MACS Service Reports - 2015 - FEB1

Total Vehicle Climate and Thermal ManagementTM

February 2015

By Paul Weissler, MACS Senior Technical Correspondent
LOGICAL THINKING SAVES TIME AND EFFORT

prodemand.com

It always amazes us how many technicians
"guess" about the problem in an electrical circuit
when it's in a system they've worked on lots of
times, even if on a number of different vehicles,
"because it's always the same." We can't totally
disagree with some of this, particularly when
the problem is as "obvious" as this one: the compressor clutch doesn't engage. A traditional basic check is to hot-wire the clutch coil terminal
from the battery positive terminal, and if the
clutch now engages and the system cools, that
tells you there's a problem in the circuit to the
relay. At this point, somewhere around 99% of
technicians who guess will pop in a new relay.
That often is the fix, but relay problems have
been on the decline, particularly since the fixing
of the handful of high-failure-rate ones on some
Japanese models. And a better approach is not
to hot-wire, but to ground the relay coil terminal, and if the compressor engages, that tells the
technician the relay is good. If the relay clicks
but the compressor doesn't engage, there's still
a possibility the relay isn't passing current, but
more likely the issue is a break in the wire
Figure 1: 1994 heavy-duty (2500 series) Ram pickup wiring diagram shows (at lower right) that A/C compressor clutch circuit is
tapped off the horn relay circuit (in particular, the relay coil). So
trying to blow the horn will identify a problem in the circuit, which
is from the 20-amp Fuse B in the Power Distribution Center.

ALSO INSIDE THIS ISSUE:
ANOTHER EXERCISE IN LOGIC .................................................................................................................................................... 3
A/C PERFORMANCE ISSUE ........................................................................................................................................................... 4
IT'S NORMAL. IT REALLY, REALLY IS (MUCH OF THE TIME ANYWAY) .............................................................................. 4
SO IT'S NOT PLUGGED, THERE'S NO AIR LOCK, WHAT ELSE CAN IT BE? ..................................................................... 5
THERMOMETERS .............................................................................................................................................................................. 7
EVAPORATOR CORE REPLACEMENT: YES OR NO ................................................................................................................. 7
MACS Service Reports is the official technical publication of the
Mobile Air Conditioning Society Worldwide, Inc., P.O. Box 88,
Lansdale, PA 19446. The material published in MACS Service
Reports expresses the views of the contributors and not necessarily that of MACS. Every attempt has been made to ensure

February 2015

the accuracy of the content of MACS Service Reports. MACS,
however, will not be responsible for the accuracy of the information published nor will MACS Worldwide be liable in any way for
injury, labor, parts or other expenses resulting from the use of
information appearing in MACS Service Reports.

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Table of Contents for the Digital Edition of MACS Service Reports - 2015