I just can't figure out why my AC doesn't work - PeachParts Mercedes-Benz Forum

dannym · #1 05-11-2008, 10:09 AM

I can't figure out why my AC doesn't work. I did everything by the book.
Flushed whole system.
Pulled a vacuum for 3 hours.
Held vacuum for 1/2 hour
Added correct oil and charged with R-12
Brand new filter/dryer

High side around 150 low side around 25. No joy but I have a good temperature drop through the condenser coil. I can't remember the exact numbers but it was like a 40 degree temp drop. When I rev the engine the low side drops to like 10 or 15.

Added more R-12. High side around 200 low side around 35-40. Still no Joy, When i rev the engine the low side drops again to around 20 or so. Is the drop of pressure like that normal??

The expansion valve is ice cold and I can hear it working. I did find the outside air damper was always open. I disconnected tha actuators and jammed it shut. But it didn't help.

I just don't understand it. It's almost like the air is completely going around the cooling coil. Is that possible?

Testing was done with ambient air at 90 or so. Revving the engine at 2000 rpm doesn't help either. There are bubbles in the sight glass but I can see refrigerant flowing. I should get SOME cooling.

What do I need to do to get cold air??

Thanks

Danny

bobodaclown · #2 05-11-2008, 11:30 AM

WOW. That is crazy. Is the air coming up and out at the defrost vent? Are all the vents open? It sounds like its a problem in your air handler box. Check all vents, Floor, Dash, Windshield. I don't have a Diagram of the air box.

dannym · #3 05-11-2008, 02:32 PM

What I meant by saying it held vacuum for 1/2 hour was I let it sit for 1/2 hour before charging and it held vacuum, meaning it doesn't leak. I did not leak test it but it's held the charge for about a week now. I'm pretty positive it's not leaking.

Pinching the heater hose is a good idea. I stuck my hand back there and felt the hoses and they didn't feel hot. Also starting from a cold engine the AC should get cold before the engine heats up if the heater lines are leaking.

I don't know how much R-12 I put in. I was going by the gauges. 225 lbs on the high side seemed like too much even though the low side was reading low.

I think someway, somehow the air is going around the evaporator. It doesn't make sense to not get any cooling at all. I'm wondering if this is even possible.

Knightrider966 · #4 05-11-2008, 02:43 PM

Danny, I would suspect the blend door is not operating properly. If your system is anyhting like my 300D there is a flap on top of the heaterbox that appears to direct air across the top toward the heater or through the evaporator. You should be able to get to this by going through the glovebox if you remove it. I had similat symptoms and this is where I'd look next, Your system pressures should be giving you some cold air by now!

dannym · #5 05-11-2008, 02:47 PM

Quote:

Originally Posted by Knightrider966

Danny, I would suspect the blend door is not operating properly. If your system is anyhting like my 300D there is a flap on top of the heaterbox that appears to direct air across the top toward the heater or through the evaporator. You should be able to get to this by going through the glovebox if you remove it. I had similat symptoms and this is where I'd look next, Your system pressures should be giving you some cold air by now!

So there is a way to bypass the evaporator! Your not talking about the fresh air door? I found that always open and jammed it shut.
Maybe whatever was causing that to not work is causing the blend door to malfunction?

Thank, the glove box is out right now. I'll look into it.

Knightrider966 · #6 05-11-2008, 02:53 PM

Quote:

Originally Posted by dannym

So there is a way to bypass the evaporator! Your not talking about the fresh air door? I found that always open and jammed it shut.
Maybe whatever was causing that to not work is causing the blend door to malfunction?

Thank, the glove box is out right now. I'll look into it.

There is on mine if I'm looking at this correctly. I had a couple of vacuum lines that were loose and one pod that was malfunctioning. Once I fixed these, I got cold air out my vents!

