MACS Monthly Newsletter - 2020 - AUG5

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uted to many of the leaks. However, it still may leak, from
many of the same causes that also afflicted unsupported
couplings, such as debris in the lines, deterioration of the
O-rings, weakening of the springs and poor finishes of
the surfaces on the interior of the piping. Unsupported
couplings that were "fixed" by installation of improved
O-rings were enhanced by A/C technicians with plastic locking devices (called "immobilizers"). They're still
available, a sign that the issue remains (along with the
fact that older cars are still in service -average age of cars
on the road is 12 years, so many cars even older are operating. See Figure 7. n

Figure 7: Unless a spring lock coupling was supported by
brackets in the engine compartment, it could vibrate and
leak. Immobilizer "clamps," such as the plastic assembly
shown, were released in the aftermarket to solve the problem.

JUST ONE FAN BUT...
It was a hot summer day and the radiator electric
fan on the 2012 Chevy Cruze (1.4-liter four-cylinder)
seemed to be operating erratically. A gauge manifold
set was connected prior to a road test and the A/C
cooled as the car rolled down the road. However,
when the car was brought to a stop, the high-side
pressures soared to about 350 psi and the radiator
fan went to high speed and stayed there bringing the
high-side pressures down. The technician changed the
expansion valve, which had no effect, but before he
executed his next guess - a plugged condenser - he
decided to check the fan circuit.
Unlike fan circuits with a straightforward computer control of a variable speed fan based on a varying
temperature sensor input, he found the Cruze has a
three-speed fan with a five-relay circuit that includes
two resistors, one for low-speed, another for medium
speed and a circuit that bypasses both for high speed.
It might seem that on an economy car like the Cruze,
the fan circuit shouldn't be all that complex to troubleshoot. But it can be.
Example: When the fan runs on high speed, but it
seems it should be at medium speed, the Chevy service manual diagnosis picks a defective speed control
relay as the likely cause. Unfortunately, the service
information doesn't provide a fan speed control strategy tied to coolant temperature and/or A/C operation, only trouble codes for coolant temperature sensor failures. And there's a lot of wiring connections in
the five-relay circuit.
If your scan tool has the coverage, you can activate
the fan for each of the three speeds, so if the fan misses a speed, that narrows the search somewhat. And
if you know the circuit operation, you can see if the
appropriate relays all are closing at the appropriate

August 2020

times and at rational coolant temperatures or A/C
pressures.
When the A/C is first turned on or shortly thereafter, the electric cooling fan surely should operate at
low speed. For this, the Engine Control Module (ECM)
should supply a ground for the coil terminal of the
cooling fan relay, which will close the switch terminal
of the cooling fan relay, which as the diagram shows,
grounds the coil terminal of the fan's low speed relay.
This energizes the relay, closing its switch terminal,
completing the circuit through a resistor to the fan
motor, which then runs at low speed. If the fan's lowspeed circuit is complete, this also indicates the fuse
circuit for the switch terminal of the medium speed
cooling fan relay is intact and the fan itself can be expected to run at medium speed if that relay is operational. You also should notice that the ECM grounding
of the cooling fan relay also activates the cooling fan
speed control relay. Its switch terminal closes, completing a circuit which supplies B+ to the coil terminal
of the high-speed relay. But the circuit stops there, and
the fan maintains low speed.
For medium fan speed, the ECM grounds the coil
of the medium speed relay, which closes the switch
terminal of the relay, completing the fused circuit
through a second resistor to the motor.
To run the fan at high speed, the ECM grounds the
cooling fan relay and as noted with the low speed circuit, this also activates the speed control relay to supply B+ to the coil terminal of the high-speed relay. But
in this case, the ECM also grounds the high-speed relay's coil terminal. So, this relay's switch terminal also
closes and completes the circuit directly to the fan motor, bypassing both resistors, so the fan runs at high
speed. See Figure 8.

MACS Service Reports

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MACS Monthly Newsletter - 2020

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