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  #1  
Old 01-14-2008, 05:33 PM
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w126 1989 300SEL heater isn't working

Where should I look for heater trouble? I get nothing but cold air coming out of the vents. If the heater valve is faulty doesn't it usually stick in the open position? (I'm hoping it is not a vacuum control issue).
Thanks
-Brett

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  #2  
Old 01-14-2008, 05:38 PM
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Mono Valve is pretty easy to check out. On my 126's they sometimes stick closed and I have to "thump" them. Although not the orthodox method of repair - some don't get driven frequently. The valve is pretty easy to inspect and clean.
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  #3  
Old 01-14-2008, 06:51 PM
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One quick way to check for vacuum issues is to see if the system responds with air out the proper vents when you ask it to:
1. Hit the recirculate button and see if you hear the door pop open behind the glove box, also, the air should get louder.
Also, if you have good residual vacuum, you should be able to turn the key on, engine off, after sitting several hours, and hear the recirc door move. You can also see it, through the little panel in the back of the glove box.
2. Turn the temp down to cold, the air should switch to the center vents.
3. Turn the temp full hot, air should switch to floor and side-dash vents, with a little from defrost.
4. Defrost is the default, so if it's on defrost all the time, could be vacuum.
Turn it on defrost mode, and it should definitely blow hard out the defrost vents. turn the temp all the way up, it it stays cold, it's probably not getting hot water through the monovalve.

A quick check of the monovalve: warm the engine, turn the system on and adjust the temp dial to the approximate ambient temp inside the car (say 70 degrees). The fan should slow down, meaning it's detecting that it's close to desired temp inside the cabin. this should put the monovalve in cycle mode.
Go under the hood and put your finger on the top of the monovalve, and feel for a click every 5 seconds. If you feel that, it is probably working (maybe).

Just a couple of things to try; these sytems can be pretty complicated; but it seems the mV, the vacuum dashpots, or the control head solder jointsa are the common bad guys.

If you fix it, then you can help me find my intermittant vent issue - works great, then suddenly switches to defrost without warning. GRRRRR.

DG
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  #4  
Old 01-15-2008, 04:35 PM
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My ignorance it going to show here. Which is the recirculate button? I didn't see one. On the right I have a column of three buttons (high, low and auto fan speeds). Across the bottom I have a row of 5 (I believe) with "0" (off) at one end, defrost at the other, and three buttons with arrows in the middle (don't know the difference between the arrow buttons).

This car has a passenger-side airbag, so there is no glove box (so no nifty window in the back of it).

I went through steps 2-4. I did have some air coming out of the center vents when on heat, and some coming out of side vents when on cold, but most of the air was coming out where it should be.

I had someone help me with the monovalve check. She turned the temperature dial up and down while I had my finger on the monovalve (that's a tight spot back there). Each up and down I could feel the click of the valve. So it seems to be passing the click test.

I grabbed the metal coolant line that goes back to the firewall (right side of the engine) which I believe feeds the cabin heater. It was reasonably warm. Not too hot to touch though. I was expecting either completely cold or very hot.

Does this get me any closer to solving the problem?

Oh, by the way, periodically, with no apparent pattern, the heat will work just a little bit. I will get some none-freezing air coming out. It is a little short of "warm" but much better than what I normally get. Normally it feels just like the external air temperature (right now about 25-30 degrees F).

Thanks for the terrific list of ideas.

-Brett

Quote:
Originally Posted by S-Class Guru View Post
One quick way to check for vacuum issues is to see if the system responds with air out the proper vents when you ask it to:
1. Hit the recirculate button and see if you hear the door pop open behind the glove box, also, the air should get louder.
Also, if you have good residual vacuum, you should be able to turn the key on, engine off, after sitting several hours, and hear the recirc door move. You can also see it, through the little panel in the back of the glove box.
2. Turn the temp down to cold, the air should switch to the center vents.
3. Turn the temp full hot, air should switch to floor and side-dash vents, with a little from defrost.
4. Defrost is the default, so if it's on defrost all the time, could be vacuum.
Turn it on defrost mode, and it should definitely blow hard out the defrost vents. turn the temp all the way up, it it stays cold, it's probably not getting hot water through the monovalve.



DG
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  #5  
Old 01-15-2008, 04:38 PM
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I like the "thump" idea. Unfortunately it didn't do anything for me in this case. What's involved in getting the valve out to clean it. It's really tight back there and I'm assuming there are hoses connected to the bottom (I couldn't even see them when looking down on it).

-Brett

Quote:
Originally Posted by TX76513 View Post
Mono Valve is pretty easy to check out. On my 126's they sometimes stick closed and I have to "thump" them. Although not the orthodox method of repair - some don't get driven frequently. The valve is pretty easy to inspect and clean.
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  #6  
Old 01-15-2008, 04:39 PM
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Quote:
Originally Posted by TX76513 View Post
Mono Valve is pretty easy to check out. On my 126's they sometimes stick closed and I have to "thump" them. Although not the orthodox method of repair - some don't get driven frequently. The valve is pretty easy to inspect and clean.
I had the same thing happen with mine and replaced the mono valve and now I am nice-n-toasty. Until you get it fixed the heat from the defroster will still work.
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  #7  
Old 01-15-2008, 08:40 PM
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I have heat again

I pulled the top off of the monovalve and found that the plunger (if that's what it is called) was rather difficult to move. I replaced it with the one from a 420SEL I have sitting here (waiting for a replacement engine to come along) and the heat is working great now. I did lose an electrical connection piece though. The plug came apart when I was taking it off. I found the top black piece of plastic, but the bottom one is gone forever. So right now I have the wires on the posts without the plastic bits around them. I don't know how much trouble that's going to cause me. But right now I have heat.

Thanks for the help.

-Brett
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  #8  
Old 01-16-2008, 10:54 AM
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Brett, good deal. Glad you found it.
Apparantly the old monovalve was kinda jammed, and it would click and try to open, but not well enough.
Sounds like the vacuum system is okay - oh, the recirculate button is higher up, near the hazard switch, has a circle on it and a little red light when it's on. Push it and you'll hear a big dif in air flow noise.

Also, that metal pipe on the passenger side shouod be really hot if the engine is warm and the heater is on.

Easy fix, I'm enthused. My system is randomly switching to defrost vents for no good reason, but it's heating. But I may just go into the monovalve and see if I get lucky. Doesn't make sense to my analytical side, but always try the easy stuff first.

These systems have some weird, complex stuff; an oldtime mechanic at the dealer told me the system does a periodic check on the auxiliary water pump avery 15 minutes or so. If the pump doesn't return the proper current/resistance reading, the system pops the monovalve wide open for 15 seconds to help heat the air. Happened to mine - AC would go full hot for a few seconds every 15 minutes. Repaired pump, all fixed.

Cheers,
DG



DG
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  #9  
Old 01-16-2008, 11:28 AM
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Quote:
Originally Posted by BrettS View Post
I pulled the top off of the monovalve and found that the plunger (if that's what it is called) was rather difficult to move. I replaced it with the one from a 420SEL I have sitting here (waiting for a replacement engine to come along) and the heat is working great now. I did lose an electrical connection piece though. The plug came apart when I was taking it off. I found the top black piece of plastic, but the bottom one is gone forever. So right now I have the wires on the posts without the plastic bits around them. I don't know how much trouble that's going to cause me. But right now I have heat.

Thanks for the help.

-Brett
You might want to wrap them with some electrical tape until you get the replacement. Glad your toasty again.

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