MACS Service Reports - 2015 - JUL5

General Motors
Figure 6: This loop configured GM Class 2 circuit drawing has each module daisy chained together for extra reliability. If there is an open circuit anywhere on the
bus, communications between modules still occurs. Diagnostics that require circuit isolation however, are far more difficult.

J1850), so scoping is easy. Some buses require a little coaxing to talk to your scope with those square
waves, such as Chrysler 's SCI (Figure 8) and the
CAN bus I ran into on a VW not long ago. See Figure 9. Simply connect a scan tool and start pulling
DTCs or requesting Data PIDs, and you'll see a flat
line go to a real network pattern on these shy buses.

Dave Hobbs

While most buses run on a 0-5 volt span or less,
the J1850 bus (GM Class 2 and Chrysler PCI) runs
on a 0-7 volt span (Figure 10). Since almost all digital electronics operate on a 0-5 volt digital signal,
the extra 2 volts in the single wire J1850 buses can
allow for a noisy ground to float up to 2 volts and
still provide 0-5 volt digital communication shifts.
You'll need to perform a dual trace on Ford's J1850
to observe their mirror image 2nd wire in pin # 10,
which is used for greater fault immunity (Figure
11).
High speed dual wire CAN circuits also require
a dual trace scope setup to capture both HS CAN
High and HS CAN Low signals. Normal patterns
also mirror each other, as with the J1850 Ford 2-wire
buses, but the shifts up and down are different. See
Figure 12. Due to the nature of high speed communications typical to HS CAN buses, there are termination resistors (120 ohms each) at each end of a
CAN bus wired in parallel with each other. These
resistors are essential for preventing reflectivity
(mumbo jumbo term for EMI, or electromag

Figure 7: Numerous DLC break out boxes are available on the market
which allow you to plug in a meter or scope to any of the 16 pins. This particular model from AES Wave features several LEDs on the data circuits
to allow a quick glance at bus status (short to power, short to ground or
normal activity) and connect a scan tool / reflash tool while the break out
box is in the circuit.

drop out in the middle of a burst of data transmissions.

Flat Line Pattern: Cat Got F.R.E.D.'s Tongue?
Some serial buses won't speak until spoken to.
Others just start chattering away when the key
is turned on (HS CAN) or door opened (LS CAN,

July 2015

5

MACS Service Reports



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

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