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Cooling fans

Tiny
RLWEBB
  • MEMBER
  • 1999 CADILLAC DEVILLE
  • 4.6L
  • V8
  • FWD
  • AUTOMATIC
  • 97,000 MILES
Car keeps over heating. The cooling fans will only come on if you turn on the heater or air conditioner. I have replaced the temperature sending unit. What would cause the fans not to come on? Fans motors are fine when you unplug the sending unit the fans run on high speed. Please help.
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Wednesday, February 15th, 2017 AT 9:32 AM

5 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,414 POSTS
Check all three fan relays for power and grounds. They work together to operate the fans based on signals from the PCM and the A/C controls.
Also, verify that the ground for the right fan is good (it should be as the fans do run)
To test further requires a scan tool with bidirectional controls to make the PCM control the fans to verify that it is sending those signals.
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Monday, June 1st, 2020 AT 2:39 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,414 POSTS
Forgot to add the wiring diagram.
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Monday, June 1st, 2020 AT 2:39 PM
Tiny
RLWEBB
  • MEMBER
  • 3 POSTS
Okay, I will try all that. Thank you so much and I will let you know what happens.
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Monday, June 1st, 2020 AT 2:39 PM
Tiny
RLWEBB
  • MEMBER
  • 3 POSTS
Ok had a friend bring home a snap on scan tool computer and we was able to run the funs with the scan tool and everything worked so where does that leave me I'm lost on what to do help please help
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Monday, June 1st, 2020 AT 2:39 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,414 POSTS
If they work with the scan tool the issue has to be sensor or BCM related. You could watch the scan tool live data to see what the sensor is actually reporting, The connector on them likes to fail as well.
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Monday, June 1st, 2020 AT 2:39 PM

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