Quadcammer

So heres the scenario:

engine warm or cold
hvac on
a/c on or off
outside temp makes no difference
internal temp makes no difference
blower speed makes no difference

The problem:

At very light throttle openings or closed throttle decel, between 900 and 1200rpm my 2005 S500 4matic makes a bit of a groaning/grinding noise. It is not terribly loud, but can be heard plain as day with the radio off. It can also be felt through the floor board if your left foot is flat on the floor. It can be felt through the gas pedal, but not as easily. If you turn off the hvac unit entirely, the noise goes away completely and does not return.

The HVAC control unit was recently replaced, with no change in the noise, and HVAC function otherwise is perfect.

At any other rpms or under heavier throttle, the noise is not apparently and can not be felt.

So, considering this is related to engine rpm, I figured it may be related to the compressor (or compressor clutch). The dealer figured it may be a slightly noisey a/c purge valve, but I don't understand how that would create such a noise.

Do any MB techs have any ideas about this noise?

I've looked at EPC, and it doesn't look like anything HVAC related runs under the drivers side floorboard.

Thanks for any help.

blas0827

yeap you need the compressor clutch the bearing is bad i do theses alot

stonedjoseph

yeah blas0827 was right i do this often too..

Quadcammer

glad to hear it.

DIY-able?

Anybody have a part number for the clutch itself. All I can find on EPC is the compressor itself.

blas0827

i work today so ill find out

Quadcammer

thank you sir.

zz4x4

Did you solve the issue? Sounds like the radiator fan to me. I had a similar issue. The fan has a variable speed and comes on when idling ( or engine hot) and when AC is on. The noise may go away at speeds as the fan turns down / off depending on temps.
In reference to the AC compressor clutch possibility, note the compressor is connected to the engine and is isolated from the chassis by engine mounts so I would not expect to feel AC clutch issues through the chassis. The fan shroud is connected directly to the radiator / chassis so you will feel and hear it going out. Hope it helps.

Quadcammer

the dealer claims they messed around with some hoses and tightened up some bolts/nuts whatever and i haven't really heard it since.

While you have a point in saying that the engine mounts should lessen the feel, I can't imagine that an electric fan would have this type of effect and turn on/off at exactly the same rpms on both accel and decel.

zz4x4

Glad it has been solved.

In reference to the radiator fan, the ECU seems to have an operating map for temp vs. car speed and whether or not the AC compressor is on or off. Add to that the variable speed the fan can run when it is on. I had similar occurences of hearing the noise at idle in warm weather , not so much in the cold, and not when accelerating or cruising at higher speeds. All related to the fan operation. Just something to keep in mind if case the problem returns.

Quadcammer

well, as the weather has warmed up a touch, the noise is back.

I can't believe this is the compressor clutch. If the bearing was bad, then all would be well with the a/c on, but when you hit the all off button, it should make the noise, which it doesn't. It just doesn't make sense. Again, as soon as you hit the "all off" button, the noise disappears immediately.

I may have to look into the fan, but I have my doubts as well.

I guess I'll have to get my mechanics stethoscope out and have someone rev the engine for me while I can listen.

I wonder if the noise could be transmitted by the hvac blower motor.

Looking at the HVAC units for moving parts, there really isn't too much.

hvac blower
a/c compressor
possibly cooling fan

Thats about it.

Quadcammer

ok, so things are still weird with this noise, but I found some time to look into it.

It is not related to the electric cooling fan.

Now, Im not terribly familiar with MB A/C compressors, but on my mustang, when you turn off the a/c, the clutch disengages the pulley, turning it into an idler. This is very obvious externally, as two protrusions stop spinning.

However, with the 220, the pulley continues to spin regardless of whether the A/C is on, off, or the entire Climate control is off. Now Im not sure if it is impossible to tell if the compressor is disengaging externally, but from what I can tell, this thing spins all the time, which is not good for power, gas mileage, or longevity.

However, that sort of eliminates the compressor if the noise goes away when you turn the Climate control off, since there is no difference in the compressor (at least externally).

Even more confused now as I though I had it pinned down.

Oxygen

Mine does the same thing, same RPM, but I can't physically feel it I don't think... maybe just barely.

If you're from NJ maybe we can meet somewhere and two people thinking is better than one.

You need to find out if the compressor setup is the same on your car and mine.

Quadcammer

According to EPC they are different:

Mine: 0002309111
Yours: 0012300111




That said, operation should be identical. I am very sensitive to noise/vibration, so I can definitely feel mine, and it is repeatable over 35 degrees.

Apparently below 35 degrees the compressor does not turn on.

