MACS Monthly Newsletter - 2022 - SEP5

you should expect to see. If this is the result of your
test, then electrically there is nothing wrong. The ECM
is happy, it has commanded the relay " on " , and the coil
is working as it is supposed to. In this case, the most
likely problem is a mechanical one in the clutch assembly.
Perhaps the air gap is excessive, or the clutch plate
is damaged in some way. In any case, the compressor
will need to be removed for closer inspection.
A common reading is current that is too high. As
Figure 15: Is it an electrical or mechanical problem preventing
the clutch from engaging?
sor clutch coil. With power applied, the magnetic field
created around the clutch coil draws in the clutch disc
and engages it with the free-spinning compressor pulley
that is driven by the accessory belt drive system.
When it's not working, there are numerous possibilities
for the malfunction, and we can determine which is the
most likely simply by measuring current in the circuits
responsible for the A/C relay.
We all understand how a relay works. In this case,
the ECM typically completes the ground side of the relay
actuation circuit to close the relay and complete the
path to the compressor clutch coil. Each circuit usually
has its own fuse so first start by identifying the fuse
that protects the compressor clutch circuit. Follow that
up with a quick check of the Power Distribution wiring
diagram to see whether the fuse also protects other
system circuits. Usually, it won't, but knowing upfront
can help prevent you from making a misdiagnosis early
on.
In my case, the fuse is a 30-amp fuse that serves
only the compressor clutch. By replacing the fuse with
a fused jumper lead or Fuse Buddy, I can use my amp
probe to measure the current flow in the compressor
clutch coil circuit. With the amp probe in place, turn
the A/C on full blast and check the meter's reading.
If the meter reads " 0 " amps, the problem could be a
failed relay or failure of the ECM to energize the relay.
We'll return to tackling this result momentarily.
There are three other possible readings you may encounter.
The first is a normal current draw for the coil,
the second is one that is too low, and the third is one
that is too high. How do you know? Generally, compressor
clutch coils will draw around 3 amps or so, but
it is best if you can locate the coil's resistance specification
and then use Ohm's law to calculate the reading
September 2022
5
Ohm's law taught us, if current has gone up (and the
source voltage is the same), circuit resistance has to
have gone down. And that typically points to a short
in the clutch coil. While it is working electrically, the
shorted windings will not build enough magnetic force
to operate the clutch.
The third possibility is that current is too low. In this
case, the circuit's resistance must have increased. Use
the voltage drop testing method I showed you earlier
to determine if the fault lies on the ground side or power
side of the circuit and repair accordingly. Keep in
mind that many compressor clutch coil circuits ground
through the compressor case, to the engine block and
from there to the battery. Improper compressor installation
or corrosion between the compressor and the
mount is enough to affect the voltage drop in the circuit.
Another common source of unwanted resistance
is corrosion in the connecting plates that make up so
many of today's " intelligent " fuse/relay centers.
Now, back to tackling the problem when the amp
probe isn't measuring any current flow to the coil. The
easiest first step is to swap the relay with a known good,
and equivalent, relay. If that fails to fix the problem, I
suggest connecting a scan tool and reviewing the data
PIDs related to the A/C command. In many cases, a
temperature sensor that is reporting incorrectly to the
ECM is enough to deny compressor operation and with
many systems offering multi-zone control, there are a
number of these sensors to check! All it takes is one to
keep the A/C off. Of course, check the pressure sensor
readings and other inputs to make sure they are in
range.
Using current to test any relay-controlled circuit is
basically done in the same way as I describe here. The
question to answer as quickly as possible is simple. Is
the ECM (or control module in charge of the system
you are troubleshooting) happy with what it sees and
is turning the relay " on " and if so, is the component
side of the relay (and the component
function as designed? v
itself) able to
MACS Service Reports
Pete Meier
MACS Monthly Newsletter - 2022

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