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 07-08-2014, 11:01 PM
'84 300D Owner
 
Join Date: Oct 2006
Location: Dallas, TX
Posts: 211
Interesting AC problem

Very weird, the AC will intermittently blast me with hot air, but only when I come to a stop at a light and idle for a minute or two, never when rolling down the road. After a minute when the AC has cooled off the heater core again do I get cool air.

I'm thinking bad monovalve or bad monovalve ground in the ACC unit, but why do you think it's only happening at idle and then fixing itself at speed?

Thanks!

__________________
cbjukraine
'84 300D
Reply With Quote
  #2  
Old 07-08-2014, 11:30 PM
vstech's Avatar
DD MOD, HVAC,MCP,Mac,GMAC
 
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,835
My guess is torn mono valve, and poor ac performance from r134 conversion.
__________________
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
  #3  
Old 07-09-2014, 03:01 AM
Posting since Jan 2000
 
Join Date: Sep 2011
Posts: 7,166
Quote:
Originally Posted by cbjukraine View Post
Very weird, the AC will intermittently blast me with hot air, but only when I come to a stop at a light and idle for a minute or two, never when rolling down the road. After a minute when the AC has cooled off the heater core again do I get cool air.

I'm thinking bad monovalve or bad monovalve ground in the ACC unit, but why do you think it's only happening at idle and then fixing itself at speed?

Thanks!
Pull out the glovebox liner. Look for a hose horizontally to the sensor at the top center of the instrument panel. This hose commonly disintegrates and disappears. If it is gone or leaking, use foam pipe insulation from Home Depot to fashion a new one. Don't us hard plastic hose, due to its thermal properties.
__________________
2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution
Reply With Quote
  #4  
Old 07-09-2014, 07:09 AM
JimFreeh's Avatar
Benz addict
 
Join Date: Mar 2007
Location: Hampton Roads, Virginia
Posts: 3,366
Quote:
Originally Posted by vstech View Post
My guess is torn mono valve, and poor ac performance from r134 conversion.
Agree.

Heat on = mono valve passing coolant.
Should be off when A/C is on.

You can test by feeling the solenoid "thunk" when +12 is applied.
At this age, it's common to find a torn rubber diaphragm inside, as well.

Variable cooling = poor A/C performance.

Best A/C conditions require compressor rpm AND a good airflow across the condenser.

You have neither when idling.

Larry suggested checking the hose that siphons air over to the ambient temp sensor. These hoses degrade over time and as a result, the sampling air comes from inside the dash.... However, since you're getting cold air when moving, I think your problem lies elsewhere.

There are a number of possibilities for your A/C issues.

Electric fan not coming on.
Poor conversion....
Moisture in system
Receiver dryer not working
System not vacuumed before conversion gas installed
Did not install R-134A throttle valve (controls liquid flow to evap)

When poor performnce is encoutered, it's best to back up and start again, replacing the receiver/drier and the throttle valve (if not already done).

A complete system vacuum, running for at least an hour, then shut off pump and close valves and moniter vacuum. Any loss requires additional work before the system will be good.

Also consider the oil. Whenever you replace a component in the A/C system, a certain amount of oil will be lost. This must be replaced. then there is the type of oil to use, search the archives for lots of opinions...

R-134a backfits typically take 85% of an R-12 fill. R-134a systems are very sensitive to fill amounts. With R-12 you got a sinusoidal wave type of cooling graph (gradually increase cooling effect as charge increases) whereas R-134a operates more as a step function (nothing, nothing, nothing, bang! cooling!).

I've found it very consistent to monitor low pressure vapor return line for temperature. When you approach the correct amount of R-134a, you can feel a noticable drop in the return temperature. Also, on the W123 cars, this return line runs right across the front of engine, uninsulated. For my W123 cars, I used a piece of pipe insulation to cover return line, to ensure a cooler compressor inlet temp.

Good luck!

Jim
__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles
Reply With Quote
  #5  
Old 07-09-2014, 09:54 AM
Registered User
 
Join Date: Sep 2004
Posts: 8,971
Quote:
Originally Posted by JimFreeh View Post

Heat on = mono valve passing coolant.
Should be off when A/C is on.
That is not correct. Unless MIN is selected on the temperature control, the ACC uses heat to regulate temperature, so it is entirely normal for the monovalve to be operating concurrent with the a/c.
__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.
Reply With Quote
  #6  
Old 07-09-2014, 01:47 PM
JimFreeh's Avatar
Benz addict
 
Join Date: Mar 2007
Location: Hampton Roads, Virginia
Posts: 3,366
Quote:
Originally Posted by tangofox007 View Post
That is not correct. Unless MIN is selected on the temperature control, the ACC uses heat to regulate temperature, so it is entirely normal for the monovalve to be operating concurrent with the a/c.

Agreed, the A/C part still stands.

I had assumed he was already on min because of the A/C issue.

Bad assumption on my part, and worse to have not specified it when suggesting a solution...

Jim
__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles
Reply With Quote
  #7  
Old 07-09-2014, 03:06 PM
Registered User
 
Join Date: Aug 2010
Location: Sacramento, CA
Posts: 3,115
If the "hot air" is hotter than ambient in cabin, it must be from the heater. Try clamping off the heater hose from the head, near oil filter. This assumes your temp dial has been set to "min".

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 02:20 AM.


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