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  #1  
Old 08-07-2006, 02:01 PM
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Join Date: May 2004
Location: Columbus, OH
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400e aux fan / AC trouble

Having some cooling troubles - aux fans never run. I shorted the connector at the sensor screwed into the front of the engine and hear the relay activate, but no voltage at the connector plug leading to the fans just behind the driver headlight. I removed the fans and tested them - both working fine. I read on one of the numerous cooling threads that someone found a fuse inline between the relay and the fans, although I've been unable to find such a fuse (ABS unit removal is less than appealing). The fuse in the fuse box is of course fine. That's problem number 1. Problem number 2 is that after I reinstalled the fans, the AC now blows warm air (worked before). Compressor doesn't activate. I took care not to damage the radiator or condenser when working on it, and the AC system still has plenty of charge. Only thing I can think of is maybe the aux fan relay is bad even though it "sounds" like it works, and maybe I fouled up some of the AC wiring - can't see how I could have. Any suggestions would be most appreciated!

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  #2  
Old 08-07-2006, 04:19 PM
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Check out the dropping resistor for the fans just behind the drivers headlite...$40 part and with the M119, they often get cooked or they rust out at the connections. Just did mine actually.
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1992 500E
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Old 07-07-2007, 12:48 AM
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Solution to aux fan problem

Thanks for the reply. For those who may review this brief thread ... the problem turned out to be no +12 voltage at the relay. The relay schematic shows two pins that are connected internally. A meter showed no voltage at either sleeve with the relay removed / key in ON position. Options appeared to be (1) pull the whole fuseblock (yikes!), or my choice, (2) create a double-insulated adequate guage wire between the protected side of the fuse and and one of the relay posts. Some may view this as a hack job, but I'm confident in it's safety and reliability.
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Old 07-07-2007, 07:29 AM
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Join Date: Aug 2001
Location: Florida
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Hey guy,
I don't understand if you can hear the relay click then there is voltage when the sensor tells the relay to deploy. Have you checked the resistor behind the headlight drivers side. That is still the likely culprit from what you first and second post indicate.Good luck.
Brian.

96 S320
92 400E
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  #5  
Old 07-07-2007, 07:27 PM
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There is a metal strip fuse right by the left shock mount in a little black plastic rectangular box. Mine gave up once, replaced it and everything was back to normal. Check this first.
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'96 E300D 162,000 miles
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  #6  
Old 07-07-2007, 08:08 PM
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The fuse in the fuse box is the feed for sensor side of the relay..[ pull-in coil]. You know that is OK b/c you jumpered a sensor and the relay clicks. The other side of the relay is the LOAD side [ fan motor Contacts]. That is the pins you have no voltage at..that has an external fuse in a seperate box out side the fuse box.
...look to the right of the fuse box .

On Relays:

A clicking relay simply means the solinoid/coil in the relay is pulling in the MAIN CONTACTOR [ that is the mechanical clicking you hear]... but if the fuse that protects the main load [ in this case , the fan motor] is no good , you have NO FAN even though the relay is clicking away..No power to the contactor section of the relay= No power to the device the relay turns ON/OFF..
The relay has a low amp side [ coil] and a high amp side
[ load/contacts] ...each side has a seperate fuse for protection of that circuit. That is what a relay does ..it lets a low amp sensor/switch turn on a High amp switch
[ contactor ] ..a relay is a switched switch..
If one did not have a relay , the sensor would not be able to handle the high amp load of the fan motor, so that is why the circuit requires a relay .. low amp sensors triggering high amp motor.
I may as well throw this in too..
Knowing this about the relays , you can see that if one jumpers a sensor and does not hear the relay clicking, you know that the sensors side [ coil side ] of the relay circuit is where to start looking..but if the relay clicks when jumping a sensor, you know the sensor side of the relay circuit is verified and the place to look is on the LOAD side [ main contacts] of the relay..just a little tip to help narrow it down real fast....The most common fuse to blow is always on the LOAD side b/c of the high amp draw . The sensor side draws very little and is less likely to have a bad feed b/c of a fuse [ but does happen]

Last edited by Arthur Dalton; 07-07-2007 at 08:45 PM.
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  #7  
Old 07-07-2007, 10:04 PM
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Join Date: Aug 2001
Location: Houston, Texas
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If your car's a/c control is like my 87, 260E, then I have posted a step by step procedure to check out the a/c compressor operation from the Klima relay socket.

http://www.peachparts.com/shopforum/showthread.php?t=158591&page=5

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