Posted to Technical Theory Forum on 12/28/2013
Ive gathered some information from several sources to piece
together a diagnostic routine/guide for myself to use
whenever I come across a communication error. Any
input/critique is welcome.
#1 double check that you have the correct vehicle info
entered into the scan tool... it happens okay
#2 check for data at the dlc using a scope. DLC breakout
boxes are nice. Ive used aeswaves before and loved it . This
will confirm no data transmission and eliminate the
possibility of a faulty scanner. This could also narrow down
which network you need to be concerned with.
#2.5 while youre there at the dlc, check for proper power
and grounds to pins 16,4&5. Some tools may require those to
work while others may not. This is sometimes why one scanner
will work and another may not.
#3 Try communicating with other modules. It is possible that
the entire network is not compromised but only one module is
down. this can be done using a suitable scan tool or by
jumpering to the suspected modules . Depending on the
system, this could be done easily at a splice pack or
#4 Check the terminating resistors. Most of these(all?) have
two terminating resistors in parallel to each other on each
end of the network. Each resistor is 120ohms so when wired
in parallel, will measure 60ohms .
#5 check for TSBs. Ive heard of vehicles where they needed a
flash to solve the problem. That confuses me because if you
cant communicate, how can you flash it? perhaps i wasn't
told the whole story.
#6 Like for a lot of issues, check for any aftermarket
equipment, modifications or prior work that could have been
done by somebody less than professional.
#7 If one wire of the twisted pair has no data transmission,
this is most commonly(not always) an issue with wiring and
not a module.
On CAN LO, anything below 1.5v is a short to ground and
anything over 2.5v is short to power
On CAN HI , anything below 2.5v is short ground and anything
over 3.5v is short to power
If one wire is compromised or a terminating resistor is
damaged, the noise on the lines will be greater than usual.
Ryan from Illinois
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