ACTION NAPA AUTO CARE - 14

P. Meier

P. Meier

Computer controls for air conditioning compressors

Use the diode test function on a DMM to test a
diode for failure. Check the meter's instruction
manual for speciﬁcs on how to perform this test.

This clutch didn't engage, but current ﬂow
was normal. The mechanical damage
wasn't visible until the clutch assembly was
removed.

P. Meier

fuse that powers the relay coil also powers the clutch coil,
so you may not have to move your clamp. If that's not the
case, locate the fuse for the clutch coil, or simply clamp
around the wire leading to the clutch coil itself. With the
operation of the relay coil confirmed, a normal clutch coil
will draw approximately 3.0 to 5.0 amps. Some OEMs will
list a specification for this, so check for one in your service information system.

Got a clutch that doesn't engage? Checking current
with a fused jumper wire and a low amp clamp will
quickly help you isolate whether it's an electrical or
mechanical fault.
A current draw reading of under 0.50 amps or so will
tell you that the relay coil side of the circuit is OK, and
you can focus on the clutch coil circuit next. Often, the

14

ACTION/NAPA * Spring 2015

No current flow indicates a bad relay contact or other
open in the circuit. Low current can indicate a problem
with voltage drop in the circuit, robbing current the clutch
coil needs to operate. High current can indicate an internal
short in the clutch coil, reducing the strength of the magnetic field and not allowing the coil to engage the clutch
plates. Normal current and no engagement means there is
a mechanical fault with the clutch - too much air gap, mechanical separation or other damage.
There are various control strategies for operating the
compressor clutch, but they all use the same basic components described here. With a good understanding of electrical circuits and of how to use a DMM, plus a wiring
diagram and even a modest description of the control strategy, you're ready to tackle any problem that might show
up in today's compressor clutch circuits. ❆

ACTION NAPA AUTO CARE

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