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  #1  
Old 04-28-2014, 07:05 AM
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Cabin temp

Where or what can I troubleshoot for lack of heat control. No matter where I positioned the temp setting on the control, all I get is hot air. Even after the turn the system off I still feel hot air coming from the vents, mainly the vents on the side towards the window.

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  #2  
Old 04-28-2014, 07:32 AM
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Quote:
Originally Posted by WMO Madness View Post
Where or what can I troubleshoot for lack of heat control. No matter where I positioned the temp setting on the control, all I get is hot air. Even after the turn the system off I still feel hot air coming from the vents, mainly the vents on the side towards the window.
I think you have Newt Gingrich trapped in your vents.
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  #3  
Old 04-28-2014, 08:38 AM
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There are a few items that can cause this:

1) Bad vacuum pods
2) push button unit problems (cold solder joints)
3) temperature regulator control unit problems
4) Monovalve problems.
5) vacuum leaks

I'd start by checking your monovalve. Pull the cartridge, make sure theres no holes in the rubber diaphragm.

Next I'd check the vacuum pods. Make sure they are all holding vacuum.
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  #4  
Old 04-28-2014, 08:49 AM
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Originally Posted by JamesDean View Post
Next I'd check the vacuum pods. Make sure they are all holding vacuum.
Vacuum controls air distribution. Temperature control is accomplished electrically, through the monovalve.
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  #5  
Old 04-28-2014, 08:52 AM
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Quote:
Originally Posted by tangofox007 View Post
Vacuum controls air distribution. Temperature control is accomplished electrically, through the monovalve.
Yes I know this. OP said even after the system is he getting air at vents. That suggest (at least on a 124 or 126..not 100% sure on a 123) that the air scoop vent pod is either bad or not being controlled properly.
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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
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  #6  
Old 04-28-2014, 12:06 PM
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Quote:
Originally Posted by tangofox007 View Post
Vacuum controls air distribution. Temperature control is accomplished electrically, through the monovalve.
Not sure where the "monovalve" is located. Do you have a photo of it?
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  #7  
Old 04-28-2014, 12:08 PM
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Quote:
Originally Posted by WMO Madness View Post
Not sure where the "monovalve" is located. Do you have a photo of it?
http://www.peachparts.com/shopforum/attachments/diesel-discussion/2319d1022593930-image-location-monovalve-1985-300d-turbo-monovalve.jpg
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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
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  #8  
Old 04-28-2014, 12:17 PM
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Quote:
Originally Posted by JamesDean View Post
There are a few items that can cause this:

1) Bad vacuum pods
2) push button unit problems (cold solder joints)
3) temperature regulator control unit problems
4) Monovalve problems.
5) vacuum leaks

I'd start by checking your monovalve. Pull the cartridge, make sure theres no holes in the rubber diaphragm.

Next I'd check the vacuum pods. Make sure they are all holding vacuum.
All that. Good advise.
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  #9  
Old 04-28-2014, 02:49 PM
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Thank you! So what goes bad on these valve? Can I take it apart and clean it?
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Last edited by WMO Madness; 04-28-2014 at 03:41 PM.
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  #10  
Old 04-28-2014, 03:44 PM
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Remember the mono valve is electrically operated such that
- no voltage = full flow
- full votage = no flow

The fused +12 V dc from the climate control circuit connects directly to the monovalve, and the controller switches the grounded side of the coil. Therfore if you measure voltage you should always see +12 V dc on the coil at least on one side.

To provide proportional control of the temperature the voltage is switched on and off with varying duty cycle. The less "on" time, the warmer the heater will become.

When the controller tells the monovalve to close, the controller grounds the wire coming from the other side of the monovalve coil. In this case you can see the voltage going between 12 V (controller says valve open) and nearly 0 V (controller says valve closed).

Putting the dial on MIN temperature results in constant ground (i.e. 100% duty cycle) and putting the dial on MAX results in no ground (i.e 0% duty cycle).

The thermal mass of the heat core smooths out the pulsed operation.
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  #11  
Old 04-28-2014, 03:55 PM
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Quote:
Originally Posted by jay_bob View Post
When the controller tells the monovalve to close, the controller grounds the wire coming from the other side of the monovalve coil. In this case you can see the voltage going between 12 V (controller says valve open) and nearly 0 V (controller says valve closed).
So...when the circuit is completed on the ground side, you can "see" the voltage go to zero? Where should one look to see this?

