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  #1  
Old 10-24-2011, 11:10 PM
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No heat on highway, but lots at idle

Alright guys, I'm having a bit of an issue with the Benz that I'm noticing now that the weather is getting colder. At idle I get loads of cabin heat, however when I'm on the highway, I get barely anything.

Coolant level is up, and the thermostat is only a few months old. Anyone have an idea on what could be the issue?

Car in question is the '85 300SD W126.
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  #2  
Old 10-24-2011, 11:15 PM
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Torn monovalve diaphragm.


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  #3  
Old 10-24-2011, 11:44 PM
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Quote:
Originally Posted by tangofox007 View Post
Thanks for the info. Is there a "how to" out there showing how to replace it? I'm mostly familiar with VW diesels, and don't have a manual for the Benz. (yet)
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Old 10-25-2011, 06:59 AM
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take out the 4 screws in the top of the valve, remove core, replace with new core.
on your car, it's below the windshield inside the fake firewall about the center of the car.
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Old 10-25-2011, 08:01 AM
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I understand a torn diaphragm will result in heat all the time.

Sounds opposite from a malfunctioning recirculating pump tho.

Any leaks in the system letting outside air in? Such as a pod flap that stays open.
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Old 10-25-2011, 08:05 AM
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Originally Posted by toomany MBZ View Post
I understand a torn diaphragm will result in heat all the time.
That would reflect a deficiency in understanding. No heat at high RPM is the classic symptom of a torn diaphragm. There are plenty of reports on this forum that support that concept.
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Old 10-25-2011, 08:49 AM
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So what would cause heat all the time? That is another issue I have. Thanks
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Old 10-25-2011, 09:41 AM
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I thought a bad t-stat would cause coolant to constantly pass through the radiator. At idle, no air moving over the radiator, so he would get heat. At road speed, it would be over-cooling the engine... right?
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Old 10-25-2011, 09:51 AM
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Quote:
Originally Posted by andrewjtx View Post
I thought a bad t-stat would cause coolant to constantly pass through the radiator. At idle, no air moving over the radiator, so he would get heat. At road speed, it would be over-cooling the engine... right?
Depends on how it failed, but the symptoms as described are classic symptoms of a torn monovalve diaphragm.
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Old 10-25-2011, 11:43 AM
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Quote:
Originally Posted by Mad_Matt View Post
So what would cause heat all the time? That is another issue I have. Thanks
heat all the time would be a stuck open diaphragm in the monovalve or it could be a defective CCU pushbutton unit. that symptom is classic for a siezed aux pump that has burned up the CCU board.
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1987 190D 2.5-5SPEED!!!

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  #11  
Old 10-25-2011, 02:43 PM
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Quote:
Originally Posted by Mad_Matt View Post
So what would cause heat all the time? That is another issue I have. Thanks
A poor electrical connection at the monovalve would be a common cause.
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Old 10-25-2011, 05:37 PM
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Quote:
Originally Posted by vstech View Post
heat all the time would be a stuck open diaphragm in the monovalve or it could be a defective CCU pushbutton unit. that symptom is classic for a siezed aux pump that has burned up the CCU board.
Sorry, can't be the first issue, ask tango.
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  #13  
Old 10-25-2011, 07:31 PM
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Quote:
Originally Posted by andrewjtx View Post
I thought a bad t-stat would cause coolant to constantly pass through the radiator. At idle, no air moving over the radiator, so he would get heat. At road speed, it would be over-cooling the engine... right?
Quote:
Originally Posted by kerry View Post
Depends on how it failed, but the symptoms as described are classic symptoms of a torn monovalve diaphragm.
My thermostat is almost new. I bought the car last April with a bad thermostat that was stuck open. Replaced it in about March.

Thanks for the info guys. I'll order up that valve here later this week along with a new water pump since mine is leaking a tiny bit from the shaft seal.
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  #14  
Old 10-25-2011, 08:02 PM
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Answer

Quote:
Originally Posted by Mad_Matt View Post
So what would cause heat all the time? That is another issue I have. Thanks
Answer:
* Blown CCU fuse.
* Bad mono valve.
* Bad ground to the mono valve.
* Failed push button unit.



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  #15  
Old 10-25-2011, 08:04 PM
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Agree

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Originally Posted by tangofox007 View Post
Textbook symptom of this issue.
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