MACS Monthly Newsletter - 2021 - OCT7

work well as a natural rodent repellent, additionally available
online at hondapartsnow.com. At $40, the 65-foot-long roll
isn't cheap but should be long enough to enable wrapping all
exposed vehicle wiring.
There also are spray-on rodent repellents, such as peppermint
essential oil, and they are inexpensive, but their use is
pretty much limited to short-term protection (about 2-4 days).
Setting rodent traps in the car is another possibility, but some
motorists may find them objectionable. v
Meet The " LIN " Bus
You're hopefully familiar with the " CAN " bus (Controller
Area Network) the twisted pair wiring that forms a data highway,
used on vehicle wiring to transmit multiple signals and
commands, since the end of the 20th century and on most
cars since 2004. This bus eliminates much hard wiring and
increases the amount of communication between all the
electronic control modules on the car. Of course, there were
earlier communication busses in automotive use, and with
the increase in vehicle electronics there are new ones on motor
vehicles. The primary bus used on a car (generally CAN)
is based on the amount of needed data transfer and the cost
of the system, i.e. not all pairs of wiring and the modules to
which they're connected can work at the same speed and do
the same amount of work at given speeds.
CAN has become " commoditized, " that is, it's so widely
used that the modules have become readily available at lower
cost than alternative networks. However, in creating the modern
vehicle's electronic network, the car maker can encounter
the need for localized communication that is simple enough
to be done with an even lower-cost communications bus. The
LIN ( " Local Interconnect Network " ) is an example.
It's a single wire configuration and the network into which
it's connected has fairly modest requirements. One of its
most popular jobs, as an example on most GM climate control
systems, is for all the switches and knobs, with a connection
to the HVAC control module (Figure 10). Most of those
switches and knobs are " smart, " which means they contain
circuitry that enables them to communicate within a network,
not merely turn something on and off.
The LIN used by GM for A/C is a bus that not only enables
the switches to communicate with the HVAC control module,
but if needed with each other, through the HVAC module.
This LIN has some important capabilities and characteristics.
One reason it's used is that it's cheap, primarily because it
works slowly. But how fast does a switch turning on/off an
A/C function really have to operate? Certainly nowhere remotely
as fast as the modules that provide engine control for
emissions.
Typically, the HVAC module wakes up all smart devices on
the LIN when the ignition is turned on.
Because each " smart " component on the LIN is not wired
through to the " DLC " (diagnostic link connector, i.e. the under-dash
diagnostic plug, which in itself is part of a data bus),
a scan tool can't read anything directly from the LIN. However,
it can read the HVAC module, so anything the scan tool
reads (including trouble codes) and commands through the
HVAC module, can be made available if the software enables
it. At the least, if any smart device loses communication with
October 2021
7
the HVAC module (or fails to establish communication during
Figure 10: This LIN diagram from Chevrolet Colorado
shows the single wire circuit to the HVAC module, but the
LIN circuit also runs through the buttons and knobs of the
HVAC control panel.
the wake-up phase), the module will log a communications
code for that device.
Each device on the LIN sends out a reference voltage, 1 volt
less than circuit voltage, which normally would be just under
12 volts if the LIN is on but not communicating. When the LIN
is in good condition, is communicating and you're testing it
with a multimeter (Fluke 87 or equivalent) set to record min/
max, the readings should be 12 V max, 1 V min. If you don't set
the meter for min/max, it will normally read 7-9 V. If a device
fails, and you have a meter connected, you'll see the voltage
toggle.
Important: because the LIN is basically a 12-V circuit
and is, whenever transmitting, under the peak voltage,
you can only rely on the voltage readings you get if
you have a battery charge maintainer ( " tender " ) connected.
The
most common failures on the LIN are opens, shorts
and poor contact of connector terminals, including connectors
not fully seated. v
MACS Service Reports
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MACS Monthly Newsletter - 2021

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