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  #1  
Old 06-17-2009, 09:21 PM
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Aux Fan Temp switch

Howdy
I dont think my aux fans for the 90 300d are turning on like they are supposed too. When coolant temp hits 105 or so, the fan doesnt turn on. When i touch the 2 wires together that go to the switch, the fans turn on and stay on till temp goes down.
The temp switch was replaced a couple months ago. The 2 wires that go to the switch lost their holder. Does it matter which of the wires goes to which post?
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1983 300SD 70,000 WVO Powered miles
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Last edited by VegStew; 06-17-2009 at 11:30 PM.
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  #2  
Old 06-17-2009, 09:23 PM
79Mercy's Avatar
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No, the wire order doesn't matter. I'd replace the switch again.
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1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

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  #3  
Old 06-17-2009, 09:30 PM
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I don't think all MB's got a coolant temp sensor to turn on the aux fan on. I think the sensor you are looking at is for the A/C drier temp, and turns the aux fan on when the A/C refrigerant gets over something like 105*F degrees.
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1983 Mercedes 240D Turbo, 131bar injectors, Cosworth intercooler and 63' Ford Falcon radiator, Ardic Parking heater, Headlight wipers, Best 38.6mpg.

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  #4  
Old 06-17-2009, 09:47 PM
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There are different temp. switches. You can get one that comes on early or late. I went through all this crap a few months ago. There are a couple other things that could be wrong. But, I forget what they are. Do a search or click on my name and "find all posts by snookwhaler". It is in there somewhere.

What I did was ohm out the switch on the car to test it. Then I removed the switch from the top of the drier (You can remove the switch without losing freon). I heated some water in a pot and with a thermometer and suspended the tip of the switch in the water. As the temp. rose I was watching the water temp. and the ohm meter. It did close eventually. But, it had to get pretty hot.

You can use a switch that comes on earlier. IE: a cooler temp.

I know on the 85's there is a switch up on the thermostat housing that turns the Aux. fan on when the engine gets too hot as well, and in conjunction with the temp. switch over on the drier. I'm pretty sure your 83 has both as well. There are wiring diagrams involved here too. That is where I found most of the answers.
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Last edited by snookwhaler; 06-17-2009 at 09:52 PM.
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  #5  
Old 06-17-2009, 11:16 PM
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Quote:
Originally Posted by VegStew View Post
Howdy
I dont think my aux fans are turning on like they are supposed too. When coolant temp hits 105 or so, the fan doesnt turn on. When i touch the 2 wires together that go to the switch, the fans turn on and stay on till temp goes down.
The temp switch was replaced a couple months ago. The 2 wires that go to the switch lost their holder. Does it matter which of the wires goes to which post?
Which car?
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  #6  
Old 06-17-2009, 11:29 PM
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Right, Its the 90 300d. Its the temp switch for the aux fan (on top of the engine where the upper radiator hose connects) to turn on when the car gets up to 105. A quick look at fastlane shows a 3 prong switch. Mine only has 2. I suppose they could have the wrong pic up.
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  #7  
Old 06-17-2009, 11:53 PM
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Quote:
Originally Posted by VegStew View Post
Right, Its the 90 300d. Its the temp switch for the aux fan (on top of the engine where the upper radiator hose connects) to turn on when the car gets up to 105. A quick look at fastlane shows a 3 prong switch. Mine only has 2. I suppose they could have the wrong pic up.
The '87 with the OM603 uses the 3 prong. As you know your car uses the 2 prong. Have you confirmed that the temp send is working? Do you have a IR thermometer you can shoot the switch with?

Have you tried jumping the switch to confirm that the fan(s) work?
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“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss
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  #8  
Old 06-18-2009, 12:38 AM
compress ignite's Avatar
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Unplug the connector of the Blue Two Prong Temp Switch

At the Top Radiator hose fitting on top of the Head.
With the KO (Key On) and the two wire connection separated the Auxiliary Fans
(IN Front of the Radiator) should operate on High Speed!

