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 07-26-2011, 02:56 PM
Richard Howard
 
Join Date: Nov 2008
Location: Western NC Mountains
Posts: 69
84 300SD - AC Compressor No-Op - Fuse 11 Hot

I've read very many of the threads in the AC Problems "Sticky" but haven't come close to finding a solution to an AC problem. Details follow:

Car is an 84 300SD (no Klima) and I the 3rd owner, finally getting around to making the AC work. Heat, fans, vacuum servos all work correctly but the compressor clutch will not pull in with CCU control.

1) I solvent-flushed and air-dried all lines, evap., condenser, exp.valve, etc. Replaced all o-rings with new, lubed w/refrigerant oil. New rcvr/dryer installed.
2) The CCU was replaced with a rebuilt unit from the Boca Raton FL company.
3) I took the car to a reputable MB indy to have the system evacuated and recharged.
4) Vacuum pull-down was good, running for an hour. Held 26" Hg with no loss. No obvious leaks.
5) With the CCU set to max cool, a new compressor clutch relay in position 6, the pressure switch on the receiver/dryer jumped, the clutch would not pull in while the engine was running. Time-delay was well beyond time with still no clutch engagement. Fuses 11 and 10 are good, contacts are clean.
7) Clutch coil resistance measured 3.8 ohms. No high-resistance short to ground in clutch coil windings.
8) The system was then charged with 2.3 pounds R134a.
9) The shop technician (not the owner) then jumped clutch relay socket contacts 80 to 87 to make the compressor clutch pull-in. The compressor clutch then pulled-in.
10) Now the interesting and puzzling problem: WITH CLUTCH RELAY PIN CONTACTS 80 AND 87 MADE COMMON TO FEED 12 VOLTS TO THE CLUTCH COIL, FUSE 11 BECAME VERY HOT, MELTING THE SOLDER FROM AN OLD-STYLE 25AMP BUSS GLASS BODY FUSE. At this point work stopped and the refrigerant was recaptured.

I do not have a wiring schematic for the AC system and am wondering what the contacts in the compressor clutch relay connect to. The fuse 11 overheating clearly indicates over-current draw somewhere, but where?

Any ideas or comments as to where the problem could be will be much appreciated. Thanks too for time spent trying to assist with ideas to work with.

__________________
Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951
Reply With Quote
  #2  
Old 07-27-2011, 10:08 AM
Richard Howard
 
Join Date: Nov 2008
Location: Western NC Mountains
Posts: 69
The ETR Failed In The Open Position

Thanks to Roy Hunter's help, I found the ETR / evaporator temperature regulator switch on the side of the evaporator case. I jumpered its wires and with the receiver-dryer pressure switch's leads also connected via jumper, the CCU set to max cool, and key on, the compressor clutch pulled in.

An ohmmeter connected across the ETR's contacts showed it had failed in the open position, as if the evaporator's temperature was below 36 degrees F.

A new ETR has been ordered from our FastLane, so at least the compressor no-op situation will soon be fixed.

Now to determine why fuse 11 overheats, or whether it will no longer do so with a new ETR in the clutch relay's circuits.

We are very fortunate to have Roy Hunter's experience available to us when forum members aren't able to provide answers.
__________________
Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951
Reply With Quote
  #3  
Old 07-27-2011, 11:09 AM
Registered User
 
Join Date: Jun 2004
Location: West of Ft. Worth. TX
Posts: 4,186
Schematic

Here's a schematic of the Compressor Clutch Control Wiring Circuit:
From what I can make of it, there is either a shorted wire (pinched) or something wrong with the temperature controller. I would suspect a pinched wire, as the input to the temperature controller is for a feedback circuit. Another possibility is something awry at the compressor relay. Probably the easiest way to check it, is to remove fuse #11, disconnect the compressor clutch and see what your resistance values to ground are. BTW, the temperature controller is above the passenger's footwell.
Attached Files
File Type: pdf IMG.pdf (567.7 KB, 316 views)
__________________
Sam

84 300SD 350K+ miles ( Blue Belle )
Reply With Quote
  #4  
Old 07-27-2011, 11:18 AM
Registered User
 
Join Date: Sep 2001
Location: central Texas
Posts: 17,281
Good work.... and described in a fashion that is readable...

A couple of notes for others about to do this sort of thing...
as Vstech has mentioned several times.... and I suspect in the sticky above... that vacuum is not a sufficient way to check for leaks before committing what may be expensive refrigerant to the system... there is a built in leak at the front compressor shaft seal... AND in a vacuum the orings are stressed in the opposite direction from the working condition .. the way you do not want them to leak... AND an hour is not generally a long enough time to really be sure all the moisture is evacuated from the distant recesses of the convoluted system...
It is recommended that pressure .... a dry inert gas be used for leak testing... nitrogen and argon being common...with four oz of R22 installed with it...so that a sniffer can be used to find possible leaks.. dye WILL NOT find a leak in the evaporator ( that you can see reasonably ).... but a sniffer will go off like Xmass tree lights...
You need to find a wiring diagram for YOUR car soon... perhaps NEXT in this process ....

But off hand it seems like a short or restriction ... meaning broken wires or such ...that increase the resistance and cause too much pull for the proper fuse to deal with is the next physical problem to look at...and that will be helped by the diagram...High resistance can occur without there being a short... just a lack of copper/or corrosion for the load to pull the needed power through...

Edit... OH GOOD... a wiring diagram delivered to you... can't beat that service...
Reply With Quote
  #5  
Old 07-27-2011, 12:51 PM
Richard Howard
 
Join Date: Nov 2008
Location: Western NC Mountains
Posts: 69
Good Information Is Valuable!

Thanks, SD Blue and leathermang, for the clutch relay schematic and helpful comments.

__________________
Respect, protect, and maintain complex systems - whether natural or of German design, to benefit from their full potential.

1982 300SD W126.120 with an OM617.950 from a W116.120
1984 300SD W126.120 with an OM617.951
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 06:17 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