickers on the A/C and the radiator support got blasted off when the engine bay was steam cleaned, so there is nothing telling me what refrigerant was in there.

Ok... I'll get highs and lows and get back to you.



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1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

Edited: Fri March 06, 2009 at 2:44 PM by curtis73

curtis73 on Fri March 06, 2009 2:57 PM User is offlineView users profile

Chick... I made a quick little drawing to show you what's happening.



Notice how the shroud (red) tries to offset the flow. On the right hand side, as RPMs reach about 4000 you can watch the shroud start to get sucked off the tank. Once it touches the fan, its all over. As the blades follow their upward arc and contact the shroud, the plastic blade is forced forward, and it pokes a hole in the inside wall of the tank just behind where the tubes attach.

Also notice how the shroud covers very little of the radiator, and leaves a 2" gap on the driver's side.

-------------------------
1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

Edited: Fri March 06, 2009 at 3:00 PM by curtis73

Matt L on Fri March 06, 2009 4:58 PM User is offline

The auxiliary fan should start if you sit idling with the AC on, unless it is very cold. 50F may be cold enough to inhibit their operation (by keeping the refrigerant pressure and temperature down), but they will run in the summer. The engine fan does not pull enough air.

Oh, also ensure that the belt between the fan blades is intact. If you spin the exposed left hub, the right side blade should turn as well. Even ignoring that you only get half the airflow when you need it, I've heard of that belt getting behind the motor pulley and putting a hole in the condenser.

Ensure that the climate control system knows the actual refrigerant pressure and temperature, as it is responsible for turning on the auxiliary fan. When you measure the high-side pressure with a gauge, also check the sensor value reading. To do this, press and hold the REST button on the pushbutton unit for >5 seconds, until the display changes to show a "1" in the left side. Press the left auto key until the left side reads "7" and the right side will show the refrigerant pressure in bar.

If this pressure is much less than the pressure seen at the service port, you have a restriction in either the condenser or the dryer. Those parts sit between the high-side service port and the pressure transducer. Such a restriction prevents the compressor from shutting off even with extremely high head pressures. It is also a cause of the hiss emanating from the center ducts, as is low refrigerant level. Either causes low refrigerant flow, which is the real problem. Normally when this is the case, there is also a measurable difference in temperature between the two halves of the center outlet.

curtis73 on Sun March 08, 2009 4:02 AM User is offlineView users profile

Quote
Originally posted by: Matt L
Ensure that the climate control system knows the actual refrigerant pressure and temperature, as it is responsible for turning on the auxiliary fan. When you measure the high-side pressure with a gauge, also check the sensor value reading. To do this, press and hold the REST button on the pushbutton unit for >5 seconds, until the display changes to show a "1" in the left side. Press the left auto key until the left side reads "7" and the right side will show the refrigerant pressure in bar.

If this pressure is much less than the pressure seen at the service port, you have a restriction in either the condenser or the dryer. Those parts sit between the high-side service port and the pressure transducer. Such a restriction prevents the compressor from shutting off even with extremely high head pressures. It is also a cause of the hiss emanating from the center ducts, as is low refrigerant level. Either causes low refrigerant flow, which is the real problem. Normally when this is the case, there is also a measurable difference in temperature between the two halves of the center outlet.

You see... I love this forum... but most of all, I love Mercedes. My BMWs were similar. If you know the right combination of things to press, the car basically diagnoses itself.

The electric fans are spinning equally, belt is intact. Matt, you are a lifesaver. I'll get some time hopefully this week to post some hi and low readings, as well as what the display reads.

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1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

Matt L on Sun March 08, 2009 1:22 PM User is offline

Hopefully it helps. I found that my system had a severe restriction. The pushbutton unit read 3 bar, the gauge was hit 300 psi and was climbing when I shut it down.

There's a TSB for a restricted dryer (due to a bad design) in the service manual, but I am pretty sure that it does not cover your MY '99.

I forgot to mention, when in the current-sensor display mode, press the left AUTO key again to get to #8, the refrigerant temperature. It will be in either C or F (I've heard of both from owners) and will be in the same units as readings 1-4 (various temperature sensors in the system which will be at ambient temperature before you start the car).

curtis73 on Mon March 16, 2009 3:37 PM User is offlineView users profile

Ok... sorta updating. Matt maybe you can help.

I haven't done any more diagnosis yet (working two jobs and my A/C gauges are in storage 20 miles away.) but I have an update. Now the display has automatically selected the EC mode (economy... doesn't run the compressor) and you can't turn that mode off (can't select the compressor to be on).

That says to me that the leak has allowed enough refrigerant out that the car has defaulted to economy to prevent damage. Matt, is that a safe assumption? I know that nothing is a definitive diagnosis without putting gauges on it, but I thought that this new clue might point toward the low refrigerant answer.

Matt, do you know where I can find that TSB? I googled and didn't find it. Just wondering if MB has an improved dryer part number. As long as there is no 134 in there, might as well use this time to put a new dryer in it and maybe flush the system.

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1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

Matt L on Mon March 16, 2009 6:00 PM User is offline

I found the TSB in the factory service manual. That manual is available from M-B on DVD for $150 plus tax and shipping.

The EC lamp will indeed illuminate if the high-side pressure is too low before the compressor starts. It will also come on if the high side drops precipitously while running, down to 30 or so PSI.

Another common fault which shuts down the compressor is the evaporator temperature sensor. That is reading #5 on the climate control unit. The failure mode is to show an unrealistically high temperature. It also should be in the same units as readings 1 through 4 (whether that be C or F).

I'm betting on low refrigerant. Bad evaporator temperature sensors don't hiss.

curtis73 on Tue March 17, 2009 2:33 AM User is offlineView users profile

Ok thanks

I gotta get the service manual for this car

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1966 Pontiac Bonneville
1973 Impala Wagon
1987 BMW 325 convertible
1996 Impala SS
1997 Toyota Tercel

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