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Where is the Cooling Fan temperature sensor located?

Tiny
DEADSET
  • MEMBER
  • 2008 LEXUS RX 350
  • 3.5L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 220,000 MILES
I know where the temperature sensor is for the gauge, and when I unplug it, the fans do come on. I would have thought there would be a different temp sensor for the cooling fans and if there is I can't find it. The problem is the fans only come on if the AC is on or if I unplug the temperature sensor that controls the gauge. The fans do not come on any other way. With my diagnostic equipment plug in when the temperature gets to 205, I turn the A/C on, the fans come on and the engine temperature drops to 190 ish. There are no codes except for the code when I unplugged the temp sensor. I assume the rx350's fans should come on at 200.
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Thursday, September 19th, 2024 AT 6:26 PM

7 Replies

Tiny
BORIS K
  • MECHANIC
  • 799 POSTS
Hello,

The ECM only utilizes 1 temperature sensor for the temperature gauge and fan operation.
The cooling fans have their own dedicated control module. The fan main relay supplies 12V on the black wire to pin 3, +B1, pin 1, E1, white/blue wire is the ground. Terminal SI, pin 2, light green wire is the control from the ECM.

As the cooling fan starts up with the coolant temperature sensor unplugged indicates that 12V supply and ground are present.

According to your description it sounds like a possible issue in the cooling fan ECU

See diagram below.

Cheers, Boris
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Friday, September 20th, 2024 AT 9:34 AM
Tiny
DEADSET
  • MEMBER
  • 4 POSTS
What temperature does the engine need to be before the fans kick on? I assume around 200. If the engine temperature should be 190-220 I might not even have a real problem if the fans won't come on until closer to 220. I am going to do a radiator flush and go ahead and replace the temp sensor mainly because I have several Toyota temp sensors on the shelf. This is my stepdaughter's Lexus that she bought without me looking at it first which she knows better than that.
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Friday, September 20th, 2024 AT 10:12 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,363 POSTS
It looks like the engine coolant temperature sensor is on the thermostat housing. Check out the images (below). Please let us know what happens.
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Monday, September 23rd, 2024 AT 10:26 AM
Tiny
DEADSET
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  • 4 POSTS
After flushing the cooling system and changing the temperature sensor engine ran fine. Fans would come on at around 190ish and the engine temp fluctuated between 186-215. I drove it up and down the mountain several times for a total of about 90 minutes of drive time. The temp never went over 215. My stepdaughter gets her car, and it starts to overheat 30 miles down the road. I forgot to mention the t-stat has already been replaced.
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Monday, September 23rd, 2024 AT 11:02 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,363 POSTS
This can be a clogged radiator (internally or externally) or the engine has a blown head gasket which coolant will not show up in the engine for 100% of the time. Here are two guides to go over to get a battery idea of what's going on:

https://www.2carpros.com/articles/engine-overheating-or-running-hot

and

https://www.2carpros.com/articles/head-gasket-blown-test

Please go over these guides and get back to us.

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Monday, September 23rd, 2024 AT 1:22 PM
Tiny
DEADSET
  • MEMBER
  • 4 POSTS
It is what I was afraid it would be from the first time she called about her car overheating a blown head gasket.
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Tuesday, September 24th, 2024 AT 8:04 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,363 POSTS
Yep, it happens especially after a hot summer. Let me know after you do the test.
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Tuesday, September 24th, 2024 AT 2:13 PM

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