Mercedes-Benz Forum banner

1 - 10 of 10 Posts

·
Registered
2007 ML350
Joined
·
56 Posts
Discussion Starter #1
I have a 2007 ML350 bought new in Jan '07. It has been virtually trouble free. Only repair so far other than tires, wiper blades, bulbs and brakes was a small leak in the PS resevoir at 62,000 miles.

Now the A/C is giving me trouble. After about one hour of steady driving with the unit on "auto" and both dials set at 74, outside temp in the 90's, there is a noticeable reduction in air flow. In another 15 minutes or so there is insufficient flow to cool the car. Switching to the highest fan speed helps for a while. Eventually the air flow is to almost zero , all vents. I can hear the fan working but almost no air is coming out. The air that is coming out is very cold. Shutting the system down for 10 - 15 minutes restores nearly normal operation.

All of the above behavior has been consistent on 5 or 6 separate occasions.
Day to day, driving for less than an hour before stopping there is no detectable problem - A/C air flow is good and air temp is fine.

I have my own idea of what is happening but I don't know for sure and I don't know what would cause it.
Any one else experience this? Any idea what the problem might be?

BillM
 

·
Registered
His 2013 RAM Cummins Turbo Diesel Laramie; Hers 2007 ML320 CDI P3; Kids 2010 "4Matic" Impreza
Joined
·
334 Posts
Try it at 72F. See if any improvement.
I wonder if the system is "dumping" the cool air like it does with the hot air when you first start up?
 

·
Registered
2008 ML320CDI Iridium Silver/Black, NAV, P2 pkg, Sirius, Harmon Kardon. Active Bi-Xenon
Joined
·
478 Posts
The evaporator is freezing up. There are a few causes that come to mind, plugged cabin air filters (should be changed every 2 yr or 20k miles), low refrigerant, bad evap temp sensor. My 06 would do this on occasion. I would replace the filters first, its most likely the culprit.
 

·
Registered
2007 ML350
Joined
·
56 Posts
Discussion Starter #4
Thanks for the replies.
I quess I left out some info that might help. As the interior starts to get uncomfortable we lower the temp settings from 74 to 72 then to 70. After a while it becomes obvious this does not help. That's when I switch the fan speed to max. That takes the system off auto.

To me it acts like the evaporator is freezing up. The air is very cold but the flow is "choked off" in spite of the blower working hard. After the first 2 or 3 times this happened I changed the cabin filters. They were very dirty. However the problem has persisted. It has happened twice since the filter change.

I had it to the dealer 2 months ago but it appears they misunderstood the problem. This was on the paper work the tech got from the advisor:
"Cust states that on a long trip 2hrs into trip air gets warm, shut unit off & turn back on after 45 1 hr it get cold again"

Tech reported:
"Ran short test and found no codes.Test drove vehicle for approx 25 miles with A/C on max cooling and it never stopped cooling. Checked actual values in A/C and found everything reading to be ok. Checked and found newer software for A/C control unit. Programmed A/C control unit. This may or may not remedy the customer's complaint. Customer will need to monitor to see if A/C stops cooling. Further diagnosis will be needed if problem persists."

So it's back to the dealer with a much better description, in writing this time, of the problem. I'll report what they find.
BillM
 

·
Registered
2019 X350d, 2017 GLE500e, 2012 ML350BT (sold), 2013 A200 Exclusive(sold), 2009 ML350CDI(sold)
Joined
·
250 Posts
Don't take it to the dealer, take it to someone who knows - an A/C specialist.

This is a very common problem. The evaporater is freezing. There is an adjustment in the system which may need tweeking, or the temperature sensor is out. The experts will have it fixed in no time. :D
 

·
Registered
2009 ML320 Bluetec, 2013 Tesla Model S 85
Joined
·
875 Posts
I think the most likely problem is low refrigerant. Don't take it to someone "with gauges". You want a pro who will evacuate the system, weigh the refrigerant, and refill with the exactly specified mass of refrigerant.

It is extremely difficult to gauge an R134a system for proper fill. If your tech says he knows how then find another because he doesn't.

EPA has long since mandated refrigerant recovery systems for service. If your A/C tech knows what he is doing then he knows he can suck all your old out, weigh it, and put it back in with the proper amount of new to fill to spec quicker than trail and error with gauges.

The gauges are handy to make sure the pump is pumping and that the expansion valve and/or drier are not blocked.
 

·
Registered
2019 X350d, 2017 GLE500e, 2012 ML350BT (sold), 2013 A200 Exclusive(sold), 2009 ML350CDI(sold)
Joined
·
250 Posts
Billm has an A/C which is freezing. This is the opposite of low refrigerant.

And I suggested an A/C Specialist.
That does not mean "someone with gauges".
It means someone who fits and repairs A/C day in day out and carries stock of evaporators dryers condensors pumps and all the other little bits and pieces.
 

·
Registered
2009 ML320 Bluetec, 2013 Tesla Model S 85
Joined
·
875 Posts
Billm has an A/C which is freezing. This is the opposite of low refrigerant.

And I suggested an A/C Specialist.
That does not mean "someone with gauges".
It means someone who fits and repairs A/C day in day out and carries stock of evaporators dryers condensors pumps and all the other little bits and pieces.
Freezing is exactly what happens with low refrigerant. Because there is less gas in the system the refrigerant expands to a lower pressure and a colder temperature.

It generally takes 20 or 30 minutes for the system to freeze up. This is why gauges take so long (possibly hours) to get the gas charge correct.
 

·
Registered
2019 X350d, 2017 GLE500e, 2012 ML350BT (sold), 2013 A200 Exclusive(sold), 2009 ML350CDI(sold)
Joined
·
250 Posts
OK I accept gas could be the cause and I will bow to your better knowledge N4HHE.

However, I still insist it can freeze for other reasons, as I have personally found.
That's why I recommend a specialist to diagnose and fix the true problem.
You suggest a "man with gauges" may not discover the problem.
I suggest the MB dealer "technician" may fall into the same category.
If the car was still under warranty, then I would go to the dealer. But otherwise, I would want it fixed promptly and economicly.
 

·
Registered
2008 ML320CDI Iridium Silver/Black, NAV, P2 pkg, Sirius, Harmon Kardon. Active Bi-Xenon
Joined
·
478 Posts
1 - 10 of 10 Posts
Top