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The CANBUS demistified....

143K views 121 replies 63 participants last post by  Old Dogger 
#1 · (Edited)
As someone that works with CAN everyday I see a lot of misinformation and confusion as to what the CANBUS is and how it is used in our JKs. So I figured I'd try and demystify it.

CAN = Controller Area Network. It's a two wire (sometimes a 3rd sheilding wire is used) communications link between control modules. Think of it as the link between your computer and your router/modem at home. All the CAN link does is allow modules to pass information back and fourth ie: vehicle speed, engine coolant temp, throttle position, switch states and so on.

The CAN Link (or CANBUS) is dumb, it doesn't know anything or do anything on it's own. It's vital to the operation of your JK because it's how modules get data from one another, which is why it can cause a whole host of problems if the wires get damaged or one module goes down.

All the inputs and outputs in your JK are controlled by the various modules (not the CAN link itself); TIPM, Cluster, Steering Control, Radio, PCM, Keyless Entry, Occupant Classification Module, ABS and so on.... You have three CAN links, all monitored by the TIPM (the TIPM acts as sort of a gateway). You have the diagnostic link that goes only to the TIPM and the DLC under the dash. Then there is an "interior" link that connects to the cluster and the radio and the non critical modules. Then there is the main link that connects between the TIPM, PCM and safety modules.

Maybe now is a good time for an example. I'll use the headlight/foglight circuit since it seems to be the most talked about.

The headlights and foglights are not directly controlled by the CAN link. Yes the CAN link plays an important role but all it does is transmit switch state.

Your multifunction switch is part of the Steering Control Module. The Steering Control Module monitors the switch position and communicates that position to the Cluster via the LIN BUS (another simpler communications network). The Cluster then communicates that position to the TIPM over the CAN link. The TIPM then decodes the switch state from the message and makes a decision about what to do. The TIPM then controls the headlights via driver circuits (transistors for the most part). These circuits have the ability to monitor current and protect the circuit by disabling the drivers. This is why there is no fuse for the headlights or fog lights. This is also why trying to run a higher wattage bulb does not work (without a relay), the system detects too much current and disables the circuit.

In the end, all your inputs and outputs are controlled by a computer module and the module uses the CAN link to share that information. Why make it so complicated? There are a lot of reasons to start with it allows for multiple modules to share inputs without having to run multiple wires (and without having to have multiple inputs - this makes controllers cheaper), more precise control, better diagnosis and better diagnostic control via a service tool for troubleshooting. Is all this necessary for a wrangler? Probably not but I am sure it allows for a lot of cross platform carry over and reduces engineering/manufacturing costs.

Different modules broadcast different periodic messages. The reason you get into trouble removing something from the link is because there are other modules expecting that message and diagnosing it. And in the case of removing the cluster, the TIPM doesn't know when to turn the headlights on.

I realize I am over simplifying it, but the principle operation of the CAN link is pretty basic. It's the interaction between modules that gets you in trouble. Though there are ways around this with the right tools and know how.

I hope this helps somewhat. Feel free to add to this, point out corrections or ask questions.
 
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#24 ·
I suppose I didn't really demystify it per se. I just tried to clear it up and make it less scary since I see misleading information.



Wish I had one. My knowledge of the JK is from digging into my own, looking at the service manual schematics and spying on the data link.

how i understand it, simplified.

jeep has x y and z computers, and f g and h sensors. y and z have 2-3 wires connecting each to x. y and z are stupids that forward info to x from sensor f, g or h. mess with g and h will mess up due to signals used in communication.
That's a better way to put it. However that doesn't mean you have to be 100% hands off on things like the TIPM, it's still partially just a fuse box. Just don't break it or cut the CAN lines :)

I hear people blaming Chrysler/Jeep engineers, but all the manufacturers are doing it.
People want gadgets, features, and options. This is what it takes to make it happen.
added options and features, ease of assembly at the manufacturing plant, better diagnostic abilities, less parts over the alternative (added wires, relays and fuses)... There are a LOT of benefits.