Report Back (as AD says)



"And that sucker don't trigger the High Fans @ 105C (It's more like 130C)

BUT you can acquire Mercedes Part # 006 545 40 24.
Which is a Red Two Pole Temperature Sensor that triggers @ 100C
(to replace the Blue one you have)"

EDIT: (Through the kind services of GSXR's forum correction)
I must point out BOTH of the above statements are Incorrect

CORRECTION:
MB # 008 542 45 17 (Blue Two pole SENSOR) operates as low as 107 C.
AND MB # 006 545 40 24 is a SWITCH not a sensor...SO the two are not
interchangeable!
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Last edited by compress ignite; 07-12-2009 at 04:41 PM.
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  #9  
Old 06-18-2009, 12:42 AM
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Yup, i jumped the switch and the fans turn on. When i jump the switch at 105 degrees the fans turn on and stay till the temp goes down.
I have not confirmed that it works, i assume it does cause its new. I suppose it could be a bad new switch though. That does happen.

Im gonna rethink this. Maybe the switch turns the fans on at a temp higher then 105. Its just a little nerve racking seeing that temp gauge go so high.
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  #10  
Old 06-18-2009, 12:50 AM
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'Putting on GSXR's hat now

Have you cleaned your radiator and condenser by back flushing with H2O ?
How accurate is the Instrument Panel Temperature gauge?

(By the way the 105C figure is the Pressure switch that screws into the Reciever/Dryer AND if you "Jumper" those two wires [with the connector still attached to the sensor] you'll get Low speed fans)
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  #11  
Old 06-18-2009, 12:59 AM
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Quote:
Originally Posted by compress ignite View Post
Have you cleaned your radiator and condenser by back flushing with H2O ?
How accurate is the Instrument Panel Temperature gauge?

(By the way the 105C figure is the Pressure switch that screws into the Reciever/Dryer AND if you "Jumper" those two wires [with the connector still attached to the sensor] you'll get Low speed fans)
And: Is your viscous fan clutch working? The aux fan isn't going to do much to keep the engine cool. It will move some air over the condenser when sitting in traffic.
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“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss
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  #12  
Old 06-18-2009, 01:03 AM
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Temp gauge accuracy... not known. I jumped it the switch(with the connector still attached to the sensor) and the fans do indeed turn on, and stay on, till temp lowers to about 100.

Im taking it in this friday to have the system flushed and coolant replaced. I have not done this since i bought it (10000miles) and i dont know when the previous owner did.

Yes the fan clutch is working. It was replaced with the aux fan switch.

Compress, according to your IM, my high speed aux fan doesnt turn on till 135. I shall look for a different switch.
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1983 300SD 70,000 WVO Powered miles
1990 300D In driveway with Head Off - Cracked

Last edited by VegStew; 06-18-2009 at 01:10 AM.
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  #13  
Old 06-18-2009, 01:56 AM
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Symptomology not Electrical

You're running an almost 20 year old cooling system
OF UNKNOWN UPKEEP HISTORY
Do you have a Known Good Thermostat (Tested in boiling water on the stove?)
If you do not see cooler operating temps AFTER cleaning the outside of the
Radiator and the Condenser AND "SHOOTING" the Radiator at Operating temps
with an Infrared Thermometer [You're looking for COOL spots in the Radiator
IF there's a cool spot in front of the Viscous fan Clutch IT WILL NEVER ENGAGE!],It MAY be time for a new Radiator.

I'll give you a Big Hint...NOT (Normal Operating Temps) run around 80-85C
(Not in Death Valley,Nor on a Five Mile Steep Upgrade with Ambient @ 100+ F)

YOU HAVE A PHYSICAL PROBLEM IN YOUR COOLING SYSTEM!
It Ain't Electrical !
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  #14  
Old 06-18-2009, 04:33 AM
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Is there a relay in the aux fan circuit like in the 300D Turbo?
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  #15  
Old 06-18-2009, 08:46 AM
LarryBible
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MOST MB's actuate the aux fan based on high side pressure or temperature. I know that on an 88 300 E there is a red switch at the receiver/dryer for this.

This is very typical because the fan is used to bring down high side pressure to prevent a/c system strain. This is why you will often here the aux fan still running after you have turned off the engine, locked the car and are walking away. The high side pressure has not had time to equalize.
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