Aircon causing hiss sound from passenger door – why?

[Paul SL320]

It’s taken me some time to narrow down, but this highly unlikely scenario has recently started on my 1997 SL320 with standard non-Bose hi-fi system.


By setting the Climate control unit to diagnostic mode I can see the temperature of the refrigerant at the aircon evaporator varying between 3°C and 8°C on my car. When the temperature climbs up to 8°C the expansion valve opens up a bit and makes a gently hissing sound for a few seconds. I can see the refrigerant temperature start to drop down 3°C again.



As soon as the expansion valves gentle hiss stops, more times than not, I get a static type crackling sound from the passenger speaker. The crackle lasts about 1 to 2 seconds. The crackle sounds like the type of sound you get from the speaker of your home stereo system if you play around with its speaker wire while the amp is powered up.


If I switch off the aircon the speaker crackle never occurs.
I have disconnected the plug to the amplifier located under the rear seat base. The speaker crackle still occurs.


I have removed the stereo head unit from the car, so that the harness that plugs into the head unit is left open ended. Still the speaker crackle occurs.
I think that somehow the speaker cable is either picking up electromagnetic interference from an electrical actuator on the aircon system or the speaker wire and aircon system wire has become bared in a place where they both touch.



However I’m not really sure what goes on in the aircon system when the evaporator temperature reaches 8°C and the system decides to cool the refrigerant down. Can anyone confirm?


Or possibly an electrical device on the aircon system has a bad earth that somehow is feeding current into the cars electrical system via the earth and manifesting itself as a speaker crackle. Does anyone know where the aircon systems earth is located?


Or, is the expansion valve controlled by vacuum, and its a vacuum leak to the central locking actuator in the door that I am hearing?



Any help or suggestions would be greatly appreciated.


Regards
Paul

 

[SJX]

I would suggest that the evaporator expansion orifice is a fixed type and the cooling control is done by modulating the AC compressor on and off. I bet the electro-magnetic clutch on the compressor is causing the noise. (as it engages at 8 deg. to apply some cooling)

Have a look at the wire to the AC compressor clutch, there is usually an inline spade connector a few inches from the pump body, give it a good clean up and it should be your problem solved (be suprised if it didnt!) if not try some suppression at this point.

All the best Steve

 

[andy27168]

Hi,
What are your electric A/C fans in front of the radiators doing? they are supposed to cut in and out as the refrigerant pressure rises, just wondered if the noise you are hearing through the speaker is one or both of the fan motors trying to start,

 

[Paul SL320]

Andy, Steve,

I've narrowed this down, and I confess made myself look a bit stupid.

With my Dad driving I was able to put my ear against the passenger door speaker. I was wrong, its not coming from the speaker, its deep from within the dash.

This simplifies the determination of the cause I believe. It also puts a different slant on the cause. You both suggested some good viable electrical interference causes, for which I thank you. But i think I'm looking for a pressure type root cause now.

I don't know how the air con system works in anything other than simple terms, if you can give me any more pointers I'd be very grateful.

Regards
Paul

 

[SJX]

Overly noisy evap is usually due to a lower than optimum system charge. Get the system serviced/regassed then assess your situation.

All the best Steve

 

[BTB 500]

I get the odd hiss from inside the dash on my SL as the aircon operates ... it's always done it in the 8 years I've had it. It's had an aircon service at a main dealer without any comment, so I assume it's normal.

 

[SJX]

As my comment was general of automotive AC, it makes good sense to listen to those with the same configuration/model as a reference,

So.......... in light of that:

Turn radio up or drive faster, or get some kids in the back !

S