Cooling fan issue

KILLERQUEEN70

MEMBER
2007 MERCURY MARQUIS
4.6L
V8
2WD
AUTOMATIC
143,000 MILES

I was waiting on line at a drive thru and my car overheated and the engine light came on. I checked the code it was P1299. I called my mechanic he came over and he replaced the thermostat because it was stuck opened and the sensor that's behind the alternator which the code refers to. The engine light and code was gone but then the cooling fan wouldn't come on so now he replaced the fan motor and module as soon as he attached everything the fan came on without the car turned on. He checked the fuses and in the process of looking for shorts in wiring he had to disconnect the fan because it won't stop running. Can you give me anything else he can possibly check? He thinks it could be the PCM but wants to make sure it's not something else first.

Friday, October 23rd, 2020 AT 6:23 PM

1 Reply

JACOBANDNICKOLAS

MECHANIC
110,176 POSTS

Hi,

The idea that it won't turn off indicates a short to power. The fan speed is controlled by the powertrain control module (PCM). The module receives signal from the PCM and determines at what speed to operate the fan.

Do me a favor. First, disconnect the cylinder head temperature sensor and see if the fan stops. The sensor signals the PCM the engine's temperature. If it has shorted, that could be the cause.

I attached a pic of the wiring schematic for you to reference. If disconnecting the sensor doesn't turn the fan off, either he got a defective module or the PCM is having issues. That will be determined.

Pic 2 shows the location of the CHT. It is behind the alternator. However, you shouldn't need to remove the alt just to disconnect it.

Let me know what you find.

Joe

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