MACS Monthly Newsletter - 2021 - DEC2

Figure 1: A good visual inspection should
include a flashlight test across the entire
section of the radiator surface area.
tem per OE instructions. After running the engine for a
few minutes, the technician realizes the cooling fans are
not coming on. Now we have an idea of what could have
caused the overheating condition and led to the head gasket
issue. The technician is now faced with diagnosing the
reason why the cooling fans are not coming on. So, good
troubleshooting practices involve being prepared by becoming
familiar with the cooling fan circuit.
Getting a description and operation of the cooling fans
The Modern Electric Cooling Fan
The electric cooling fans on modern engines are
usually controlled by the PCM (powertrain control module).
The PCM monitors certain parameters such as engine
coolant, vehicle speed, A/C on-off status, A/C pressure
and others to determine engine cooling fan needs. When
the engine temperature gets up to a certain setpoint, the
PCM enables either a relay or cooling fan module to turn
on the cooling fans. Once the engine temperature is reduced
to a lower setpoint, the PCM disables the relay or
cooling fan module and the fans turn off. The PCM will
also turn on the cooling fans when the A/C is requested.
This cycle will keep repeating and be modified as coolant
temperature, vehicle speed, and air conditioning requests
are changed. See Figure 2.
should also be on the shopping list of good service ready
essentials. When examining the circuit, we notice that
both cooling fan motors are grounded at the same location.
Upon closer inspection, we notice that the vehicle
looks like it has been in an accident in the past. The main
ground bolt for the fans is found to be loose, causing the
fans to be inoperative. The customer didn't tell the shop
owner that the car was recently involved in a frontal collision,
never thinking to mention it. However, examining the
electrical schematic before even looking at the car eliminated
many steps and saved valuable shop time.
Let us analyze some of the various cooling fan circuits
and determine the methods manufacturers use to control
their speeds. Using some basic tools and test equipment,
we will discuss what diagnostic tests can be made to locate
any electrical circuit deficiencies that may arise.
Resistor and Relay Control of Cooling Fans
In the beginning, man created resistance. Like most
schematics, interpreting the power flow is from the top of
the page to the bottom of the page. I call it stages, and it
breaks down like this.
1. Where is the voltage originating from? Point
of origin-Battery, battery junction box, modules,
etc.
2. What is the current path going through? Components,
fuses, fusible links, relays, and control
devices, etc.
3. Where does it go to? Point(s) of termination.
Ground locations.
All circuit examination should include a thorough visual
Figure 2: The electric cooling fans on modern engines are usually
controlled by the PCM (powertrain control module).
Many systems use separate modules with many different
names to control the cooling fans. Depending on the
system, there could be one, two or even three cooling fans
on the vehicle. These systems may be controlled with resistors,
relays or a combination of the two to control the
fans speeds. Based on the complaint, we may be able to
narrow down what could be causing a problem and what
cannot.
Field Example: If the vehicle overheated and blew the
head gaskets, the head gaskets are the end result of the
problem, not the problem. The technician replaces the
head gaskets, installs a new thermostat, fills the system
up with the correct coolant and bleeds the cooling sysDecember
2021
2
inspection.
Example: 2013 Ford Transit Connect
Look at the names of the components in Figure 3 that
make up the cooling fan circuit. Inside the Battery Junction
Box (BJB), reading from left to right, we notice we have a:
* Cooling Fan Diode
* F16 30A fuse
* Low Fan Control Relay
* F17 50A Fuse
* High Fan Control Relay
* PCM Power Relay
* F14 10A fuse
Like many wiring diagram conventions, the rectangular
dotted line inside the Battery Junction Box (BJB) indicates:
Not all wiring or information is shown as drawn. It is incomplete.
In this circuit that is okay, because everything
MACS Service Reports
Peter Orlando
Peter Orlando
MACS Monthly Newsletter - 2021

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