Quote:
Originally Posted by jay_bob View Post

Putting the dial on MIN temperature results in constant ground (i.e. 100% duty cycle) and putting the dial on MAX results in no ground (i.e 0% duty cycle).
That is correct in the case of specific modes and not correct in others.
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  #12  
Old 04-28-2014, 07:10 PM
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Quote:
Originally Posted by tangofox007 View Post
So...when the circuit is completed on the ground side, you can "see" the voltage go to zero? Where should one look to see this?
Using a digital voltmeter you will be able to see this. If you apply voltage to a coil but do not connect it to ground (or neutral), you will read line voltage on both sides of the coil. This is because the meter does not draw enough current (due to its high source impedance) to cause a voltage drop through the coil windings.

If however the controller puts a ground on the other side of the coil, the voltage on the controlled side of the coil will go to zero (or very nearly so, it depends on whether it's a relay (123/early 126) or solid state switch (124/late 126/210).

I have been an industrial controls engineer for the past 25 years. I see this in practice every day if we lose a neutral on a relay coil in a control panel. We normally switch the "hot" side, so if we have the switch 'on' and the relay not pulled in, and I read line voltage on both sides of the coil, it's time to find the bad neutral.

If you ever talk to an electrician, they will likely tell you the worst time they got shocked is off the neutral wire, where they opened the connection between the panel and a load, and got their hand in between the two wires. There is definitely full voltage on the load side of the neutral when measured against the panel neutral.

Quote:
Originally Posted by tangofox007 View Post
That is correct in the case of specific modes and not correct in others.
That is correct. I should have been more clear. This operation is true when either the "hi-lo", "normal" or "EC" buttons are pushed. This is the 3 buttons in the center of the 5. "Off" grounds the monovalve to keep it [edit: shut], and "defrost" removes the ground so it is maximum heat.
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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Last edited by jay_bob; 04-28-2014 at 10:27 PM. Reason: Last sentence, I'm used to electrical "closed" means on and "open" means off
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  #13  
Old 04-29-2014, 07:45 AM
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Quote:
Originally Posted by jay_bob View Post
Remember the mono valve is electrically operated such that
- no voltage = full flow
- full votage = no flow

The fused +12 V dc from the climate control circuit connects directly to the monovalve, and the controller switches the grounded side of the coil. Therfore if you measure voltage you should always see +12 V dc on the coil at least on one side.

To provide proportional control of the temperature the voltage is switched on and off with varying duty cycle. The less "on" time, the warmer the heater will become.

When the controller tells the monovalve to close, the controller grounds the wire coming from the other side of the monovalve coil. In this case you can see the voltage going between 12 V (controller says valve open) and nearly 0 V (controller says valve closed).

Putting the dial on MIN temperature results in constant ground (i.e. 100% duty cycle) and putting the dial on MAX results in no ground (i.e 0% duty cycle).

The thermal mass of the heat core smooths out the pulsed operation.
Thank you for the detail explanation, even though it is over my head. Anyway, I located the valve and unplugged the connector. Noticed some corrosion so I cleaned it and applied some grease to it and worked in and out several times. Started the car and let it warmed up, I was able to control the heat at will.

Again thank you for the education.
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  #14  
Old 04-29-2014, 02:17 PM
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Quote:
Originally Posted by WMO Madness View Post
Thank you! So what goes bad on these valve? Can I take it apart and clean it?
With the engine cold remove the four screws on top and look inside. If it s flooded above the diaphragm, get a repair kit and just slip in a new diaphragm/plunger.
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  #15  
Old 08-14-2014, 07:59 PM
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Quote:
Originally Posted by Air&Road View Post
With the engine cold remove the four screws on top and look inside. If it s flooded above the diaphragm, get a repair kit and just slip in a new diaphragm/plunger.
I removed the Mono valve today and inspected it for damaged, the diaphragm look good. so I applied power to the valve, it actuated like a electric shut off valve. The latest symptom is, full hot hair when before I was able to control it specially after I got the A/C fixed. Even when the A/C is on the cold is not overriding the warm air coming out.

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