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My cooling fan only runs with the A/C on

Tiny
CSM1992
  • MEMBER
  • 1987 BUICK CENTURY
  • 83,000 MILES
I recently acquired a 1987 Century with the 2.5L engine. The car runs great however the cooling fan only runs with the A/C on. I checked a basic wiring diagram and find that the wire from the ECM to the fan is grn/yellow. There is a plug near the fan hanging in space that has two wires: grn and grn/yellow. The problem is I can't find a home for it. The plug that goes on the fan has a blk and blk/red wire and there are no other plugs. I even checked at the parts store to see if it had the wrong fan and no it apparently has the correct fan. Traced the black and red wire up the harness a bit looking for another plug but no luck. Any suggestions?
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Tuesday, September 25th, 2012 AT 9:12 PM

14 Replies

Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
Does your temp gauge work?Also do you have a multimeter to do some testing?
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Tuesday, September 25th, 2012 AT 9:18 PM
Tiny
KHLOW2008
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Check the coolant fan switch located on upper left side of engine, at end of exhaust manifold. It has a Dark Green/White wire.
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Tuesday, September 25th, 2012 AT 9:24 PM
Tiny
CSM1992
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  • 6 POSTS
I am an above average shadetree mechanic and have tools etc. The plug in question has grn and grn/yellow wires. The coolant temp sensor has one wire attached.
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Tuesday, September 25th, 2012 AT 9:33 PM
Tiny
SATURNTECH9
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Post a pic of the connector showing it and where its located. The single wire is the gauge the two wires is the coolant temp sensor connector
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Tuesday, September 25th, 2012 AT 9:44 PM
Tiny
SATURNTECH9
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It says the temp sensor is on left side of engine bellow coolant outlet.
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Tuesday, September 25th, 2012 AT 9:47 PM
Tiny
CADIEMAN
  • MECHANIC
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Find the fan relay try swapping the relay with another 1.I would replace the coolant sensor that is what controls the fan oper. It grounds the fan relay to turn the fans on when the a/c is off.
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Tuesday, September 25th, 2012 AT 11:24 PM
Tiny
SATURNTECH9
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If the fan comes on with the ac I would say the fan relay is working wouldn't you cadieman?
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Tuesday, September 25th, 2012 AT 11:49 PM
Tiny
CADIEMAN
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Yes but the coolant temp. Sensor tells the cpu when to turn the fans on when the temp. Is to high. The fans are turned on auto. As soon as u turn the a/c on.
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Wednesday, September 26th, 2012 AT 1:37 AM
Tiny
SATURNTECH9
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Correct but the same relay is used to turn the fan on at the correct temp or when the ac is turned on. So when you said to check the relay that's why I said what I said.
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Wednesday, September 26th, 2012 AT 1:48 AM
Tiny
CADIEMAN
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I was thinking the a/c grounds the relay directly from the a/c head when turned on. When u let the sensor ground the relay thats from the temp. Sensor. So u have 2 circuts controlling the fan funshion by grounding the relay.1 is the cts and the other is the a/c head. Seeya tomarrow im falling asleep. Ltr
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Wednesday, September 26th, 2012 AT 2:54 AM
Tiny
SATURNTECH9
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The wire diagram shows one ac trigger and again it uses the same relay.
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Wednesday, September 26th, 2012 AT 3:00 PM
Tiny
CADIEMAN
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1 relay controls both this is true saturntech9. Only different is one comes directly from the a/c head directly to the relay. The other uses the cts to cycle the fans. Hope this guy comes back. We need to know if it has a coolant temp. Readout in the dash. For the price of cts just replace it.
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Wednesday, September 26th, 2012 AT 9:35 PM
Tiny
SATURNTECH9
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Actual in the diagram it shows one trigger point for the fan relay when.I looked up the fan diagram looks like the trigger point comes from the computer if it turns on from ac being on or the computer turning it on from the coolant temp input.
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Wednesday, September 26th, 2012 AT 10:02 PM
Tiny
CADIEMAN
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We you turn on the air it sends power to the ac clutch directly from the a/c head. The trigger is must be from the cts. Buddy im done on this 1 and im going on to greener pastures. Talk to u ltr.
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Thursday, September 27th, 2012 AT 3:16 AM

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