Recently I read a Chrysler Technical Service Bulletin about "new cooling fan strategy." After reading that bulletin, I was motivated to write this article in an attempt to convey how complicated automotive technology has become. 

Traditionally cars were rear wheel drive, meaning that the engine turned the rear wheels to power the vehicle. Back then, the engines were mounted in the car "in line" with the drivetrain, meaning, from front to back. The fan belts were on the front of the engine and the belts spun the engine cooling fan, which cooled the coolant in the radiator. From the 1980's and on, front wheel drive cars started to dominate the marketplace. On front wheel drive cars, the engine is mounted transversely, meaning that the engine is mounted "sideways" under the hood. Due to this configuration, the drive belts are now behind the right front or left front wheel and not anywhere near the radiator. As a result, a belt driven engine cooling fan cannot be used. Since then, front wheel drive cars have had electric fans mounted on the radiator. The engine computer senses when the engine coolant is getting hot, (about 225F), and powers on the cooling fan. When the coolant temperature drops, the computer shuts the fan off. This cycle repeats itself as needed while you are driving. Typically the fan never comes on above 30 MPH since at those speeds there is sufficient airflow passing through the radiator, so fan operation is not required. The fan will normally come on at low speed, stop-and-go, or idle type driving. 

Well, that old strategy just got a lot more complicated thanks to Chrysler. Recently we came across Chrysler service bulletin # 07-003-01, which explains new cooling fan operating characteristics on some of their 2001 model year vehicles. Now fan operation will also depend on windshield wiper speed operation. What you ask, do the wipers have to do with the engine cooling fan? Well I'll let you read it in their own words. The copy of the bulletin follows: 

"Owners may complain that the engine cooling fan runs constantly, coming on just as the vehicle comes to a stop, or that the cooling fan may be intermittently running under cool weather or cool engine conditions.  Technicians performing routine diagnostic procedures may notice that the cooling fans are running under conditions that would not normally require cooling fan operation.

Just before the 2001 model year launched, a radiator steaming condition (water vapor rising from the front grill opening) was detected following a drive in the rain. The water vapor is a result of moisture (rain) build up on the radiator under certain specific conditions and not the indication of a vehicle overheat problem. To prevent owner concern over this water vapor, the cooling fan strategy in the Powertrain Control Computer was revised to command on the cooling fans under the following conditions:

1.  Ambient air temperature is greater than 25F.
2.  The engine coolant temperature is less than 195F.
3.  Windshield wiper motor speed and vehicle speed is above a certain threshold.
4.  Under these conditions the cooling fan will turn on at vehicle speeds less than 20 MPH.

This strategy will turn off if any one of the following conditions are met:

1.  The vehicle must travel more than 100 miles without the windshield wiper motor on.
2.  The engine coolant temperature drops below 100F. 
3.  There are 10 minutes of accumulated time with the engine coolant above 220F."

It's obvious that diagnosing the cooling fan circuit of these cars is not going to be as easy as it used to. As these vehicles age and develop problems, we will have to look at the wiper motor and vehicle speed sensor as possible trouble when diagnosing the cooling fan circuit. Who would have ever thought cars would have become this complicated? 
 

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