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#1
05-10-2003, 09:55 AM
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Aux fan not coming on, relays look good, what to check next?
Hello everybody.
94 E420 I noticed that the air coming out the the ducts was getting warm, once I stopped at a red light for more than a minute. Once moving again, the air was nice and cold again (cold enough to give me an "ice cream headache". So, I figured I need to check if the fans are running. ... Nope. Then I checked the fuses....All of them are good. Then I took of the relay cover, and eyeballed relays B and C. They looked good too (including the fuses. I even took the relays apart...they still looked good. Now I have come to the end of my rope. I did notice that the fans would come on once the engine temp. was way up there (around the 100° mark)...but that doesn't help me any when they turn back off. What should I check next? (on a somewhat related note... Once I pulled off the relay cover, the largest module there (it has a 126xxx part#) had a hole on the top of it... as if someone had pushed on it, and broke the plastic. Is it imperative that I cover this hole? and if so, with what? electrical tape? JB weld?) thanks! thanks for any help, -Larry
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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein 09 Jetta TDI 1985 300D 
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#2
05-10-2003, 10:10 AM
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Sound like you are slighly short of Refrigerant...
Fans come on if you jumper the high pressure sw at reciever/drier ?? If not, check dropping resistor..
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#3
05-10-2003, 01:09 PM
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Could be a few things...
I have the same vehicle and had the same problem. It could be 1 of 3 things.
1) Refrigerent low 2) Bad coolant level sensor  3) Aux. Fan Resistor, failing or has a rusted connection  4) Bad Coolant Temperature Sensor (picture borrowed from JimF's site :-) It's the blue one in the middle.  Check out the following interesting links: Auxillery Fan Problem and http://pages.prodigy.net/jforgione/MB_CTS.html When you disconnect the coolant Temp sensor, do the fans go on? If I remember correctly, they should if your relay is good. A new relay is around $25 and a temp sensor is also cheap. Get them both here. Willy
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Willy '94 E420 White Mods ------- Berg Werks Keyless Entry Buzzer replaced with Chime Dash Light Upgrade Burlwood/Leather shift knob Alpine CDA-9815 Head Unit 2002/2003 16" C Class 7-Spoke Wheels Updated "Chip" to fix Code 19 problem
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#5
05-10-2003, 05:12 PM
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One wants to remember that there are two fan circuits...
They both share the same fan motor/s, but have nothing to do with one anothers basic operation..each circuit has it's own fuses, relay. sensors.. The high fan is switched by the ECT circuit and is an aid to the radiator in controlling high Engine running temps. The low speed is switched on by a high pressure switch at the reciever/drier of the a/c unit and it purpose is to aid the a/c systems high side pressure from getting too high by pushing some air throught the a/c condenser.. So, a couple of easy test and what they tell you.. Does the Fan motor work??? By removing the ECT connector, the fan should come on. *note: [ this will not work if one has installed a temp lowering resistor bridge, see JimF site] If the fan comes on, you have tested the integrity of the high speed fan circuit. But the main reason for this test when looking into a/c fan circuit is just to verify that the fan motor/s are good.. So, you now go to the pressure sw at the a/c drier. jump this sw and look for fan run. If good , do NOT assume you have a bad pressure sw. This sw only comes on when the high side reaches a hefty reading [ as in high ambient temp/load]. The #1 cause here can be LOW freon [ enough to cool, but not enough to get the high side up to cut-in pressure called for by the sw specs]. The common symptom here will be crap cooling at stop/idle [ no air flow]..better cooling at speed BC you have air flow from car movement Jumping the sw also test the rest of the low fan circuit. No fan- start looking at fuses Another easy test [ this applies to both circuits]: If you dissconnect the ect for high fan test or jump high pressure sw for low fan test and hear the relays CLICK, but NO fan.. you more than likely have a fuse blown on the load side of the circuit [ these are the large amp ones , usually on top of the relays or outside the fuse box] The exception to this is that the low fan also has a dropping resistor [ behind driver headlamp] that can be open [ common to see wires burnt off here].. A note here .. Benz has allowed increasing the value of the low fan fuse , as the original was under-rated. [ I always check this first on No Low a/c fan BC of this] You would assume low fan to draw less , but BC they are using a dropping resistor in the circuit to accomplish the lower speed, it actually draws more.. They allow 25A replacement.. Last edited by Arthur Dalton; 05-10-2003 at 05:49 PM.
