Radiator fan only turns on when the A/C is on

Tiny
MADDOGMIKEY
  • MEMBER
  • 1996 LINCOLN MARK VIII
  • 4.6L
  • V8
  • 2WD
  • AUTOMATIC
  • 140,000 MILES
With A/C running radiator fan comes on. Without A/C on fan motor refuses to come on. I think the fan motor has two speeds. Perhaps one does not work or perhaps sensor is bad. If I jumper both leads going to sensor should the fan come on? Will that be an easy test that the sensor or the fan motor is bad? Could still be in the wiring? The fan motor does not default and come on when the sensor is disconnected.
Friday, May 24th, 2019 AT 10:23 AM

10 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
Good afternoon,

Yes, try jumping the leads together?

If they come on, then the sensor is bad.

If they do not then there is a wiring issue.

I attached a wire diagram for you to view. Verify the 3 voltages to the unit. If you have all 3 signals, the control is bad. That is a very common failure. It is on top of the fan shroud

Roy
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Friday, May 24th, 2019 AT 12:18 PM
Tiny
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Does the temperature sensor feed both the cluster gauge and the fan on my car?
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Friday, May 24th, 2019 AT 1:40 PM
Tiny
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No, it feeds nothing.

It sends a signal to the PCM of the coolant temperature for fuel mapping.

Roy
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Friday, May 24th, 2019 AT 1:56 PM
Tiny
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Without AC on at what temperature should the fan come on?
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Friday, May 24th, 2019 AT 6:56 PM
Tiny
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212 degrees.

Roy
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Friday, May 24th, 2019 AT 7:02 PM
Tiny
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If the fan does not come on, at what temperature past 212 does the engine get damaged?
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Friday, May 24th, 2019 AT 8:00 PM
Tiny
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220 and above. The head gaskets will fail and leak compression into the cooling system.

Roy
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Saturday, May 25th, 2019 AT 4:24 AM
Tiny
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I clamped a cooking thermometer to the pipe that houses the sensor and ran the engine at idle up to 195 degrees without fan coming on. The car is 23 years old and I am reticent to press my luck to 212 degrees. Is there a way to fool the sensor to believe it is 212 degrees? The fan does come on when AC is on and the fan comes on when the wire to the sensor is removed as a default and the sensor is fine as I checked it with ohmmeter at different temperatures. Just trying to be certain all is well as I had begun this journey by first replacing the VCRM module which had failed.
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Saturday, May 25th, 2019 AT 11:18 AM
Tiny
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Thank you for your time and knowledge and willingness to share both. I bit the bullet and ran the temperature up to 205 when the fan came on. All is good and working. Thank you for your patience. Kindest regards, respectfully, Mikey.
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Saturday, May 25th, 2019 AT 8:46 PM
Tiny
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What was the resistance of the sensor at the different temperatures?

No, there is no way to fool it.

If you removed the wires and the fan came on, replace the sensor.

212 is a normal temperature for it to run. If you run it cooler, the fuel mapping will be off and the engine may run different and you will get less fuel mileage.

Roy
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Sunday, May 26th, 2019 AT 5:22 AM

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