MACS Service Reports - 2015 - JUL6

Dave Hobbs
Dave Hobbs

Dave Hobbs

Figure 8: It may be normal to have a flat line showing no activity on a scope, until you use a scan tool to ask a module to begin speaking. This 1999 Chrysler
did not have any kind of pattern at the PCI terminal in Pin 2 of the DLC until a scan tool was connected and data was requested from the PCM via the
SCI bus.

Figure 10: The SAE J1850 for GM (Class 2) and Chrysler (PCI) use a 0-7
volt square wave that pulses sporadically as modules either communicate
with one another or send out periodic SOH (State of Health) messages.
The baud rate for both OEMs is just over 10 Kbps; not fast enough for
Powertrain to Chassis communications required in stability control systems. A fault on the bus would be indicated with a flat line (low or high)
or the pulse train shown above erratically breaking down in the middle of
a burst of signals.

Figure 9: VW's later model CAN bus behaves the same way (flat line
CAN bus) as the older Chrysler PCI bus prior to putting a scan tool onto
the DLC. Use your scan tool to request Data PIDs, DTCs, Mode 9 (VIN
& Calibration Info) and the PCM will begin making communications activities on the CAN bus.

netic interference) of the super-fast bus transitions. In many cases, these 120 ohm termination resistors are built into a module at each end of the
bus, but in some cases they are built into plastic
boxes that resemble relays, and are located under
the vehicle near the fuel pump. It's near the same
location as the CAN bus controlled module that
runs the pump on some GDI vehicles. Other times
it's simply soldered into the wiring harness and
taped up near the Telematics module in the trunk.
In these types of resistor location arrangements,
the resistors can be subject to damage or corrosion,

July 2015

and not have the correct resistance as a whole in the
parallel circuit containing the two 120 ohm resistors. The 60 ohms total (Figure 13) that the electrical
formula for adding resistances in parallel (the inverse of the sum of the inverses) is never wrong, so
if you see 80 ohms, one of those resistors has some
crummy connections adding resistance into the circuit. If you see 120 ohms measuring between HS
CAN High and HS CAN Low (either at the actual
circuits or at pins 6 and 14 of the DLC), you've got
an open on one of those resistors. The bus may

6

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
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