MACS Service Reports - 2016 - DEC2

about that already. I've been diagnosing vehicles
for 40 years now. I've seen fads in published diagnostics and the approaches to training. I've also
seen methods that are simple, timeless and REALLY work! Over those four decades I've taken OEM
dealer classes where we studied neutrons, protons
and how free electrons break free of one atom and
make it to another. I've been made to study where
exactly within a serial bus message structure the
module address is located. I've had to learn about
binary and hexadecimal numbering systems. Let
me just say this; most of it (probably 90 %) was a
total waste of time. I wouldn't trade the 10 % that
wasn't a waste of time for anything though! One
very important bit of that 10 % that I benefit from
regularly is the general knowledge of Ohm's Law.
Ohm's Law can be put into your brain much like the
Ideal Gas Law. Who cares about what "PV=nRT"
means? Engineers? Mobile HVAC techs just need
to have a general awareness that when the pressure of a gas goes up or down, the temperature of
that gas goes up or down with it. The same is true
with Ohms / Watts Law. (Figure 1) So, if you've
had an aversion to learning any simplified version
of Ohms Law, please reconsider because that's one

piece of knowledge that you HAVE to have if you're
ever going to be able to go 'free style diagnosing.'

Analog SWC Problems
Using your voltmeter / ohmmeter to make some
measurements along with the information / general concept you've mastered from Ohm's law (you
memorized it or just downloaded the smart phone
app) you truly can determine what each button press
should read when back probing the SWC (steering
wheel control) with a voltmeter. When might you
do such a diagnostic test? When the radio / HVAC
intermittently doesn't do what it's supposed to do
when the customer presses a button on the SWC
(steering wheel control). Poor connections, switches breaking down, even the circuitry in the radio,
HVAC head or BCM can cause crazy problems that
you can diagnose accurately!
Analog steering wheel controls consist of switches
and a series of resistors. The resistors are in series
so each switch has a unique resistance value. Voltage is supplied to the steering wheel control switches. (Figure 4) When a button is pressed, a unique
voltage value is seen by one of the modules on the

Figure 4: In addition to SWC switch values, the schematic has given us part of what we need to know;
1.SWC supplied B+ by BCM. Note: OCV is "Open Circuit Voltage" measured with no SWC switch applied.
2.BCM contains a resistor in series between the SWC resistors and ground. Knowing this we can turn off the key, disconnect the SWC connector and
measure that resistor's value (probe between BCM pin 6 and ground) to see that last resistance in this series circuit. This arrangement is referred to as a
'Voltage Divider Network'. They are used extensively throughout EVERY vehicle on the road!

December 2016

MACS Service Reports

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