MACS Service Reports - 2015 - JUL4

General Motors
Figure 4: This star configured, single wire bus (GM Class 2) uses two star connectors (called splice packs in service manual) and 1 wire per module. Circuit isolation
for diagnostics can be as simple as removing the splice pack and using an ohmmeter to test for shorts to ground.

Dave Hobbs

to ground, but that is the very lowest we allow before we condemn the bus as shorted to ground.
As far as using a voltmeter on the bus, some of
the slower pre-CAN buses can be observed to be active with a very jittery meter measuring DC, but the
technique is not very reliable. However, if you are
attempting to detect a bus going to sleep, a DVOM
is often adequate to simply display whether or not
there is voltage or activity on the bus. No activity
would show up as a very steady meter at 0 volts.
This is essential in battery drain diagnostics, as often times a "F.R.E.D." is waking up on the bus with
activity that can run the battery down. On today's
vehicles with Key Off Evap systems and Telematics
such as OnStar or Lexus Link, occasional activity
on one of the vehicle's data buses is normal.

Figure 5: GM Star Connector is typically located just a few inches from the
DLC and is wrapped up in black electrical tape. Chrysler calls it a "Joint"
connector. Regardless of title or construction, isolating the nodes on a serial bus to determine which module is shorting the bus or causing unstable
activity is necessary in diagnosing stubborn intermittent bus communications problems. Kent Moore and Snap-On both make break out boxes for the
GM Star Connectors, allowing technicians to troubleshoot far more quickly
than pulling this 'comb' looking shorting bar and jumpering module circuits one at a time.

and for me, back probing from the rear of the DLC
requires too much work and contortionist moves.
I prefer to use a break out box instead (Figure 7).
They can cost as little as $100 and range up to over
$600 depending on features. Using a DVOM is limited to determining if the bus is shorted to power or
ground. A rule of thumb is if you measure between
a data bus and ground (key off and the bus asleep)
there should be greater than 35 ohms. Typically
you'll see numbers several times that in resistance

July 2015

Scoping Is the Best Choice
Scopes can be your best friend in data bus diagnostics. You are not really looking for how many
square waves line up in a millisecond, but rather
wanting to find out if the pattern is normal. What
exactly is normal? Practicing with known good vehicles is the perfect way to define normal. Abnormal would be a flat line (low or high) or an erratic

4

MACS Service Reports



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

MACS Service Reports - 2015 - JAN1
MACS Service Reports - 2015 - JAN2
MACS Service Reports - 2015 - JAN3
MACS Service Reports - 2015 - JAN4
MACS Service Reports - 2015 - JAN5
MACS Service Reports - 2015 - JAN6
MACS Service Reports - 2015 - JAN7
MACS Service Reports - 2015 - JAN8
MACS Service Reports - 2015 - FEB1
MACS Service Reports - 2015 - FEB2
MACS Service Reports - 2015 - FEB3
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MACS Service Reports - 2015 - FEB5
MACS Service Reports - 2015 - FEB6
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MACS Service Reports - 2015 - FEB8
MACS Service Reports - 2015 - MAR1
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MACS Service Reports - 2015 - MAR3
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MACS Service Reports - 2015 - MAR5
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MACS Service Reports - 2015 - MAR7
MACS Service Reports - 2015 - MAR8
MACS Service Reports - 2015 - APR1
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MACS Service Reports - 2015 - MAY8
MACS Service Reports - 2015 - JUN1
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MACS Service Reports - 2015 - JUN3
MACS Service Reports - 2015 - JUN4
MACS Service Reports - 2015 - JUN5
MACS Service Reports - 2015 - JUN6
MACS Service Reports - 2015 - JUN7
MACS Service Reports - 2015 - JUN8
MACS Service Reports - 2015 - JUL1
MACS Service Reports - 2015 - JUL2
MACS Service Reports - 2015 - JUL3
MACS Service Reports - 2015 - JUL4
MACS Service Reports - 2015 - JUL5
MACS Service Reports - 2015 - JUL6
MACS Service Reports - 2015 - JUL7
MACS Service Reports - 2015 - JUL8
MACS Service Reports - 2015 - AUG1
MACS Service Reports - 2015 - AUG2
MACS Service Reports - 2015 - AUG3
MACS Service Reports - 2015 - AUG4
MACS Service Reports - 2015 - AUG5
MACS Service Reports - 2015 - AUG6
MACS Service Reports - 2015 - AUG7
MACS Service Reports - 2015 - AUG8
MACS Service Reports - 2015 - SEP1
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MACS Service Reports - 2015 - SEP6
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MACS Service Reports - 2015 - SEP7
MACS Service Reports - 2015 - OCT1
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MACS Service Reports - 2015 - OCT7
MACS Service Reports - 2015 - OCT8
MACS Service Reports - 2015 - NOV1
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MACS Service Reports - 2015 - NOV4
MACS Service Reports - 2015 - NOV5
MACS Service Reports - 2015 - NOV6
MACS Service Reports - 2015 - NOV7
MACS Service Reports - 2015 - NOV8
MACS Service Reports - 2015 - DEC1
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MACS Service Reports - 2015 - DEC3
MACS Service Reports - 2015 - DEC4
MACS Service Reports - 2015 - DEC5
MACS Service Reports - 2015 - DEC6
MACS Service Reports - 2015 - DEC7
MACS Service Reports - 2015 - DEC8
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