MACS Service Reports - 2016 - APR3

Dave Hobbs

Dave Hobbs
Figure 5: When looking at scan tool data on a vehicle with no cooling
due to a no A/C clutch or A/C variable displacement compressor control
valve not letting the compressor put out sufficient pumping, always try
to think W.W.F.D. (What Would F.R.E.D. Do)? What would one of the
many Frustrating Ridiculous Electronic Devices do in order to: 1.) Prevent Engine Overheat; 2.) Prevent Transmission Overheat; 3.) Keep the
engine stable during start; 4.) Prevent 12-volt battery rundown; 5.) Give
full power on hard acceleration; 6.) Keep the compressor from failing.

Figure 7: Graphing (scoping) PIDs. This late model Chrysler HVAC
blower circuit works well now thanks to a new control head. The manual
input from the blower speed knob (potentiometer PID top) now has a good
span of voltage when gradually clicked clockwise from off to full blower.
The knob has detents you can feel. With each click another resistance step
is incremented, thus changing the voltage output of the HVAC head's
blower control request signal.

ture on why the high pressure might be higher than
normal, Figure 3 contains some additional PIDs you
may want to investigate including ECT, IAT, OAT
(ambient), along with a complete inspection of the
engine cooling fan and air flow efficiency across
the condenser. Figure 4 dives into the world of "it's
made that way" with the no A/C compressor operation simply due to the heavy foot of the driver.
Also to be considered as possibilities to inhibiting
proper HVAC operation are all the checks and balances put into place by engineers to prevent everything from 12-volt battery rundowns overnight to
battery discharges during idling (see load shedding
PID in Figure 5). Always keep in mind that the more
PIDs you display on most scan tools, the longer it
takes to get a complete update on the tool's screen,

so make sure you know how to build a custom list
of PIDs on your tool to give you only the information you want when you need it. Graphing select
PIDs (Figure 7) is essential in seeing trends in both
the natural responses we should observe while revving the engine or driving the vehicle during a road
test.

Bi-Directional Scan Tool Operation -
Command, Monitor, and Observe

Dave Hobbs

A tool's bi-directional controls (Figures 6 & 7) can
be used to bypass the decision making process of
various electronic control modules on a vehicle. For
example, if you would like to see if the PCM has
the ability to command an output to the compressor clutch relay, you can set up your tool to perform
a compressor clutch actuation. If pressing the A/C
request on the HVAC control head didn't result in
an engaged compressor clutch, and the system is
fully charged thanks to a quick gauge check and
a manually jumped clutch, you would now want
to determine if the problem is computer related,
wiring related or related to yet another unknown
cause. Setting up the tool to read data PIDs such
as "A/C Request Signal" and "A/C Clutch Command" while in the A/C compressor bi-directional
command mode (Figures 2 & 3) will help you determine the reason for clutch that just can't get turned
on via normal operation of the HVAC controls. In
the data captured during bi-directional control of
the clutch (Figure 3), we see some additional PIDs
for clutch disengagement history. ■

Figure 6: Customer complaint verified - blower motor inoperative / barely
working. Bi-Directional Command and Observe; top PID list in this Dodge
minivan's front blower bi-directional command at 100 % on was accompanied
by a lot of air and noise. Bottom PID list when 10% blower speed requested
was accompanied by a very quiet blower motor at a speed lower than expected.
Rotating the blower speed control knob however had no effect on the front
blower pot voltage monitored in the control head. Now you can have confidence that if you change the control head you will get the blower control
working again.

April 2016

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2016