Cooling System Diagnostics (008-020)
B3.9 and B5.9 Series Engines
Section 8 - Cooling System - Group 08
Some applications use thermatic fans. These fans operate
only as needed to keep the coolant at the correct tem-
perature. If the fan does not operate when the coolant
temperature increases, the engine will run hot. If the fan
does not shut off when coolant temperature decreases, the
engine will run cold.
NOTE: Make sure the coolant temperature sensor is func-
Shutters are designed to control airflow across the radiator.
If the shutters fail to open when needed, the engine can run
hot. Failure of the shutters to close can result in too much
airflow and the engine running cold.
NOTE: Make sure that the air temperature sensor is func-
tioning correctly. Check the air-operated shutter controls.
Check for air leaks.
Winterfronts can be used on a charge air cooled engine,
but must be designed to cover the frontal area of the
cooling system partially. A minimum of 120 square inches
of charge air cooled frontal area must be left open to
An incorrect fan shroud or obstructions can reduce airflow
and cause the engine to run hot.
NOTE: Check to be sure air is not recirculating. Check for