It opens a lot of doors for the aftermarket too and those knowledgeable enough to play with it. Maybe not so much in a 3.8L Jeep Minivan, but it sure has made it easier for everyone to change tire size and gear ratios and tweak the power band(what little there is).

There is a big research project going on headed up by BMW I believe where they are looking at replacing the entire CANBUS system with something that runs on the TCP/IP protocol (Thats what the internet is running on in case anyone was wondering). Every module in your car would have an IP address and communicate to one another much like computers communicate over a network in an office.

Then we are all in for a real treat :thefinger:
There is also Flexray. They all have their limits. CAN is fairly robust, but won't stay around forever.
 
#9 ·
how i understand it, simplified.

jeep has x y and z computers, and f g and h sensors. y and z have 2-3 wires connecting each to x. y and z are stupids that forward info to x from sensor f, g or h. mess with g and h will mess up due to signals used in communication.
 
#10 ·
Thanks for the simplified explanation there Z... I understood how the thing worked before and considered getting a CAN bus snoop tool to watch the message flow... then I might figure out WHY IN THE !$@#^#&@ MY CRUISE CONTROL QUITS WORKING UNTIL THE NEXT FREEKING POWER CYCLE!!!! (which is doable at 70MPH. pop into neutral. turn ignition off. wait three seconds. restart. put back into gear and go. Depending on the terrain I usually lose less than 5mph doing that.) It has something to do with the aftermarket alarm module for the remote start wired into the bus circuit for the column computer. The cruise "cancel" function does not always work when the cruise lever is pulled and held it causes the cruise module to turn off until the power cycle. Its a real PITA. I'm thinking I have the wires reversed in my connection to the bus....
 
#11 ·
Thanks for straightening us out.

Now can someone do a global "find and replace" on this whole site and replace the word "canbus" with "that ******* unnecessarily complicated computer system jeep engineers decided to use to control everything"
 
#13 ·
Yeah. Plus our Big Brother is working us all towards a future where they will be able to remotely disable your vehicle for "official reasons/official use only" of course.

Not saying that having tech in the vehicle is necessarily a bad thing, just more things that can go wrong. After all, my 8-track is no longer modern tech by any means and the car-puter is so last decade.
 
#16 ·
That sounds about right, multiply that by three or four times and you'll get what I played with at BMW. Don't forget the fiber optic audio system :bounce:
 
#18 ·
I understood the system as well, CanBus is a lot like the EIA485 token ring and Echelon FTT10a systems I have worked on in the past. Works great and fast when everything is in proper order. I understand it's use and need with all of the driving "nanny" services (ERM, ABS, ESP, etc.) built into the new DC vehicles... That being said, it still pisses me off that I had to buy a 70 dollar adapter to install an aftermarket radio. :pissed:
 
#41 ·
That's when u Blame the Aftermarket, Vehicle had the system before The aftermarket was involved. 1st come...LOL....That's almost like buying frozen Dinners and bitch that you have to throw the food away to get the Dish...

AfterMarket needs to get up with it...No reason that can make them universal etc because there isn't that many Network systems that are that different. Unlike the way Apple does Business...LOL
 
#21 ·
There is a big research project going on headed up by BMW I believe where they are looking at replacing the entire CANBUS system with something that runs on the TCP/IP protocol (Thats what the internet is running on in case anyone was wondering). Every module in your car would have an IP address and communicate to one another much like computers communicate over a network in an office.

Then we are all in for a real treat :thefinger:
 
#28 ·
i didn't realize canbus needed to be demystified....but thanks for doing it....maybe there won't be so many people afraid of it because they don't understand it.

i don't think people understand how much more complicated and unreliable our vehicles would be without it. there would be lots more wires, connectors and sensors to have problems with.

:beer:
 
#34 ·
Any of the generic OBDII scanners should work. Well any of the modern ones. Only ones I have heard problems with is some of the programmers out there only read codes from ECMs and not from the TCM as well. Others will read codes from even non OBD modules if it is specific for your vehicle.