I didn't have to add any more refrigerant to get cold air, so It would be logical there is a blend door in there somewhere.

vstech · #7 05-11-2008, 11:54 PM

what prep did you do to your coolant lines? evap? expansion valve? condenser? receiver/dryer? did you TOTALLY clean the condenser? inside AND out? even if you did not get any air in the system by letting it sit in a vacuum for 1/2 hour... bubbles indicate inadequate condensing. either additional 12 is needed, or more airflow through the condenser, or more heat transfer out of the condenser.

dannym · #8 05-12-2008, 09:18 AM

Quote:

Originally Posted by vstech

what prep did you do to your coolant lines? evap? expansion valve? condenser? receiver/dryer? did you TOTALLY clean the condenser? inside AND out? even if you did not get any air in the system by letting it sit in a vacuum for 1/2 hour... bubbles indicate inadequate condensing. either additional 12 is needed, or more airflow through the condenser, or more heat transfer out of the condenser.

I flushed everything except the compressor. I cleaned the condenser coils with coil cleaner. They got cleaner but I still don't get air conditioning.
If the condenser was bad would I have a 40 degree temp drop across the coil? I was thinking the condenser was bad but it really seams like the air is going around the evaporator.
I'm in Atlanta now I'll do some checking when I get back.

Danny

rrgrassi · #9 05-12-2008, 09:56 AM

Did you replace the expansion valve? Have you removed your blower to acutally feel the evaporator core?

dannym · #10 05-13-2008, 10:32 AM

No i didn't replace the expansion valve. It is ice cold though. I'll try to get a shot of the evaporator coil with my infra-red thermometer.

Could the fresh air recirculating flap being in the wrong position cause the air to bypass the evap coil?

nhdoc · #11 05-13-2008, 11:04 AM

I am convinced you have an airflow problem not an A/C cycle problem. You need to find out how the air is directed by the ACC system and resolve why it is bypassing the evaporator. The pressures you are reporting are OK and should provide at least some cooling at the evap...I would bet it is ice cold but just not seeing any air. It's even possible the evap coil/fins are so clogged up with crud air can't go through. I have seen some like that.

dannym · #12 05-13-2008, 03:57 PM

Quote:

Originally Posted by nhdoc

I am convinced you have an airflow problem not an A/C cycle problem. You need to find out how the air is directed by the ACC system and resolve why it is bypassing the evaporator. The pressures you are reporting are OK and should provide at least some cooling at the evap...I would bet it is ice cold but just not seeing any air. It's even possible the evap coil/fins are so clogged up with crud air can't go through. I have seen some like that.

Yeah, I'm pretty much convinced it's an air flow problem myself. I thought about the evap being clogged too. I'll be back home tomorrow, maybe I'll have time to look into it then. If not tomorrow then definitely Thursday.

dannym · #13 05-23-2008, 06:52 PM

Come to think of it the heat didn't work very well either. Something is definitely not right.
What is this crap in my vents? It looks like some kind of foam padding that came loose from somewhere. These are the center vents over the center console:

I found the switchover valves. Can anyone tell me what each if the 6 valves do?

Thanks

danny

vstech · #14 05-27-2008, 11:11 AM

I am going to say this again and I will keep saying it untill all you jackseed a/c techs get it through your head... test for leaks UNDER PRESSURE, DO NOT TEST FOR A LEAK IN A VACUUM, AND DO NOT VACUUM A SYSTEM UNTIL YOU ARE SURE THERE ARE NO LEAKS!!!
reclaim the original charge. or if it's already empty... YOU KNOW THERE IS A LEAK! pressurize the system with nitrogen and VERIFY IT DOES NOT LEAK OUT. if it does find the leak before you try and evacuate. look for oil stains, look for bubbles, but LOOK!
evacuating a unknown tighness system is just asking for the lines to acid fail. moisture from the air mixed with oil and refrigerant will turn into an acid and eat your seals and lines from the inside out.

Cr from Texas · #15 05-27-2008, 11:11 AM

No one mentioned the heater control valve. If it doesn't get 12V, it sends hot water to the heater core. If there is a bad diaphram in the valve, hot water also goes to the heater core. Blend this with the cold air from the evaporator and it seems like the AC is not working.

I'd agree there is a good chance the evaporator core is plugged with the same junk that's in the vents.