Radiator fan turns on only when the AC is on but overheats when is off…

[Demustang]

Hello everyone. I’m pretty new to this forum. Before posting this thread, I have tried looking everywhere on this forum and others about this problem but no luck.

Anyways…

About a month ago, I went to go start up my 2014 Mustang V6 so I can go on a cruise and let it warm up for a while and headed back to my house. Came back 15 mins due to my time management. I assumed that the car was already warmed up at operating temperature (it was around 50 degrees and raining). So, I got in the car, fasten my seatbelt, and started driving out of my driveway.

About 10 seconds after driving out of the driveway, I looked down to see the dashboard and saw the engine temperature warning light on and the temperature gauge light turn red. I immediately found a spot to safely park the car to shut it off and cool off (temps were around 259 degrees Fahrenheit).

Once the temps drop enough to drive back to my house, I noticed that the radiator fans never turned on at all. Hours later and the whole car being cooled off, I decided to investigate the problem. Whenever I turn on the A/C, the fans will kick in. When I turn it off, fans will stop.


Things I checked:
• The Radiator fan itself
• Thermostat (was bad and was replaced, but that didn’t solve the problem)
• fueses and relays
• disconnecting and reconnecting the battery
• coolant
• water pump (no leaks)
• Serpentine belt (worn out and was replaced but still didn’t solve the issue)
• radiator

Anyways I hope I can get an answer. My replies may probably take a while so I’ll try to answer them as soon as possible.

 

[cavero]

Did you check the cooling fan resistor?

 

[Dino Dino Bambino]

When the fan turns on with the AC but doesn't turn on at the appropriate coolant temperature, the problem is almost certainly a bad engine coolant temperature (ECT) sensor. I don't know where it's located on the 3.7 V6 but if you do, disconnect the wiring harness connector. If the fan turns on immediately, you've confirmed the diagnosis.

 

[Robert Larimer]

Hello everyone. I’m pretty new to this forum. Before posting this thread, I have tried looking everywhere on this forum and others about this problem but no luck.

Anyways…

About a month ago, I went to go start up my 2014 Mustang V6 so I can go on a cruise and let it warm up for a while and headed back to my house. Came back 15 mins due to my time management. I assumed that the car was already warmed up at operating temperature (it was around 50 degrees and raining). So, I got in the car, fasten my seatbelt, and started driving out of my driveway.

About 10 seconds after driving out of the driveway, I looked down to see the dashboard and saw the engine temperature warning light on and the temperature gauge light turn red. I immediately found a spot to safely park the car to shut it off and cool off (temps were around 259 degrees Fahrenheit).

Once the temps drop enough to drive back to my house, I noticed that the radiator fans never turned on at all. Hours later and the whole car being cooled off, I decided to investigate the problem. Whenever I turn on the A/C, the fans will kick in. When I turn it off, fans will stop.


Things I checked:
• The Radiator fan itself
• Thermostat (was bad and was replaced, but that didn’t solve the problem)
• fueses and relays
• disconnecting and reconnecting the battery
• coolant
• water pump (no leaks)
• Serpentine belt (worn out and was replaced but still didn’t solve the issue)
• radiator

Anyways I hope I can get an answer. My replies may probably take a while so I’ll try to answer them as soon as possible.

Does the fan come on and STAY on when you turn on the A/C? Or does it cycle off and on? Our cars have 2 speed fans.

 

[Robert Larimer]

When the fan turns on with the AC but doesn't turn on at the appropriate coolant temperature, the problem is almost certainly a bad engine coolant temperature (ECT) sensor. I don't know where it's located on the 3.7 V6 but if you do, disconnect the wiring harness connector. If the fan turns on immediately, you've confirmed the diagnosis.

Wouldn't that throw a code?

 

[Dino Dino Bambino]

Not necessarily. If you left it disconnected, yes. That's because the fan would stay on all the time and delay engine warm up.

 

[Demustang]

Does the fan come on and STAY on when you turn on the A/C? Or does it cycle off and on? Our cars have 2 speed fans.

It’ll stay on for a while but eventually will do it’s on and off cycle

 

[Robert Larimer]

It’ll stay on for a while but eventually will do it’s on and off cycle

Cycling when A/C is on is an indication that you low speed resistor or the low speed circuit has failed and you have high speed fan only. The cycling is because of high pressure in the A/C compressor that triggers the high speed fan. The low speed fan is supposed to run continuously when A/C is on to cool the condenser. Failure of the low speed fan is quite a common problem and may be the result of melting of the fan connectors or a bad resistor. It seems that the OEM fan motor starts drawing more current as it ages so I replaced the fan assembly when my low speed fan died.

 

[golkhl]

Had the same issue on my 08 Bullitt, low speed electric fan resistor was fried. Buying just the resistor is $$$, good time to upgrade to the GT500 cooling fan assembly.

 

[Demustang]

When the fan turns on with the AC but doesn't turn on at the appropriate coolant temperature, the problem is almost certainly a bad engine coolant temperature (ECT) sensor. I don't know where it's located on the 3.7 V6 but if you do, disconnect the wiring harness connector. If the fan turns on immediately, you've confirmed the diagnosis.

Took me a while to find where the ECT was located (under the intake manifold). The car did exactly what you said when disconnecting the harness connector, but after replacing the sensor, I accidentally broke the harness protective covering which probably led me to have an engine code P1289.

 

[Demustang]

Had the same issue on my 08 Bullitt, low speed electric fan resistor was fried. Buying just the resistor is $$$, good time to upgrade to the GT500 cooling fan assembly.

My OEM fan resistor was In pretty bad shape LOL. Decided to just replace the whole cooling fan as I already bought another fan from eBay.

 

[JC SSP]

GT500 fan upgrade should always be considered when replacing a stock GT fan.