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sprinkles08, ASE Certified Technician
Category: Mercedes
Satisfied Customers: 21418
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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2004 ML350 a/c blows cool, had the freon levels and line

Customer Question

2004 ML350 a/c blows cool, had the freon levels and line pressures checked all ok. The a/c starts blowing warm when the 2 electric fans shut off. I have replaced the relay and the fans don't come back on. After a while of the car being off and it is restarted the a/c blows cool it has stayed cool for an hour sometimes and 10 minutes the other times. I live in SW Florida and it is hot here in the 90's, but the engine temperature never goes out of acceptable range. Got any ideas?
Submitted: 3 months ago.
Category: Mercedes
Expert:  sprinkles08 replied 3 months ago.

Hello and welcome to JustAnswer!

I'm assuming pressures were checked during the time that the system was operating correctly. If pressures are actually correct when it's blowing warm then that would indicate a blend door issue, which I doubt is the case.

If the cooling fans don't run for a/c operation but the engine doesn't overheat at stops then odds are the fans are running for engine temperature but it doesn't know to turn the fans on for a/c operation. If the fans don't run at slow speeds or stops then high side pressure will climb and the vent temperature will warm up. It relies on signal from the a/c pressure transducer to know when to run the fans for a/c operation, I suspect the transducer signal is inaccurate and that's why the cooling fans aren't turning on.

Customer: replied 3 months ago.
Can you elaborate on what needs to be checked, tested or replaced, the generic answer is not what I expected for a specific question for a specific make and model car.
Expert:  sprinkles08 replied 3 months ago.

As I mentioned it appears that the a/c transducer signal is not accurate. You need to compare the a/c transducer pressure on a scan tool against a manifold gauge set to start.

Customer: replied 3 months ago.
if the readings are not the same what then, I would like to know a bit more about the troubleshooting and what the component(s) that would need to be addressed.
Expert:  sprinkles08 replied 3 months ago.

If they are not the same then the reference voltage, sensor ground and pullup voltage at the transducer will need to be tested and if ok then the transducer will need to be replaced. If you aren't setting a fault code for the transducer's circuit then there isn't much chance of a wiring issue.