'91 300D 2.5 minor heating issue...where to start? - Page 3 - PeachParts Mercedes-Benz Forum

dablk1 · #31 11-19-2010, 08:05 PM

Not that I can recall. I find my self constantly raising the temp.

biopete · #32 11-19-2010, 08:08 PM

What about coolant level? If the coolant is low, the heater circuit gets cool. Happened on my 93. Topped off coolant and I got my heat back at all speeds. Also, you can just unplug the monovalve to take it out of the equation. No need to buy a new one. You will get straight hot water going to heater core that way. Then if your heat works nice and hot at all speeds you know its not a coolant flow problem.

dablk1 · #33 11-19-2010, 08:16 PM

Well I unplugged the monovalve earlier today to ensure it wasn't a coolant flow problem. By removing it from the equation it will just give max heat at all times. I'm really trying to get my climate control working correctly. I won't be able to adjust the temp without the monovalve in the equation.

biopete · #34 11-23-2010, 09:56 AM

Quote:

Originally Posted by dablk1

Well I unplugged the monovalve earlier today to ensure it wasn't a coolant flow problem. By removing it from the equation it will just give max heat at all times. I'm really trying to get my climate control working correctly. I won't be able to adjust the temp without the monovalve in the equation.

This can be tough. What all have you replaced on the system now? If you could get a hold of a known good Push button unit to try out that would be what i would do next maybe.

dablk1 · #35 11-23-2010, 10:01 AM

I was also thinking about that. I haven't had a chance to replace the blower regulator yet. I'm going try and get that done today. Then I'll try to find a "known" working CCU.

shertex · #36 11-23-2010, 10:05 AM

Have you replaced the monovalve? If not, what effect might that have?

dablk1 · #37 11-23-2010, 10:46 AM

I had replaced it with one from my local PNP just to test the system. Maybe it wasn't good , but I have another to try. As far as I understand, the monovalve help control the water flow into the heater core???? It's worth a shot.

vstech · #38 11-23-2010, 10:47 AM

Quote:

Originally Posted by biopete

What about coolant level? If the coolant is low, the heater circuit gets cool. Happened on my 93. Topped off coolant and I got my heat back at all speeds. Also, you can just unplug the monovalve to take it out of the equation. No need to buy a new one. You will get straight hot water going to heater core that way. Then if your heat works nice and hot at all speeds you know its not a coolant flow problem.

this only works if your monovalve functions. I had one seized in it's bore. would not open for anything.
I've also had them seize open... another problem is the aux pump. it can plug up with gunk. and when it's not pumping it's an impediment to coolant flow.

dablk1 · #39 11-23-2010, 10:48 AM

I'm going to replace the monovalve and blower regulator while I'm changing the oil. Hopefully within the next couple of hours.

dablk1 · #40 11-23-2010, 11:16 PM

Just an update. A/C works flawlessly(including blower response), defrost mode works flawlessly. Now back to the heat...doesn't maintain proper temperature and blowe constantly blows at one speed while in auto mode, it starts blowing cool air. I think it's time to pull the auxiliary waterpump to check operation and then perform surgery on the CCU. Did I miss anything?

dablk1 · #41 11-23-2010, 11:22 PM

How can I test the monovalve aside from giving it 12V? How can I tell if it's seized?

shertex · #42 11-24-2010, 08:06 AM

Quote:

Originally Posted by dablk1

Just an update. A/C works flawlessly(including blower response), defrost mode works flawlessly. Now back to the heat...doesn't maintain proper temperature and blowe constantly blows at one speed while in auto mode, it starts blowing cool air. I think it's time to pull the auxiliary waterpump to check operation and then perform surgery on the CCU. Did I miss anything?

Didn't you just replace AWP? Did you use a used part, is that why you wonder?

biopete · #43 11-24-2010, 09:51 AM

Quote:

Originally Posted by dablk1

Just an update. A/C works flawlessly(including blower response), defrost mode works flawlessly. Now back to the heat...doesn't maintain proper temperature and blowe constantly blows at one speed while in auto mode, it starts blowing cool air. I think it's time to pull the auxiliary waterpump to check operation and then perform surgery on the CCU. Did I miss anything?

Try monovalve first. you already know your coolant flow problem is at the monovalve because when you unplug it it gets straight heat. Someone correct me if i'm wrong but I believe the aux water pump runs all the time regardless of temp setting. You can check this by unplugging it with heat off and see if you have 12 v at the plug.

Either the CCU is telling the monovalve the wrong signal or monovalve is defective. I'm not sure how to test monovalves. Mine failed by staying open regardless of temp setting.

dablk1 · #44 11-24-2010, 12:10 PM

Yes it was a used AWP, I was just check to see if it was possibly clogged. It works fine. I'm leaning towards the CCU, monovalve has already been replaced. I have to pull it part and take a look. By the way, the AWP does run all of the time.

dablk1 · #45 11-30-2010, 08:47 PM

Recap; Car will not maintain chosen cabin temperature when windows are rolled up.

replaced cabin sensor
replaced aspirator
replaced waterpump
replaced auxiliary water pump
replaced monovalve
swapped CCU with a "known" working unit

The coolant level is fine and the car runs great. This problem happens after is reaches selected temperature...the blower motor continues to run but doesn't variate speed and eventually it starts blowing cool air from from the vents. If I roll down the window to let the cooler/colder outside air inside the CCU will respond accordingly(correct the temperature). When I roll the window back up it starts the same routine all over again. It seems as though the system goes into a bypass mode or something.

While fooling around with the system today I noticed another meter/sensor located directly behind the CCU located at about the 9 o'clock posiiton...it has a plug on it with a blue tip...it also reponds to temperature change. Can anyone tell me what it is and what it's function is? How often does the outside temperature sensor fail?(the one under the wiper motor)