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 07-21-2020, 06:20 PM
Registered User
 
Join Date: Nov 2018
Location: Dallas, TX
Posts: 395
Suddenly no ground to AC

Hi,

1992 300D.
Out of the blue my AC turned off at a stop light, then came on a minute later.
The next day it worked, but cut off after a few minutes of driving. Now it works for a minute then dies. Comes on for a few seconds while driving.
I performed the diagnostic from FSM.
There is no ground from the CCU (pushbutton unit) to pigtail on high pressure switch. I tested that with 3 different CCUs.
I replaced the evaporator sensor, no change.
What should I do next?
AC charge is good, checked. If I provide ground to high pressure switch, everything is cold.
Aux fans are operating good. Belt is not slipping.
Thanks

Reply With Quote
  #2  
Old 07-21-2020, 07:13 PM
Diseasel300's Avatar
Registered User
 
Join Date: May 2016
Posts: 6,025
Does your tach work? OVP relay good? Next stop is the Klima relay.
Reply With Quote
  #3  
Old 07-21-2020, 07:24 PM
Registered User
 
Join Date: Nov 2018
Location: Dallas, TX
Posts: 395
Quote:
Originally Posted by Diseasel300 View Post
Does your tach work? OVP relay good? Next stop is the Klima relay.
Yes, it works well. OVP is new with two good fuses.

I don't understand how Klima relay would be affected, since the signal is interrupted before reaching that relay
Reply With Quote
  #4  
Old 07-21-2020, 07:38 PM
Diseasel300's Avatar
Registered User
 
Join Date: May 2016
Posts: 6,025
Do you actually have 12V coming from the Klima relay? The CCU switches via a transistor, it requires current to flow. You'll get a better idea what's going on by measuring voltage with the wires still connected to the pressure switch. Measure to ground. If you have 12V present, the CCU isn't commanding the compressor on or you have a wire broken somewhere.
Reply With Quote
  #5  
Old 07-21-2020, 08:22 PM
Registered User
 
Join Date: Nov 2018
Location: Dallas, TX
Posts: 395
Another thing is that aux fans are on high setting (bypassing pre-resistor) when the compressor cuts out. Even when I have jumped the switch for fans, it runs on low setting, then compressor cuts out and fans go on high. So it seems CCU is reading high temperature out of sudden and decides to cut out the AC and cool down the system?
I took off the connector from temperature sensor and nothing happened. Maybe it is bad, giving incorrect signal? Engine temp about 80C, ambient 110F.
Reply With Quote
  #6  
Old 07-21-2020, 08:29 PM
Registered User
 
Join Date: Nov 2018
Location: Dallas, TX
Posts: 395
Quote:
Originally Posted by Diseasel300 View Post
Do you actually have 12V coming from the Klima relay? The CCU switches via a transistor, it requires current to flow. You'll get a better idea what's going on by measuring voltage with the wires still connected to the pressure switch. Measure to ground. If you have 12V present, the CCU isn't commanding the compressor on or you have a wire broken somewhere.
Yes 12V on the pigtail from Klima.
Reply With Quote
  #7  
Old 07-21-2020, 08:45 PM
Diseasel300's Avatar
Registered User
 
Join Date: May 2016
Posts: 6,025
The fans on high is a big clue. The CCU is in charge of the high fan speed on these later cars. If the temperature goes high, the compressor is locked out and the fans are forced on high. The sensor in the head may be bad, or you could have a bad wire in the wiring harness. Has your harness been replaced, or is it still the biodegradable junk?
Reply With Quote
  #8  
Old 07-22-2020, 12:44 AM
Registered User
 
Join Date: Nov 2018
Location: Dallas, TX
Posts: 395
Quote:
Originally Posted by Diseasel300 View Post
The fans on high is a big clue. The CCU is in charge of the high fan speed on these later cars. If the temperature goes high, the compressor is locked out and the fans are forced on high. The sensor in the head may be bad, or you could have a bad wire in the wiring harness. Has your harness been replaced, or is it still the biodegradable junk?
Old harness AFAIK.
Yes, I will measure sensor resistance on cold engine then place a resistor to the connector once engine is hot and AC is dead. That should kill the fans and turn on the AC if the sensor is faulty.
Update: replaced the coolant temp sensor and everything is fine. Looks like $6 chinese sensor lasts about a year.


Last edited by filp; 07-22-2020 at 01:43 PM.
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 05:45 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