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CAN Bus diag basics
Posted to Technical Theory Forum on 12/28/2013 91 Replies

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 gateway module.

#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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