The BMW cooling system's temperature controls
include coolant-temperature sensors, thermostat,
radiator or expansion-tank cap, cooling fan(s)
and fan clutch (if your BMW model is so equipped).
These parts largely function independently of
the engine, but perform a critical function by
cooling the engine or sending critical control
signals to the car’s electronic systems.
The BMW thermostat is a spring-loaded valve.
It opens and closes in response to the temperature
of the coolant that flows through it. Common
early signs of a sticking thermostat include
a high temperature reading followed by a drop
to normal temperature, or a continuous low temperature
reading. Because many other conditions can cause
these problems, you need to know how to eliminate
each possible cause.
The radiator or expansion-tank cap is a spring-loaded
valve that reacts to the pressure within the
cooling system. The cap maintains proper coolant
level at predetermined pressures. Always replace
this cap with an exact replacement cap with
the same pressure setting. Never use other caps
except on a short-term basis in an emergency!
A belt-driven fan blade pulls air through the
radiator and is usually mounted on the water-pump
pulley. A fan clutch allows the fan to turn
with the belt at low engine speeds, and then
"free-wheel" at higher speeds. A malfunctioning
fan clutch will either not let the fan spin
at low speeds (causing overheating in traffic)
or won't let the fan “free-wheel”
at high speed, causing overheating on the highway
and/or a reduction in gas mileage.
An electric fan may be installed by itself
(as is common on front-wheel-drive vehicles)
or as an auxiliary used with a mechanical fan.
Both fan types are controlled by a temperature
sensor located in the radiator or upper radiator
hose, or on the thermostat or the water pump
housing. This sensor usually has an on-off switch
with a fixed temperature setting. Some cars
may have two or three settings for multi-speed
fans, and a sensor of this type is usually called
an "auxiliary fan switch."
Other commonly used temperature sensors include:
A gauge sender, with variable output
A warning-light sender, an on-off type
of sensor
Lambda and/or fuel-injection sensors,
which are variable to control fuel injection
settings
A thermo-time switch, for controlling
the cold-start valve
Other sensors may also be installed
in your BMW model
While temperature control is critical to your
car’s performance and emission control
devices, this system is the hardest to troubleshoot
without proper diagnostic equipment and manufacturers’
diagrams. Diagnostics of temperature-control
systems can be made even more difficult in vehicles
that use computers to regulate timing, idle
speed, vacuum and fuel delivery because they
automatically adjust for what may be faulty
temperature-sensor signals.
Maintenance of cooling-system sensors is made
almost impossible because there is nothing to
"maintain." Keeping components clean
both internally (via regular coolant replacement)
and externally, by cleaning the engine, is the
best way to ensure problem-free driving. Checking
and/or replacing parts at the recommended intervals
and/or mileage will also help.
