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Cooling fans not turning on

Tiny
YOUNGDEREK811
  • MEMBER
  • 1994 FORD MUSTANG
  • 3.8L
  • 6 CYL
  • 2WD
  • MANUAL
  • 170,000 MILES
The cooling fans won’t come on when at operating temperature or when overheating. I replaced the 2 wire engine coolant temperature sensor still nothing. But if I plug it the old one a put a torch to the tip it will turn on and off with the torch, and stay on while unplugged. What could be the problem here? Can it be the ecm, bad wiring?
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Saturday, July 17th, 2021 AT 12:09 PM

11 Replies

Tiny
SQM
  • MECHANIC
  • 6,383 POSTS
Hello,

So just to clarify, if you take the sensor off of the car (while it is still hooked up to the factory harness), then apply external heat, the fan comes on?
Is your check engine light on?
If there is a issue with the PCM the engine light should be on.

https://youtu.be/vMwTzURWXmk

https://youtu.be/ZY9IOfVDfSI

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

https://www.2carpros.com/articles/replace-electric-fan-motor

Let me know of any questions.

Thank you.
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Saturday, July 17th, 2021 AT 12:19 PM
Tiny
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Yes, I took the sensor off and applied external heat. That’s the only way I can get the cooling fans to kick on besides leaving the connected unplugged for continuous fans. There are also no codes
Where should I be looking?
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Saturday, July 17th, 2021 AT 12:48 PM
Tiny
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Make sure you do not have any type gunk built-up around where the sensor goes in. Just in case any restriction blocking coolant to reach the sensor.
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Sunday, July 18th, 2021 AT 10:27 AM
Tiny
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  • 6 POSTS
Seems likely that could do it. After playing around with the multimeter on sensor with car running, sensor will just drop down to like 1.33-1.77 volts after hitting half indicating 104-170 degrees about then it with flat line and ever so slowly continue dropping closer to that.35-.60 volts required to trigger the fans. Not sure what would block it there’s coolant under the sensor.
I’ll update if that does any thing.
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Monday, July 19th, 2021 AT 5:42 AM
Tiny
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  • MECHANIC
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I have attached the temperature chart for your reference.

Keep me posted.

Thank you.
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Monday, July 19th, 2021 AT 6:51 AM
Tiny
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Okay, really don’t believe there is a clog, but I did buy my new ECT sensor from eBay, could it be an inferior part? Also could there be a bad ground for the fan motor? That I’m not aware of because at the fan connecter at running temperature with ECT plugged in I get just a bit over 4v from red wire. Then when I disconnect the ECT it jumps up 4 volts at the red wire never even getting a full 12 volts to the fan.
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Wednesday, July 21st, 2021 AT 11:03 AM
Tiny
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  • MECHANIC
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Hi,

I noticed it has been a couple of days since you have been contacted. Has any progress been made? As far as 12v, you won't get that to the sensor. It receives a 5-volt reference. You shouldn't see more than that amount of voltage.

Now, the changing coolant temperature affects the voltage drop across the sensor terminals, providing electrical signals to the PCM. As the temperature increases, the resistance decreases. Once a predetermined voltage is received by the PCM, the cooling fans should turn on.

I read through the posts and recognize that when you heated the sensor, the fans turned on. What was used to heat it? Also, the idea that the fan turned on leads be to believe the system is working, but the sensor isn't working properly. If you heated it with something like a torch, you got it hotter than the coolant should. Also, I can't tell you how often online parts fail or just don't work properly.

Let us know if progress has been made. Also, you may want to try a different sensor.

Take care,

Joe
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Friday, July 23rd, 2021 AT 7:45 PM
Tiny
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Sorry, it went back it the back burner to work. But it was not the sensor voltage I’m talking about, it’s the 3 wire fan connecter that gets 4 volts when the ect is plugged in and not telling fan to come on. When I take away the ECT signal, back at the fan connecter I now get 8.6volts from the low speed wire (red wire) while at the 2 wire ect connecter I get around 4-5 volts unplugged. I talked to a mechanic of mine and determined that maybe earlier replacing the upper and lower radiator hoses could have left a big air pocket blocking coolant flow to the sensor. So I bled the system with a funnel at ground level not raised and managed to get a little bit of air but still problem hasn’t been solved so I’m waiting until I can have this guy come try his luck with it.
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Tuesday, July 27th, 2021 AT 1:44 PM
Tiny
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Hi,

Thanks for the update. Question. You indicated checking voltage at the ECT connection. Does a signal out change when the engine is hot?

Let me know.

Joe
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Tuesday, July 27th, 2021 AT 7:32 PM
Tiny
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Yes, voltage changes starts at like 4.6 volts cold and gets to 1.3-1.7 volts hot.
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Thursday, July 29th, 2021 AT 3:09 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Do me a favor. Check the gound circuit. See the diagnostics below. It's a simple test.

Let me know what you find.

Joe
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Thursday, July 29th, 2021 AT 7:45 PM

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