MACS Monthly Newsletter - 2020 - MAY7

work at all. Two out of three relays click as you
sequence through the commands of the fans. The
third relay that's not clicking is the high-speed fan
relay. The right fan motor will work on both low
speed and high speed, however. You check all the
fuses (they're good) and swap the low speed and
high-speed relays. No change in the condition.
Could it be the ECM's switched ground circuit
coming from terminal 42 or the ECM itself? No.
Following the highlighted small font parallel dashes from ECM Pin 42 upward, you can see that it
controls the activation of both the fan speed control relay and high-speed relay. Since the right fan
motor will work at both low and high speeds, the
triggering of a ground at ECM Pin 42 has to be taking place. How else could the right fan get a full
power ground supplied to it in order to run at full
speed? The fan speed control relay has to be getting
its ECM switched relay trigger ground from Pin 42
all the way to that relay's Pin 85.
So, what could keep the high-speed fan relay (and

ciated circuitry all work by using a fused jumper
between the high speed relay's terminals 87 and 30
in the BEC. If the left fan runs at high speed - case
closed. Install a new BEC.
As previously admitted, when I first looked at
this radiator fan schematic, I was a bit intimidated in spite of electronics being one of my stronger
skills. Then I started highlighting each circuit on a
separate page for each fan combination. Suddenly it
became much clearer and critical thinking to make
a faster and more accurate diagnosis became much
easier.

Higher Tech Fan Motors?
Yes - But with Simpler Schematics!

BMW

The Chevy's fan motor wiring schematic would
make Rube Goldberg blush with envy, but the complexity of a BMW's cooling fan motor schematic
(Figure 10) is actually a bit simpler. How ironic. Using the same highlight theme used on the Chevy's
fan motor schematic, you can highlight the B+ pow-

Figure 10: Some vehicles such as BMW's X7 xDrive50i (hybrid) use multiple cooling fans with built in modules. A single wire
LIN bus allows for a slightly less complicated schematic compared to multiple fans controlled with three relays with multiple
state combinations. Note the LIN bus passing through both fan modules indicated by the thin dotted line. The two LIN bus
terminals on each module should show continuity when measuring between each other even with the module disconnected.
left fan motor) from working? The problem would
most likely be internal to the fuse panel (BEC,
Bussed Electrical Center). Before going to the trouble and expense involved with replacing a component like a BEC, it would be easy enough to double
check your diagnostics with a replacement "new"
relay AND verify that the left fan motor and asso-

May 2020

er feeds as well as power grounds.
Wire size (metric) listed for each circuit helps a
schematic on paper (or a computer screen) come to
life when you move from looking at your "roadmap" to actually getting on the road as you attempt
to locate the actual circuits on the vehicle for live
testing. If you grew up working with AWG (Ameri-

MACS Service Reports

MACS Monthly Newsletter - 2020

Table of Contents for the Digital Edition of MACS Monthly Newsletter - 2020