16 AIR REPAIR OBDII REVIEW 2008
Figure 2
Pin #5 is the sensor signal ground and may not be used on all
vehicles. Check your wiring diagram for confirmation. The volt-age
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 multimeter 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:
www.scantool.net
www.obd-2.com (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.
How To Check A Vehicle That
Won’t Communicate
(Continued)
How Important is Wiring?
From the July 2005 issue of Air Repair.
By Pat Weber, Owner Weber Automotive, Glenview, Illinois
A frustrated customer came into my shop with an OBDII fail
because of no communication. Another shop had been unable
to get it to communicate. Because I had little repair history on
this vehicle, a 1996 Jeep, I asked the customer for a history. He
did inform me that the vehicle had been in an accident. He had
very little problems with the vehicle and it was well-maintained. I’m
glad I found out the vehicle had been in an accident as I wouldn’t
have even suspected it. The body job was one of the best I have
ever seen, and I probably wouldn’t have thought to look for other
damage that could have an effect on an emissions test.
Upon further investigation, I found that the body shop missed
a few important items such as missing ground connections and
wiring harness damage. So I spent a few hours fixing damaged
and ungrounded wiring.
The result was we got the vehicle to pass the test after
another repair facility was unable to find the problems. The
lesson learned from this experience is that spending time with the
customer to get a history of the vehicle can give clues as to how
to go about approaching the problem. Knowing that the vehicle
had been in an accident, we checked for ground and power, which
are needed to establish communication. As OBDII vehicles age,
accidents and a vehicle’s wiring will be areas that will need to be
Damaged wiring affecting 1996 Jeep OBDII test. checked more thoroughly.
