Can “Add-ons” affect the CAN systems?

   Get a teenager their first car and what’s the first thing they want 
to do… modify it…  new wheels, different exhaust, seat covers, or 
the most important  and popular one… the stereo.   You know, when 
you have to get to school with all your buddies in the car … hey, ya 
gotta have those tunes.  You can’t be seen around town with a dull 
factory radio or wimpy stock wheels… ya gotta have some flash, 
some “bling” … lots of cool stuff… all that and then some…. 

  These days however, adding on some accessories can leave you stranded in dad’s driveway.  When it comes installing that new stereo system a lot of pre-thinking needs to be accomplished before reaching into that dash and ripping out the old one.   Many of the cars these days have the stereo system tied into the buss line.  The CAN system or in other words the “buss” line allows communication to and from the BCM and other modules.  Some manufacturers will use the left front speaker as the warning chime speaker for all the necessary functions such as “door open-lights on”, “key in the ignition”, etc.  The BCM will tell the stereo to produce the appropriate bell or “ding” to inform the driver of the immediate problem that they should be aware of.  Some will keep the radio in check with the security systems; others incorporate the stereo and the climate control into one unit and display them together on a screen in the dash.

   The problem comes down to the fact that the communication between these units needs to be there or at least in some way recognized so that the buss line is not lost or shorted.   It could lead to a service light on the dash or even worse… a no start condition.   

  Here’s an example; you get a car that comes in with a no-start condition.  Checking the service codes you find that the class 2 serial data (buss) line shows one to be shorted to ground.  Tracing down the shorting bar for the buss line you can then individually test each bus line for a grounded signal.  In this example the grounded buss was on the stereo buss line.  One glance at the dash and it wasn’t hard to tell it didn’t have the factory radio in place.  

  After tearing out the aftermarket radio system and locating the buss wiring the chase was on.  Whoever installed the aftermarket stereo knew enough that the factory radio still needed to be hooked up in order for the door chimes to work.  The factory radio was still wired in and was jammed under the back seat cushion, which is where I found the stereo system case crushed into itself from someone sitting in the back seat.  

    Apparently, as soon as the aftermarket radio was installed there was enough time to cruise down the Boulevard to show off for all their friends with the boom box cranked to max, and of course, a car full of their buddies jammin’ to the music.   Everything was fine until they stopped at the local hang out to look cool next to the car with the windows down and stereo still cranking out the latest tunes.  When they all piled back into their “concert on wheels”… it wouldn’t start.  That’s when it was towed to the shop where I had to find out what was wrong.

  Stereo systems are not the only thing that can cause this problem.  But it’s sure the most common one.  Theft systems that try to “piggy-back” the factory alarm system, GPS units, van conversions, and after market add-ons of all types can cause a problem when it comes to the CAN systems.  

  Be sure to read, follow, and understand the circuits before you try to change or add something to these types of systems.  And if you get a no start in the shop don’t be afraid to ask the question… “So when did you install the stereo” …  it might be the “bling” in your diagnostics.