I'm going to try to continue to narrow down the problem, but trying to figure it out together would be useful.

Let me know if you have any ideas.

it sure is annoying.

whoover

Your car has a variable displacement compressor. Rather than a magnetic clutch, the compressor receives a control signal (PWM or pulse-width modulation) that varies the stroke volume from 2% to 100% of compressor capacity. This setup is much better than the old "on or off" compressor of your Ford.

An on-board computer combines data from the following sensors:

• temperature setting
• in-car temperature sensors
• cabin humidity
• sun intensity sensor
• outside temperature
• evaporator temperature
• engine coolant temperature

and determines the level of cooling (compressor displacement) to dial in. You don't get uncomfortably cold air and you don't have to heat overcooled air (which is what the premium versions of the old system actually did) to prevent it. When you are using A/C (which is in humid weather for defogging as well as hot weather), dialing in exactly the amount of cooling you need uses much less energy than 100% on / 0% on, plus gives more even cooling and is much easier on the system than cycling the compressor on or off. Finally, having the compressor run at 2% displacement, rather than "off," keeps the seals lubricated and prolongs compressor life, and uses very little energy. Variable-displacement A/Cs are a huge improvement.

Quadcammer

well call me a tard and slap me silly.

Very good information sir.

I'm going to need a few minutes (and a few guinnesses) to figure out how that plays into my dilemma.

Also of note, I ran A/C diagnostics and there are no strange numbers or error messages.

Whoover, you seem to be an educated individual, have you any thoughts on this?


Thanks for the help guys.

Quadcammer

I guess this brings me to a question:

What exactly changes when you shut the climate control unit off:

1. blower motor shuts off
2. compressor runs at 2%
3. ???????

The difference from this to switching just the a/c off is:

1. blower still on
2. ??????


Basically, this would point to the blower motor/regulator, but strangely enough, the noise doesn't change regardless of what the fan speed is.

Sometimes I wish these things were a bit simpler...

whoover

By "a/c off," do you mean pressing the "0" button?

Quadcammer

no.

I consider the CC to have 3 statuses:

CC on, A/C on (light on the A/C off button is not lit)
CC on, A/C off (light on the A/C off button is lit)
CC off (done by pressing the "0" button)

Noise occurs in status 1 and 2.

You can be sitting there, at 1100rpm, hearing the noise, and then push the "0" button, and the noise disappears.

Quadcammer

I'd also care to point out that my engine and tranny mounts do not appear to be defective. The engine/drivetrain are very smooth from idle to redline.

Also, battery voltage is perfect at 14.2 volts.

Could it be something like a bad engine ground that somehow transfers noise to the blower motor?

whoover

It could be the compressor, or its drive pulley (which incorporates rubber cushions between the driving and driven elements). I would guess that the compressor is totally disengaged in "0" mode. Can you make the noise appear while stationary? Perhaps by having the car in gear with the brakes held firmly and bringing the revs up to 1100 or so? Obviously don't go much more than that but the motor mounts can absorb that much output.

If you can create the noise without moving, you can pinpoint it with a length of hose. Hold one end to your ear and move the other to suspect components. Have the in-car assistant press the "0" button on your command. Just make sure you can trust the assistant to keep a firm foot on the brakes.

Quadcammer

Yes, the noise can be recreated while stationary (park is magnificent for this).

Just to confirm, on setting "0", the pulley will still spin, the compressor piston just won't run, correct?

The drive pulley can not be purchased separately, which sucks. Basically, I'd DIY the compressor just to see if that was the problem, as its not that pricey if you get the parts online. But when you take dealer prices for parts, and add in labor, it gets expensive for diagnosis purposes.

Do you happen to know anything about the expansion valve, i.e. when it is used, what it does, what sounds it makes, etc?

thanks.

whoover

No idea what kind of sound it makes when it fails, but it controls the flow of refrigerant to the evaporator, based on evap outflow temperature and pressure. Think of it as the analog of the thermostat in your cooling system. It is in the low pressure refrigerant circuit, so will be passing refrigerant whenever the compressor is on. However, it has no moving parts (other than a diaphragm) so I don't see how it could be your culprit. It's also far removed from the compressor (in the rear left corner of the engine compartment, near the driver), so a hose and an assistant should help you eliminate it or the compressor.

whoover

BTW, lots of good information here:

http://www.mercedestechstore.com/pdf...2010-30-02.pdf

Quadcammer

That link is AWESOME!

Thanks a lot Sir.

Quadcammer

Hmm, after looking at that link, Im wondering if it could be associated with the rear a/c unit.

Oxygen, do you have rear a/c?