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#6
05-11-2003, 11:29 AM
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Another point of interest concerning the high pressure a/c high side sw..et al...
The only way to accurately diagnose this sw is with gauge reading/spec. chart comparision.. Many DIYers jump the sw and see the fan come on, so they go ahead and replace the sw. They then recharge the sytem with the proper amount of charge and everthing seems to work, justifying the sw replacement .. In reality , they have simply gotten the low freon level back to spec. with the re-charge and the sw change-out was not needed.. the old sw would have reacted the same as new once the freon charge was up to spec for fan operation... Kinda like the loose belt/need alternator gig... Relaced alt and everthing working well again..musta' needed it .. Nope, the belt got tightened back to spec when you put the new alt on... you had a loose belt... Last edited by Arthur Dalton; 05-11-2003 at 12:10 PM.
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#7
05-11-2003, 10:04 PM
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Thank you for the replies!
 I jumped the pressure switch, and nothing happened. Couldn't tell you if the relay(s) clicked, since the engine was running. I know for sure that the high speed fan works, as I could clearly hear it running earlier today. I will check the other gadgets that Willy pointed out. I do have an old fashioned meter to do some testing with. Since I know that at least one relay is working, I suppose I could swap them with each other, to see if the fan comes on, once I jump the pressure switch. Agreed? thanks! -Larry
__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein 09 Jetta TDI 1985 300D
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#8
05-11-2003, 10:21 PM
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Agreed
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#9
05-31-2003, 05:48 PM
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okay guys,
here is where I stand. I too have been having the running-hot problem with my 94 E420. I've been searching around this board and reading about the apparently infamous aux fan issues. so, my fans weren't turning on. I disconnected the CT sensor and presto, both fans kicked on. Thus my relay is good (..right?). And just for clarification the relay that is involved in this is the dark olive green one that has an integral 30A spade fuse (PN 001 542 9619 if I am not mistaken). In any case it DOES click. I then proceeded to the dropping resistor. The top terminal was rusted and during the removal process the wire broke. No big deal, I removed the resistor, cleaned up the terminals with a little help from the dremmel, and then crimped a new connector on the wire. Screwed everything back in place and then let the car stand, running. The temp crept up and then BAM! at around 100ish both fans turned on! YES! Thus I can deduce that the CT sensor itself is okay (once again...right?). Here is what’s happening next. I have full charge on the AC because I just had it refilled (2.2 lbs. of R134 I believe) BUT the AC isn’t really cold. I am assuming the other half of the aux fans job is not working. Correct me if I am wrong, but isn’t one of the aux fans supposed to turn on to help the AC? What should I check now? what and where is the high pressure sw [sensor?] and where is the receiver/drier? Which relay controls this circuit? thanks
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#11
05-31-2003, 06:48 PM
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Arthur> are you saying that the fans won't come on if the refregerant isn't at full charge even if the a/c is OFF? That's what it sounds like you are saying, and you may be right, but how does that make any sense? If the coolant temp is too high there should be a separate system that turns on the fans.
Another question - one person says the fans turn on by disconnecting the CT sensor, and you said its the ECT. Which one, if you happen to know, would control the fans on my '90 300CE with the 104 engine? Thanks, GregS '84 300D, 174k '90 300CE, 163k
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#12
05-31-2003, 07:01 PM
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<
>> Correct ...There are TWO fan circuits They just share the same fan/s motor.. The low speed fan is the a/c aux fan and is activated by the preresistor relay/ a/c pressure sw. So , if you are shy of freon, don't expect to see this fan come on...[ that is why we always jump the pressure sw first thing when concerned about a/c aux fan .. very common to have no fan w/freon shortage--this quick test tells the tech the integrity of the fan circuit] The other is high fan--- activated by high aux fan relay/ECT sensor ... this aux fan is for cooling the engine and has Nothing to do with low a/c fan circuit...different relay, sensor, fuse , etc,] ECT and CT are different names for Engine coolant temp/collant temp sensor ... Last edited by Arthur Dalton; 05-31-2003 at 07:29 PM.
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#13
06-01-2003, 01:21 PM
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okay, so here are a couple picts
Temp sensors With the car running, if I pull the middle plug (the round one) nothing happens. what SHOULD happen? When I pull the plug on the far right, both fans come on. High Pressure? Is this the high-pressure switch (the one with the red cap) that regulates the AC cooling circuit? If so, how exactly should I jump it? When I jump it, what SHOULD happen? If that doesn’t happen, what does it mean?
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