W126: Climate control problem-constant heat problem. - PeachParts Mercedes-Benz Forum

Homeline · #1 11-30-2005, 04:52 PM

I'm starting to work on several issues my old '82 SEC has. Frist I'm looking into the climate control system. I have constant heat unless I roll the dial all the way up to get the AC to come on, and then it blows cold. I've checked the monovalve inside and all looks good; rubber is in fine shape with no cracks or deterioration, and the solenoid works the plunger. I did notice that the solenoid is active (plunger extended) no matter what setting the system is on. Is this correct? Are there any other rubber components within the monovalve body itself that I couldn't see?

My next stop in this diagnosis is resoldering/cleaning the control board. The blower works on high/low speeds, but not all the time in all switch settings. This seems like a needed thing to do on a 20+ year old car. What else can I look for if this doesn't take care of the problem?

Cliff

Two560Two · #2 11-30-2005, 05:04 PM

I think you should check the push button control board. I had the same problem on my first 420SEL that I had. Although I pressed the "off" button, I would still get air coming in. I didnt replace it becuase the unit is so darn expensive. I just sold the car and bought another benz. GL

derryb · #3 11-30-2005, 05:43 PM

An interesting point. Maybe we need a thread where members can fess up to why they really got rid of a benz.

Homeline · #4 11-30-2005, 05:54 PM

Yep, Two560Two, I'll be looking at the whole module real close.

engatwork · #5 11-30-2005, 06:01 PM

My guess would be either the temp sensor that measures the air temp inside the evap housing or the cabin air temp sensor is bad IF it is not the pushbutton unit. Not sure where these sensors are on your car. Find them, disconnect them and see what kind of resistance you get accross them. If you don't get any then it is bad.

Homeline · #6 12-01-2005, 10:47 PM

Just this evening I took dash apart and removed the control box. It's got a date code of 2001 and a "www" address, so it's obviously been replaced. Thats not to say it could still have a problem, but I kind of doubt it. Since I can see behind dash now, I'll investigate some of the vacuum valves to see if they are working correctly. Maybe broken diaphrams in there.

I'll locate the evap sensor and test. I think the cabin sensor is in the roof panel by the dome light. There's a hole there I can stick the tip of my finger in, looks like a cover or something is missing. What would it look like, this sensor?

donbryce · #7 12-02-2005, 10:41 AM

Quote:

Originally Posted by Homeline

I think the cabin sensor is in the roof panel by the dome light. There's a hole there I can stick the tip of my finger in, looks like a cover or something is missing. What would it look like, this sensor?

The cover plate should look like the top picture, and the sensor itself, like the next picture down (says starting 07/81). The sensor for the heater box is attached to the 2-prong plug on the exchanger box itself.

mctwin2kman · #8 12-02-2005, 11:31 AM

On my W201 there are three sensors for temperature for the HVAC system. One in the car in the roof as stated above. One for the EVAP which is only used when the AC is engaged. And one on the fan housing that measures outside air temperature. If I disconnect the outside temp one I get full heat except when the wheel is at the full cold setting. From what I can see on the Maint CD the interior one when bad will just make the temp wheel default to its setting and not be automatically adjusted. I would find that outside one first and make sure it is indeed plugged in and test for it resistance. Although it also sounds like your interior one may be missing or not connected any longer. Mine is a 1986 so I have to assume that they used a similar setup back in 1982.........

mctwin2kman · #9 12-02-2005, 04:09 PM

Quote:

Originally Posted by donbryce

I believe the outside temperature sensor is behind the front bumper, in the license plate mount area....

That is the temp sensor for the dash display not the HVAC!

Homeline · #10 12-02-2005, 04:27 PM

Quote:

Originally Posted by donbryce

The cover plate should look like the top picture, and the sensor itself, like the next picture down (says starting 07/81). The sensor for the heater box is attached to the 2-prong plug on the exchanger box itself.

