February 2006 Air Repair OBDII Review 9
How To Check A Vehicle That Won’t Communicate
From the July 2004 issue of Air Repair.
By Jim Wellman, Envirotest Repair Industry Liaison
Editor’s note: There are two sides to an OBDII system, the
manufacturer’s or vehicle-specific side and the generic or global side.
The OBDII test performed in the test lane only deals with the generic/
global side, and this article looks at the generic/global side in case you
encounter a vehicle that will not communicate with the test lane and/or
your scan tool.
Occasionally a vehicle may leave the test station with a
failed test because there was a communication problem
between the vehicle and the test lane. This happens about
0.1 percent of the time. If your scanner is not self-powered
with its own internal battery, you may not be
able to read the data coming from the computer. The
following information on testing the OBDII connector
(Figure 1) will help you deal with this problem.
The first thing you want to verify is power and ground.
To check the ground circuits, disable the vehicle so that it
will not start. Do a volt drop
check on the system ground
(which is Pin #4) to the
battery negative with the
engine cranking. This will
give you a good dynamic test. The volt drop should be less
than 0.2 volts. Then perform the same test on Pin #5. Pin #5
is the sensor signal ground and may not be used on all
vehicles. Check your wiring diagram for confirmation. The
voltage reading you get (while cranking) on Pin #5 will
depend on the amount of resistance across the computer and
should be close to what you see on Pin #4. Pin #16 should
have system (battery) voltage with the engine running.
The next step is to verify the data signal with a DSO.
(The data transmission rate occurs so quickly that a multim-eter
will not see the signal. Even if it does recognize it in
“peak detect” mode, the value is averaged). Connect the
DSO between signal and ground or Bus+ and Bus-. The
signal will resemble a square waveform pattern (Figure 2)
and should be 5 to 7 volts high, and may require a time base
less than 100 microseconds per division to recognize the
signal as being present. This is done with the engine running.
The pins that are used for this will vary depending on the
manufacturer and the protocol that is used. The protocol
that is used can usually be determined by what pins are
present in the OBD II connector. There are four protocols
we are concerned with presently. The four protocols are:
J1850 VPW (variable pulse width) (10.4kbps)
The VPW system is used on most GM vehicles and
sends data out on Pin #2 (SAE J1850 line (Bus+))
only. This system is also used on some Fords and
Chryslers and may use Pin #10.
J1850 PWM (pulse width modulation) (41.6kbps)
The PWM system is used on most Ford vehicles and
sends data out on Pin #2 (SAE J1850 line (Bus+))
and Pin #10 (SAE J1850 line (Bus-)) This signal will
be inverted on Pin #10 and is not used on all PWM
systems.
ISO 9141-2 (International Standards Organization)
(10.4kbps)
KWP2000 (Key Word Protocol) (Newer Version of
ISO 9141)
The ISO and KWP systems are similar. The ISO
system is used on most Chryslers and Imports. The
KWP system is used on some Imports. These
systems send data out on Pin #7 (K line of ISO 9141-
2 & KWP 2000). Pin #15 may also be a part of this
system (L line of ISO 9141-2 & KWP 2000). Pin #15
is not always used.
The following web sites may be helpful in making a
determination of the type of protocol that is being used on
the vehicle you are working with:
http://www.scantool.net/scanned_list.htm
http://www.obd-2.com/dlc.html this site also has
OBDII connector locations.
http://www.obd-2.de/dl/probs.txt
http://www.etools.org/files/public/generic-protocols-
04-1-04.htm
Thanks to George Generke, Al Santini and John Thorton for the help in
verifying the information in this article.
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
Figure 1. OBDII connector pins
Figure 2.
