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1991 300D 2.5 Turbo 305k, 2001 S600 110k, 2007 GL320CDI 4Matic 155k, 2007 E320 Bluetec 160k
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139 Posts
Discussion Starter #1
My cooling fans have not come on for the past 5 years or so. Recently, I tried checking the fuses and unplugging the plug next to the thermostat - neither worked. Does anyone have any suggestions as to what is wrong? (besides broken fan motors)
 

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american 1986 300E 3.0l
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107 Posts
5 years i would have to say they might be siezed. check to see if they spin by hand.
mine are siezed now after 2 yrs of not coming on.
 

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1991 300D 2.5 Turbo 305k, 2001 S600 110k, 2007 GL320CDI 4Matic 155k, 2007 E320 Bluetec 160k
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139 Posts
Discussion Starter #3
The fans spin easily.
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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17,368 Posts
Replace the aux fan resistor behind the driver headlight before replacing anything else. Use BERU brand only. After that, start troubleshooting sensors, relays, temp sender units, aux fan wiring harness.

The fans do have a finite lifespan, but the passenger side tends to go first. If both fans are inop, aux fan resistor is usually #1 suspect.

I assume you did check the fuse on the AC relay, correct???
 

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Registered
1991 300D 2.5 Turbo 305k, 2001 S600 110k, 2007 GL320CDI 4Matic 155k, 2007 E320 Bluetec 160k
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139 Posts
Discussion Starter #5
Where can I pick this part up?
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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17,368 Posts
MB dealer, autohausaz.com, germanstar.net, Rusty (do a search on this forum for contact info).

Be sure you checked/replaced the blade fuses on the high & low speed relays before replacing the aux fan resistor.

If the fans work at high speed, the problem is usually NOT the resistor.
 

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Registered
1991 300D 2.5 Turbo 305k, 2001 S600 110k, 2007 GL320CDI 4Matic 155k, 2007 E320 Bluetec 160k
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139 Posts
Discussion Starter #7
I'm sorry I'm so unfamiliar with the aux fan system... Where are the high/low speed relays/fuses?
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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17,368 Posts
I'm sorry I'm so unfamiliar with the aux fan system... Where are the high/low speed relays/fuses?
Rear compartment of the fuse box. Front compartment contains the vehicle fuses, rear compartment has the relays. You will need a stubby and/or adjustable philllips screw driver to get the rear compartment lid off.

If you have not done so already, I highly recommend replacing ALL vehicle fuses with the updated non-corroding copper core type. Many electrical issues have been solved with new fuses.
 

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Registered
1991 300D 2.5 Turbo 305k, 2001 S600 110k, 2007 GL320CDI 4Matic 155k, 2007 E320 Bluetec 160k
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139 Posts
Discussion Starter #9
Thank you for your help. I will check the fuses tomorrow and go from there.
 

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91 500SL 37k,90 300SE sold, 91 300E sold , 92 300E137k ,02 Kia Sedona ,I know..... its a Mini van.
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8,143 Posts
Hey don't forget to pull the connection on center C temp sensor,once the car is warmed up,pull the plug and the fans should come on,easy test!
 

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1991 300D 2.5 Turbo 305k, 2001 S600 110k, 2007 GL320CDI 4Matic 155k, 2007 E320 Bluetec 160k
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139 Posts
Discussion Starter #11
What center console temperature sensor?
 

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384 Posts
I suspect it's a diesel, in which case it's a switch instead of a sensor, by the upper radiator hose. Shorting it on should put the fan on high, which bypasses the resistor.
 

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91 500SL 37k,90 300SE sold, 91 300E sold , 92 300E137k ,02 Kia Sedona ,I know..... its a Mini van.
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8,143 Posts
What center console temperature sensor?
Not console ,center coolant temp sensor top of head in engine bay, 2 prong,pull it,the electrical connector,the fan's will kick on,remember to warm up the car first,both should come on,if only one does,the other one is bad and you can replace it
Your Parts Search Returned 1 Part(s)
 

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Registered
1991 300D 2.5 Turbo 305k, 2001 S600 110k, 2007 GL320CDI 4Matic 155k, 2007 E320 Bluetec 160k
Joined
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139 Posts
Discussion Starter #16
I found the sensor but could not get the connector off. I tried everything, screwdrover, pliers, my hands.... I could not get it off. I know it is something simple but I'd like to know before I break it off.
 

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Registered
91 500SL 37k,90 300SE sold, 91 300E sold , 92 300E137k ,02 Kia Sedona ,I know..... its a Mini van.
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8,143 Posts
Its the electrical connection your pulling off 2 wire,if it breaks I know how to fix the connector,mine fell apart! available at Lowe's!
 

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1991 300D 2.5 Turbo 305k, 2001 S600 110k, 2007 GL320CDI 4Matic 155k, 2007 E320 Bluetec 160k
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139 Posts
Discussion Starter #20
Still can't get it off... it looks like its corroded on.
 
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