Thanks for the pic. I wonder why mine is missing....who knows. Now I just have to locate a replacement, which shouldn't be difficult.
Cliff

Homeline · #11 12-02-2005, 05:24 PM

Quote:

Originally Posted by mctwin2kman

On my W201 there are three sensors for temperature for the HVAC system. One in the car in the roof as stated above. One for the EVAP which is only used when the AC is engaged. And one on the fan housing that measures outside air temperature. If I disconnect the outside temp one I get full heat except when the wheel is at the full cold setting. From what I can see on the Maint CD the interior one when bad will just make the temp wheel default to its setting and not be automatically adjusted. I would find that outside one first and make sure it is indeed plugged in and test for it resistance. Although it also sounds like your interior one may be missing or not connected any longer. Mine is a 1986 so I have to assume that they used a similar setup back in 1982.........

Wow, thats some advice and guidence! Thank you very much. I'll run with this info and start the search for the O/S temp sensor. Not to concerned with the roof sensor at this point, but it would be nice to put one back in.

Stop by tonight and you'll here me say.....here sensor, sensor, sensor.....

I'll keep you all posted

rschleicher · #12 12-02-2005, 09:13 PM

It still may be possible for this to be a monovalve problem. The plunger on this is full closed when doing max. cooling, and is full-open when doing max. heat, but is basically pulsing between full open and full closed at all other temperature settings. (Pulse width modulation.)

It is not unheard of for monovalves to be degraded such that they don't respond to the pulses, but still function enough to eventually go to full-open or cull-closed. This might give you the sypmtom of full heat at all but the max. cool setting of your ACC control.

One way to crudely test the monovalve is to pull the electrical connection. I forget now whether no power corresponds to the max. cool or max. heat condition, so try putting the temp. wheel in both states. Then touch the electrical connector to the pins - you should be able to feel the monovalve move as you make and break contact. For that matter, you should also be able to feel the pulses of the monovalve at any temperature setting (by placing your finger on the top of the valve).

Homeline · #13 12-03-2005, 09:29 AM

Quote:

Originally Posted by rschleicher

It still may be possible for this to be a monovalve problem. The plunger on this is full closed when doing max. cooling, and is full-open when doing max. heat, but is basically pulsing between full open and full closed at all other temperature settings. (Pulse width modulation.)

It is not unheard of for monovalves to be degraded such that they don't respond to the pulses, but still function enough to eventually go to full-open or cull-closed. This might give you the sypmtom of full heat at all but the max. cool setting of your ACC control.

One way to crudely test the monovalve is to pull the electrical connection. I forget now whether no power corresponds to the max. cool or max. heat condition, so try putting the temp. wheel in both states. Then touch the electrical connector to the pins - you should be able to feel the monovalve move as you make and break contact. For that matter, you should also be able to feel the pulses of the monovalve at any temperature setting (by placing your finger on the top of the valve).

The monovalve was my first stop in this endevour. When bench tested, with no power, it was in the open (plunger retracted) position and closed when powered. I cycled between open/close about a dozen times in one minute and it worked fine. As I recall, no matter what setting the controls were set to, the solenoid was powered (plunger extended). Although I don't recall moving the wheel to MAX cool and testing, this kind of contradicts your explanation. Or perhaps I'm just not understanding correctly. I have constant heat no matter the temp wheel setting unless it's in the MIN setting then it's cool. Will have to recheck this tonight.

Homeline · #14 12-03-2005, 09:37 AM

Quote:

Originally Posted by rschleicher

It still may be possible for this to be a monovalve problem. The plunger on this is full closed when doing max. cooling, and is full-open when doing max. heat, but is basically pulsing between full open and full closed at all other temperature settings. (Pulse width modulation.)

Now I just had a thought (watch out!)... while I didn't test this and I might be moving in the wrong direction, but how much resistance would it take to move the plunger when it's powered. I'm talking about system pressure over ridding the solenoid and forcing it in the open position. Is this a possible theory?

mctwin2kman · #15 12-03-2005, 12:13 PM

Interesting... I forgot your car has that silly mono valve.. Mine has an either open or closed heat valve that is vacuum operated. Whenever heat is needed it is open fully, only closes when the temp wheel is set low enough and the outside temp is above the selected temp. I just fixed my heat problems today with a used HVAC control unit I picked up on E-Bay. But doesn't your car use the standard blend air flap to control heat and cool operation as well so even if your monovalve is full open that the blend air flaps would open to suck in cooler outside air to mix with the heater core air? That is what mine does and all other cars I have ever seen..... Mine now works perfect and operates the Aux pump and Heater Valve as it is supposed to. My old unit would not close my heat valve and would not power my aux pump properly.