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 > Tech Help

LinkBack Thread Tools Display Modes
Old 05-20-2009, 07:32 PM
Registered User
Join Date: Dec 2005
Location: Arizona
Posts: 24
Compressor relay bypass??

Hi have 1988 300SE W126. The a/c is cold and works fine but the compressor disengages after a few miles and won't come back on until I turn off the car and restart it. Again in a few miles (if I'm lucky) it is off again. After reading several posts about all the reasons the compressor will disengage, I tried a few but it keeps happening. I then thought I would wire in a temporary switch between terminals 15 and 87 at the relay connector and leave the compressor relay out. Turning the switch on made the compressor start, but it still quit after a few miles. Now I can't get it to come on at all. There seems to be at least 5 wires going into the terminal on the compressor. Does anyone know if I can bypass everything and supply 12volts to the compressor energizing coil to engage it? I live in Phoenix at 105 degrees. I would really like to just force it on when I need it. Thanks!!
Reply With Quote
Old 05-20-2009, 07:52 PM
Registered User
Join Date: Oct 2005
Posts: 4,263
Don't run it without an evaporator temperature sensor in the circuit, or you could freeze your evaporator into a block of ice.

Check for 12V at the compressor when it fails. If you have 12V, tap the clutch with a screwdriver handle to see if it engages.
Reply With Quote
Old 05-20-2009, 08:24 PM
Registered User
Join Date: Dec 2005
Location: Arizona
Posts: 24
Which wire at the compressor do I check for 12V? There is at least 5 of them.
Reply With Quote
Old 05-20-2009, 08:43 PM
Registered User
Join Date: Oct 2005
Posts: 4,263
You did mention that; I missed it.

You'll have to find out which of them goes to the clutch coil. Of course, that many wires indicates that you have a speed sensor, so if the clutch slips the current to the clutch coil will be cut.

How big is the clutch gap? It should be around .020". If that's fine, another possibility is a dirty speed sensor.
Reply With Quote
Old 05-20-2009, 09:20 PM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
If you have a sw bridging 15 and 87 @ the relay , then speed sensors , pressure limit cut-out, temp cut-out, etc. circuits are all out of the circuit b/c that jumper is direct 12v from fuse F/7 to clutch and by-passes all sensor inputs to relay.
So, if the clutch is dropping , you have a poor contact at fuse 7 or a bad upstream feed to that fuse, or a bad clutch/gap.
The test would be to put a test lamp at 15 and ground and see if that goes out when clutch goes out. IF YES, that would indicate fuse/feed upstream problem..
If NO, that would indicate downstream problem....clutch/coil/wiring..
A Dalton

Last edited by Arthur Dalton; 05-20-2009 at 10:00 PM.
Reply With Quote
Old 05-21-2009, 09:07 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Send me a pm-mail OR e-mail & I will send you instructions.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator
Reply With Quote
Old 05-28-2009, 02:08 PM
Registered User
Join Date: Dec 2005
Location: Arizona
Posts: 24
Sorry, I haven't got back on this. That day after work I went out and the clutch engaged when I flipped my switch. It stayed on all the way home (45 miles) and seems to be working ever since. However if I turn the switch off while it is running, it doesn't seem to engage when I turn the switch on again unless the car has sat for quite a while. It sure is nice to have it work! I'll send a pm to MBDOC.

I'm not too worried about creating ice on the evap coil because the air here is very hot and dry.

Thanks for the input!
Reply With Quote


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 03:58 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page