W140 M104 Hessing Noise
 

I have a 92 W140 with the stright six M104 engine, a 300SE.

When the car is cold, I fire it up with the air-con "ON", it will make some hessing noise for about two minutes. It sounds like sliping belt or something.

When the car is cold, I fire it up with the air-con "OFF", it will not make the sound at all.

When the car is warm/hot, its normal with no special noise. This also applies to starting from a hot engine with air-con "ON".

So guys, what is it? The compressor about to go? or something less expensive? Should I be worried? Should I get it looked at?

Any advice will be appreciated. Actually other than the above, everything is working fine, well at least to my understanding.

Thanks

guys??? any help?

When you start the car when cold, an air pump will run for a minute or two. They make a lot of noise when they get old, but really don't require replacement unless they stop working entirely.

Thanks for the reply. Just wondering, if there are anything I can do to make sure its the air pump and not anything else? Also what happen if this pump fail? anything drastic?

Thanks for the advice.

cheers

Nothing drastic happens when the air pump fails. It is designed to come on far a minute or two after a cold start to reduce cold start emmissions. It is fairly obvious and definitive noise that comes on and goes off when its time is up. It will not come on if the engine is started warm or hot.

I don't think it's the air pump, as the presence of the noise varies with a/c activation (or not). Besides, air pumps make more of a whine than a hiss.

I'm guessing it's the expansion valve, and that he's low on refrigerant. Expansion valves generally make some noise, more when the system is undercharged. It will hiss more or less with changing engine RPM. After a minute or two the evap will have settled down at its minimum temperature, the expansion valve will cut the refrigerant flow, and the noise ceases.

- JimY

thanks guys... i am more worried now as I read! low on refrigerant could mean a leaking system and may mean $$$$ oh well...

so a trip to the tech you rackon?

One more note, if I press the "EC" button to "on" during the first XX seconds of the car start-up, i.e. during the noisy period, the noise immediately cuts out. If I turn "off" the EC immediately after that, the noise kicks back in.

What do u guys think??

and sorry to ask but how can I test with a BIG screw driver?

Looks like the noise is going away with activation of the A/C compressor then. I agree that if this is the case, that it is not the air pump that you are hearing.