Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 10-24-2011, 11:10 PM
Cruger's Avatar
Registered User
 
Join Date: Mar 2011
Location: Sherwood Park, Alberta, Canada
Posts: 31
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.

__________________
'85 Mercedes 300SD W126
'99.5 Jetta TDI
Reply With Quote
  #2  
Old 10-24-2011, 11:15 PM
Registered User
 
Join Date: Sep 2004
Posts: 8,971
Torn monovalve diaphragm.


PeachParts FastLane Mercedes Parts Store
Reply With Quote
  #3  
Old 10-24-2011, 11:44 PM
Cruger's Avatar
Registered User
 
Join Date: Mar 2011
Location: Sherwood Park, Alberta, Canada
Posts: 31
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)
__________________
'85 Mercedes 300SD W126
'99.5 Jetta TDI
Reply With Quote
  #4  
Old 10-25-2011, 06:59 AM
vstech's Avatar
DD MOD, HVAC,MCP,Mac,GMAC
 
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,835
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.
__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
Reply With Quote
  #5  
Old 10-25-2011, 08:01 AM
toomany MBZ's Avatar
Registered User
 
Join Date: Nov 2006
Location: central Va
Posts: 7,820
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.
__________________
83 SD

84 CD
Reply With Quote
  #6  
Old 10-25-2011, 08:05 AM
Registered User
 
Join Date: Sep 2004
Posts: 8,971
Quote:
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.
Reply With Quote
  #7  
Old 10-25-2011, 08:49 AM
Registered User
 
Join Date: Aug 2009
Location: Clarksburg MD
Posts: 174
So what would cause heat all the time? That is another issue I have. Thanks
__________________
Mad_Matt's Current Stable:
Oil Burners:
09 Jetta TDI The highway queen @ 105k
83 300D. Sold
Gassers:
16 A4 sport line
10 Passat
83 Recaro Trans AM WS6 Bone Stock Survivor
11 Harley Ultra
Reply With Quote
  #8  
Old 10-25-2011, 09:41 AM
Vice President of Snark
 
Join Date: Nov 2009
Location: Austin, TX
Posts: 1,230
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?
__________________
'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter
Reply With Quote
  #9  
Old 10-25-2011, 09:51 AM
Registered User
 
Join Date: Jan 2002
Posts: 18,350
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.
__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
Reply With Quote
  #10  
Old 10-25-2011, 11:43 AM
vstech's Avatar
DD MOD, HVAC,MCP,Mac,GMAC
 
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,835
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.
__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
Reply With Quote
  #11  
Old 10-25-2011, 02:43 PM
Registered User
 
Join Date: Sep 2004
Posts: 8,971
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.
Reply With Quote
  #12  
Old 10-25-2011, 05:37 PM
toomany MBZ's Avatar
Registered User
 
Join Date: Nov 2006
Location: central Va
Posts: 7,820
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.
__________________
83 SD

84 CD
Reply With Quote
  #13  
Old 10-25-2011, 07:31 PM
Cruger's Avatar
Registered User
 
Join Date: Mar 2011
Location: Sherwood Park, Alberta, Canada
Posts: 31
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.
__________________
'85 Mercedes 300SD W126
'99.5 Jetta TDI
Reply With Quote
  #14  
Old 10-25-2011, 08:02 PM
whunter's Avatar
Moderator
 
Join Date: Dec 2003
Location: Metro Detroit, Michigan
Posts: 17,416
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.



.
__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/
Reply With Quote
  #15  
Old 10-25-2011, 08:04 PM
whunter's Avatar
Moderator
 
Join Date: Dec 2003
Location: Metro Detroit, Michigan
Posts: 17,416
Agree

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

Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -4. The time now is 05:23 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2024 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page