When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
When I bought my 2003 E320 W211.065 sedan, there was a whine that everyone thought was power steering, until the AC compressor finally melted down. Opened the hood to white hot glowing molten steel where the sprocket had been. Having not burned down the whole gas station, limped home. Compressor was locked so removed and flushed the lines, some black stuff so new condenser/receiver/drier, expansion valve, lines and compressor. Installation went smooth until... Nothing. No compressor. Low side and high side same ambient pressure, no compressor so can't get all the 134 into the system. Enough in there to start it but it's not going. Center of pully is spinning so what could it be? Turns out it's a clutchless compressor, so the center always spins. Pressure is modulated by an electronic control valve mounted in compressor. Control valve signal is frequency based so have to be careful with VOM on supply side. First had to reset the low refrigerant code in the AC module. That allows the compressor to energize, send a signal to the control valve. So the ]AC off] light finally went out, indicating the compressor is energized, but still no compressor. The signal to the control valve goes through driver's side SAM. So a full scan and SAM is throwing a code that makes the recirc button blink (indicates compressor fault).
Reset it and it just comes back. After a day or two of digging, ran across a magazine article deep in mbworld that says some systems require a diode in the AC compressor clutch or control valve. Current flows in to the solenoid to energize the control valve. When the solenoid is de-energized the field collapses and current flows back the other way. In some systems it's enough to arc at the relay contact, reducing the life of the relay. So the diode prevents that back flow. If there's no diode the Mercedes system sees ground/open, kills the control circuit and throws 9028 in the driver's side SAM.
The compressor is grounded to the block and there's a single green wire from the engine harness to the compressor plug. Pulled the plug apart and connected to the tiny prong in the compressor (load) male side of the plug with a small alligator clip. 11.2 ohms to ground. Reversed polarity on the VOM and still 11.2 so this thing has no diode. Unplugged control valve (easy access from below) and took off the c-clip. Five seconds later the control valve shot outta there like a titan missile, with a BANG, fresh PAG oil everywhere. Found the control valve fifteen minutes later on the other side of garage under the shopvac. Tested it and found polarity both ways, no diode.
So now I need the right compressor, or actually just the right control valve. $50 or less online. But since the compressor is a cheap off-brand (VEVOR), can't find a control valve for it. Hours of research trying to figure out which major mfr from China is marketing compressors under Vevor brand. Turns out Denso valves look exactly like my Vevor valve. Also turns out that control valves for mercedes (with diode) have a big blue or yellow dot on the top of the valve. Those without diode have no dot. I was sent a compressor "direct fit for mercedes" with no dot on the control valve. And yeah it "fit" but it would never work without a diode. Maybe that's why it was only $122? So $44 for a control valve and it will be here by Friday. We'll see how that goes.
Doing a little research...so it's the A/C "sprocket" making that "whine" sound? I'm looking at replacing my OEM A/C compressor/drier soon due to a slow leak and it sounds like the sooner the better. 2003 E500, 61k miles.
When I bought my 2003 E320 W211.065 sedan, there was a whine that everyone thought was power steering, until the AC compressor finally melted down. Opened the hood to white hot glowing molten steel where the sprocket had been. Having not burned down the whole gas station, limped home. Compressor was locked so removed and flushed the lines, some black stuff so new condenser/receiver/drier, expansion valve, lines and compressor. Installation went smooth until... Nothing. No compressor. Low side and high side same ambient pressure, no compressor so can't get all the 134 into the system. Enough in there to start it but it's not going. Center of pully is spinning so what could it be? Turns out it's a clutchless compressor, so the center always spins. Pressure is modulated by an electronic control valve mounted in compressor. Control valve signal is frequency based so have to be careful with VOM on supply side. First had to reset the low refrigerant code in the AC module. That allows the compressor to energize, send a signal to the control valve. So the ]AC off] light finally went out, indicating the compressor is energized, but still no compressor. The signal to the control valve goes through driver's side SAM. So a full scan and SAM is throwing a code that makes the recirc button blink (indicates compressor fault).
Reset it and it just comes back. After a day or two of digging, ran across a magazine article deep in mbworld that says some systems require a diode in the AC compressor clutch or control valve. Current flows in to the solenoid to energize the control valve. When the solenoid is de-energized the field collapses and current flows back the other way. In some systems it's enough to arc at the relay contact, reducing the life of the relay. So the diode prevents that back flow. If there's no diode the Mercedes system sees ground/open, kills the control circuit and throws 9028 in the driver's side SAM.
The compressor is grounded to the block and there's a single green wire from the engine harness to the compressor plug. Pulled the plug apart and connected to the tiny prong in the compressor (load) male side of the plug with a small alligator clip. 11.2 ohms to ground. Reversed polarity on the VOM and still 11.2 so this thing has no diode. Unplugged control valve (easy access from below) and took off the c-clip. Five seconds later the control valve shot outta there like a titan missile, with a BANG, fresh PAG oil everywhere. Found the control valve fifteen minutes later on the other side of garage under the shopvac. Tested it and found polarity both ways, no diode.
So now I need the right compressor, or actually just the right control valve. $50 or less online. But since the compressor is a cheap off-brand (VEVOR), can't find a control valve for it. Hours of research trying to figure out which major mfr from China is marketing compressors under Vevor brand. Turns out Denso valves look exactly like my Vevor valve. Also turns out that control valves for mercedes (with diode) have a big blue or yellow dot on the top of the valve. Those without diode have no dot. I was sent a compressor "direct fit for mercedes" with no dot on the control valve. And yeah it "fit" but it would never work without a diode. Maybe that's why it was only $122? So $44 for a control valve and it will be here by Friday. We'll see how that goes.
What was the end of your A/C repair, spare Denso valve was ok?
You can buy electronic diodes anywhere for peanuts... ebay mouser digikey and stick it anywhere in the wiring away from heat ! 😊
The whine that everyone thought was power steering turned out to be the AC compressor. I think the vanes went bad after I had gone to a quick charge AC guy who added some freon, maybe overcharged it a bit, without adding the right oil. When it finally went, I opened the hood to see white hot slag boiling at the center of a melted pulley on a locked-up AC compressor. While waiting for new compressor delivery I ran the car for a couple weeks with a shorter serpentine belt, without the AC.
08-18-2021, 06:50 PM
AC Compressor
