MACS Monthly Newsletter - 2021 - DEC4

High Speed Fan Control Operation
The PCM Power Relay receives power from the F14 10A
fuse, which is fed to terminal 1 of the PCM Power Relay.
The Power Relay coil terminal 2 receives ground at the
PCM on pin 38. Once this relay is energized, power waiting
on terminal 3 will latch (close) and travel to terminal 5.
With power on terminal 5 of the PCM Power Relay, voltage
is now available to terminal 1 of the High Fan Control
Relay and terminal 1 of the Low Fan Control Relay. Do you
see it? Hopefully, you said yes.
To achieve high speed fan control, the High Fan Control
Relay coil terminal 2 must be grounded by the PCM
at pin 18 (HFC Circuit). Once the High Fan Control Relay
is energized, current will now flow from terminals 5 to 3
of the relay and exit the Battery Junction box on the RDYE
wire over to Splice 108 and continue to both Engine
Cooling Fan Motors 1 and 2 (terminal 1). Both cooling fan
motors are grounded via the BK wire that terminates at
ground location G103. In this case, each cooling fan motor
will receive full charging system voltage so they will be
running on high speed. The Engine Cooling Fan resistor is
no longer in the circuit.
High Fan Control power flow
OE schematic is drawn with all the power flow arrows
indicating direction of current for High Fan Control (Figure
5).
disables the Low Fan Control Relay, the fans will turn off.
This can create enough voltage spike in the electrical system
to cause issues with low voltage electronics. So, the
diode will dissipate this unwanted voltage to ground and
prevent that condition.
2) More importantly, look carefully at the placement
of the diode, if the diode was shorted, the voltage on the
GRN/BLU wire coming from terminal 5 of the Low Fan
Control Relay would be shorted directly to ground and
blow the F16-30A fuse. If the diode is open, there may a
popping noise heard through the audio system of the vehicle
associated with the cooling fans turning off or worse,
there could be damage to other electronics on board. The
technician needs to be aware of these possibilities for diagnostic
purposes. Always ask WHY. Why did this component
fail? If it failed due to age degradation and simply
wore out, fine. However, if something else caused it to fail
prematurely, we need to find the cause. If we don't, we
may be attempting to fix the result of the failure.
Example: I resurrected this one from the archives. Look
at the burned cooling fan resistor and connector from
a 2011 Ford Transit Connect. See Figure 6. I wish I were
making this up; however this was the THIRD one that
was replaced in a months' time. It was July, so guess what
the complaint was? " Check the A/C, it's not blowing cold
enough. " If we do not investigate this one carefully, as one
shop found out, we can seriously mis-diagnose a problem,
spend a ton of money throwing parts at the problem
and not fix anything! Not to mention lose the customers
confidence.
Figure 6: Burned cooling fan resistor and connector from a
2011 Ford Transit Connect
Figure 5: OE schematic is drawn with all the power flow
arrows indicating direction of current for High Fan Control.
Cooling
Fan Diode
We know the function of a diode is to act as a one-way
check valve in the electrical system. Analyzing the schematic,
we can see the use of a diode in the Battery Junction
Box (BJB). The diode is placed in parallel with the Low
Fan Control circuit and, in this case, it is being used for
voltage suppression. Much like we have seen spike protection
diodes placed across A/C clutches in the past, this
cooling fan diode is being used in a similar manner. This is
important to the technician for two reasons:
1)
In the operation of the circuit, each time the PCM
December 2021
4
Bulletin Issued: There was a bulletin (TSB-12-114)
issued for this problem by Ford. The proper fix
on the Ford Transit was to look at the amperage on
the cooling fans and determine if it was excessive.
If it failed a certain amperage threshold, the entire
fan assembly needed to be replaced along with the
cooling fan resistor and the PCM required reprogramming.
Here is the kicker, the Technical Service
Bulletin (TSB) is wrong!
They state in the TSB to check the LSF (Low
Speed Fan) resistor for an open circuit. Is the LSF
MACS Service Reports
Peter Orlando
Peter Orlando
MACS Monthly Newsletter - 2021

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2021
