A/C DIAGNOSIS QUESTION - PeachParts Mercedes-Benz Forum

rodm · #1 04-05-2012, 12:31 PM

I have a 1997 E300D....The a/c has always blown cold however it recently stopped blowing cold and only ambient temp. is obtained through the vents. The compressor runs when system is on The clutch turns freely when spun by hand. Within the last year however the system developed a high piercing whistle through the vents. The higher the fan switch the louder the whistle. The ambient pressure on an 82 deg. day reads around 85 psi with engine OFF on the low side . With engine on and fan and a/c on max. the pressure drops to about 73 psi with engine above IDLE. The high side is around 210 psi. Looking at the various charts on the web it shows if LOW side is HIGH and High side is LOW the compressor is probably bad. However all of the charts show HIGH side around 100 psi for this condition none show my situation of 73 psi low side and 210 psi high side any one who can shed some light on this please respond.

THANKS Rod

fdm_alex · #2 04-05-2012, 12:46 PM

It sounds like your pressures are a little low. Often times when they start to drop there will be a buzzing or whistling sound behind the dash due to the a/c line that runs to the evaporator core inside the cabin. I would have the system evacuated to check the charge level and then have it leak checked. DO NOT just put in more refrigerant from the can you get at Wal-Mart or wherever, there is no way to know how much is going in that way and an over-charge will do some serious damage to the system.

rodm · #3 04-05-2012, 07:47 PM

Alex, Thanks for the reply. An indy mechanic who I trust is going to fix the system for me. He advised me to buy a new Expansion Valve and Dryer.... It seems from your response you feel it is best to evacuate then recharge I will ask him why not try that first....By the way what will evacuating the system do if the pressures are already know?

Thanks Rod

fdm_alex · #4 04-06-2012, 05:05 PM

If it's done with a good machine, which I would figure your guy has, an evac will tell you how much came out. That way you will know if it was undercharged. If it was undercharged it's good to put it under vacuum for a while and see if it holds. If not, there is a significant leak. It is normal for these to lose some refrigerant over time, so even if it's low there may not be a major leak to be fixed.

curtmaxwall · #5 04-10-2012, 10:48 AM

It sounds like you have lots of high pressure gas equalizing into the low side. The pressure you shown in both the condition, is quite just controversial but though check the user workshop manual where you can find the exact figure for the ac.

rodm · #6 04-20-2012, 10:05 PM

My mechanic advised it was the thermal expansion valve. So he changed it along with the dryer and cabin air filters. He has recharged the system however he called to say after recharging the system it does NOT WORK. Have not had a chance to go by his shop. He now says its the compressor.......Any advise anyone.??????

Rod

fdm_alex · #7 04-20-2012, 11:41 PM

You didn't show at idle pressures in the first post, I should have asked about them. If the compressor is bad, it usually shows about 100psi on each side. The pressures should never be equal on both sides. I'll look at my cheat sheet tomorrow, I'm just coming out of winter up here in MN and you tend to forget some exact particulars when you only do A/C work 4 months a year...