Only hot air? - PeachParts Mercedes-Benz Forum

81veggiebenz · #1 04-23-2009, 12:14 AM

I have been having some trouble with my air conditioning. The blower and all of the buttons for the vents work fine but the system always blows out heat.

I recently took apart the ccu and re-soldered all of the pins thinking that it would help but it did not. When the dial is set to blow out cold air all of the air conditioning lines under the hood are ice cold. It seems as if the heat and the air are on at the same time and that the heat is just overpowering the cold air. When it is set to heat everything works as it should.

Do I need to get a different ccu or is there something I am missing?
Is there something else that tells the heat to stop blowing and the cold air to come on?

Thanks

toomany MBZ · #2 04-23-2009, 08:23 AM

I don't know if your car has a mono valve, but if it does, pull it and see if the diaphragm is torn.
It's located on the fire wall, passenger side, has a heater hose going to it and an electrical line.

Squabble · #3 04-23-2009, 08:42 AM

check those wires too. i have a brand new monovalve but broken wires that have yet to be repaired, and i only get hot air as well.

tbomachines · #4 04-23-2009, 11:30 AM

Same problem here, will be checking this thread closely especially as summer comes. Also none of my windows currently work

81veggiebenz · #5 04-23-2009, 11:48 AM

Yes the car does have a monovalve. I am pretty sure that the monovalve is stuck open because even with the climate control off I still get a little hot air blowing through the vents.

I am thinking about an alternative to the electrical and monovalve.

By putting a ball valve in the heater hose coming from the monovalve I can manually shut off the hot water from getting to the blower. Close the ball valve in the summer and open it in the winter. No more expensive crap to break and I dont mind doing this twice a year.

I believe that the water from the engine comes up through the bottom of the monovalve and out of the top into the firewall. Is this corrrect?

If so putting the ball valve just after the top outlet of the monovalve should be able to cut off all of the hot water flow so my cold air can do its job.

Tell me what you think.

leathermang · #6 04-23-2009, 12:00 PM

It is not unusual for the AC and heater to be hooked up in some mode so that moisture is dropped out of the air by the AC .... then dry warmed air is delivered to the passengers...

Graplr · #7 04-23-2009, 01:11 PM

Quote:

Originally Posted by 81veggiebenz

By putting a ball valve in the heater hose coming from the monovalve I can manually shut off the hot water from getting to the blower. Close the ball valve in the summer and open it in the winter. No more expensive crap to break and I dont mind doing this twice a year.

I believe that the water from the engine comes up through the bottom of the monovalve and out of the top into the firewall. Is this corrrect?

If so putting the ball valve just after the top outlet of the monovalve should be able to cut off all of the hot water flow so my cold air can do its job.

Tell me what you think.

After chasing hot air coming from my vents for a long time I finally decided to do what you stated. I have a ball valve and by pass of the heater core all together. In fall I open it so the hot coolant flows into the heater core, in the spring I close it so hot coolant stays out of the heater core.

I changed out monovalves with a brand new one. No change. Changed out aux water pumps. No change. Checked all the electrical. Everything worked. The only thing I can come up with is a fault CCU or stuck flaps/pods but I even checked them with vacuum and they all held as well as visually watching some actuate. So I decided to forget it and put a ball valve in.

I know many purists don't like it but after hitting so many dead ends I felt like the only option I had left was to check it with a new push button unit. Mine is at least somewhat functional. AC works (when heater core bypass), defrost works, and foot pods actuate. So I decided to keep my CCU until it completely dies.

Graplr · #8 04-23-2009, 01:12 PM

Quote:

Originally Posted by leathermang

It is not unusual for the AC and heater to be hooked up in some mode so that moisture is dropped out of the air by the AC .... then dry warmed air is delivered to the passengers...

According to the FSM, that should only happen in defrost mode in our cars. There is something malfunctioning here. He gets hot air when NOTHING is on. I had the same symptom.

81veggiebenz · #9 04-23-2009, 02:29 PM

Graplr can you explain where you placed your ball valve to stop the flow. Picture would be great if you have one.

I was thinking that the best place was right after the monovalve but if yours is working then that is probably how I will do mine.

Graplr · #10 04-23-2009, 03:27 PM

Since you run WVO in your car I'm going to assume you already have a bypass coolant loop for your WVO system.

Simply buy a 3/4 PEX ball valve from Lowe's (or the such) and put it anywhere in the line between where the other coolant loop tees in and out to divert all of the flow from the heater core to the other loop.

tbomachines · #11 04-23-2009, 04:18 PM

Quote:

Originally Posted by Graplr

Since you run WVO in your car I'm going to assume you already have a bypass coolant loop for your WVO system.

Simply buy a 3/4 PEX ball valve from Lowe's (or the such) and put it anywhere in the line between where the other coolant loop tees in and out to divert all of the flow from the heater core to the other loop.

Do you worry about the coolant temperature damaging the valve at all? I'm not familiar with temperature ratings for plastics at all, but it would be a source of concern for me.

Graplr · #12 04-23-2009, 04:58 PM

Quote:

Originally Posted by tbomachines

Do you worry about the coolant temperature damaging the valve at all? I'm not familiar with temperature ratings for plastics at all, but it would be a source of concern for me.

PEX valves are metal. Brass. No plastic.

81veggiebenz · #13 04-23-2009, 05:04 PM

Thanks for the response.

I do have the coolant bypassed but it comes out of the hose close to the thermostat, through the heater and back in near the oil filter housing.

Do you think that I could put the ball valve in before the monovalve since it seems to be stuck open?

It just seems like it would be hard to get a ball valve in any of the hoses right next to the firewall.

Graplr · #14 04-23-2009, 06:41 PM

It doesn't matter where it is as long as it is inbetween your two splice points for your auxilliary coolant loop.

81veggiebenz · #15 04-24-2009, 08:53 PM

Ok so today I checked all of the hoses with coolant to see which get hot and which dont.

The hose going from the bottom of the thermostat to the heat pump are hot. The hose going from the heat pump to the monovalve is not hot. The hoses coming from the monovalve and into the firewall are not hot either. However when I turn the system to cold air all of the lines in the entire system get hot. Through the monovalve, firewall, everything.

Does this seem like the ccu is telling the heat pump to come on at the wrong time? ie: when i turn it to cold air?

Also the return hose from the heater core back to the engine block is hot. This is where my bypass is connected. Could my bypass be giving the hot coolant a way into the heater core through the return?