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  #1  
Old 03-26-2002, 10:57 PM
pmizell's Avatar
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Location: Houston, Texas
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Aux fans not starting when "opening circuit"

Hey folks --

As I understand it, the coolant temp sensor on the M103 head that kicks in aux fans and cuts out AC compressor is an "open circuit" sensor which means when disconnected, the aux fans should come on.

I disconnected mine today with AC running and it did not start the aux fans. It does, however, start high speed aux fans at 115c when connected.

Is this sensor faulty or is there a bad relay switch somewhere?

Thanks as always for any insight!

~Paul

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'91 300E, 208k miles
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  #2  
Old 03-26-2002, 11:37 PM
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Location: Jax, FL
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Paul,

I think you have it backwards. If you have a "closed circuit" they should come on. Touch both leads, the fan should come on.
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'86 300E
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  #3  
Old 03-27-2002, 10:46 AM
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DOH! Thanks for clearing that up for me J.H.

I'm still learning about the electrical system of these cars and how they work, and how to test them. My next purchase is a multimeter.

Thanks again,

~Paul

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  #4  
Old 03-27-2002, 11:44 AM
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The aux fan for the a/c circuit has nothing to do with the sensor on the engine. It is not temp sensed, it is pressure sensed [ high side freon pressure]
It is located in the high side line near the drier/reciever. Jumper these will run low fan when a/c is on.
The high speed aux fan is temp sensed and is a thermistor circuit. When the sensor gets to the proper resistance [ temp], it triggers the relay circuit for high speed fan. These two fan operations are independent of one another.
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Old 03-27-2002, 12:00 PM
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Thanks Arthur for pointing that out ... I was referring to the '86 model 300E shop manual, so I suppose this circuit has changed since then. The sensor appears to be a 3 pole in the diagram, as opposed to the 2 pole that's on my '91.

The manual notes:

"As a protection against engine overheating an emergency cut-out is provided. After reaching approximately 115c engine coolant temperature, the switch is closed (grounded)(diagram pointing to head temp sensor) and the A/C compressor via control unit is cut-out. The switch opens at approx. 108c and the A/C compressor is then re-engaged."

~Paul

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  #6  
Old 03-27-2002, 12:26 PM
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Your '91 has the newer 2 wire thermistor circuit [ 105-107 C cut-in temp. Approx. 265 ohm]
An added note.
If the low speed fan works by jumpering the a/c pressure switch with a/c on, then it is advisable to check freon level, as a low level will not allow the high side pressure to reach the cut-in pressuer required to trip the switch.

[ in other words, don't codemn the switch until you know the level]
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