Can the stock fogs be replaced with LED equivalents with no issue since they draw less power in the Canbus system or will that create a problem too?
Yes as long as they draw less current than the factory bulbs you're fine.


umm so can you tell me how to make my jeep think my cat sensors are passing so I can pass inspection? :laughing:
I assume the JK uses O2 sensors before and after the CAT to see if they are working? An old trick in the tuner world was to take those threaded spark plug adapters and stack one or two together and put those in the hole for the rearward O2 then thread the O2 sensor in there (may have to drill the adapter for the O2 to fit. That pulls the O2 sensor out and it reads false values. May not work on all cars. Can also buy dummy sensors.
 
#31 ·
Can the stock fogs be replaced with LED equivalents with no issue since they draw less power in the Canbus system or will that create a problem too?
 
#35 ·
Taxx,

I have a 2012 JK Sport & just bought the Mopar Heated Seat Kit and the factory switch assembly with the factory heated seat buttons. This switch assembly is attached with a CanBus connector.

Assuming I can get the computer re-flashed to turn on this feature, is it possible to wire this kit to these factory switches?
 
#36 ·
You should be able to wire that kit right in per instructions if it was for a 12. Then you need to take it in and have the Starscan hooked up so they can turn that option on. You should install any other options you want so you only have to have this done once example fog lights. On the Cannbus try integrating a ls chevy into this. Glad Motech and JJP are doing this so I can get me ls installed!
 
#38 ·
Were you really serious saying it does nothing? LOL....if you were to replace the Network with traditional wiring, you wouldn't have room for passengers....lol....


There's way too much to explain to you, But the most Critical work that is does is make all this safety technology a Reality....Like ESP, ESP doesn't more then One thing.

Each wheel's speed, direction of steering wheel, transmission, engine, brake position etc etc etc, all this info is gathered,processed and commands/action are all done in a blink of an eye. That is just a very small safety feature..

LOL....so how is that Dumb and does nothing? Infact with all the technology expanding the network system is also expanding and improving right now.....

You must have been bored at work when you posted this because the network at work had nothing to do, and when it has nothing to do, it does nothing.

Unless you work on one Chrysler car everyday, then you have no idea what it does. Every network is unique and not the same programing for all jeeps or all trucks. It waits for it was designed to do, Work fast and work well.

This is not new Technolgy....LOL
 
#39 ·
As someone that works with CAN everyday I see a lot of misinformation and confusion as to what the CANBUS is and how it is used in our JKs. So I figured I'd try and demystify it.

CAN = Controller Area Network. It's a two wire (sometimes a 3rd sheilding wire is used) communications link between control modules. Think of it as the link between your computer and your router/modem at home. All the CAN link does is allow modules to pass information back and fourth ie: vehicle speed, engine coolant temp, throttle position, switch states and so on.

The CAN Link (or CANBUS) is dumb, it doesn't know anything or do anything on it's own. It's vital to the operation of your JK because it's how modules get data from one another, which is why it can cause a whole host of problems if the wires get damaged or one module goes down.

All the inputs and outputs in your JK are controlled by the various modules (not the CAN link itself); TIPM, Cluster, Steering Control, Radio, PCM, Keyless Entry, Occupant Classification Module, ABS and so on.... You have three CAN links, all monitored by the TIPM (the TIPM acts as sort of a gateway). You have the diagnostic link that goes only to the TIPM and the DLC under the dash. Then there is an "interior" link that connects to the cluster and the radio and the non critical modules. Then there is the main link that connects between the TIPM, PCM and safety modules.

Maybe now is a good time for an example. I'll use the headlight/foglight circuit since it seems to be the most talked about.

The headlights and foglights are not directly controlled by the CAN link. Yes the CAN link plays an important role but all it does is transmit switch state.

Your multifunction switch is part of the Steering Control Module. The Steering Control Module monitors the switch position and communicates that position to the Cluster via the LIN BUS (another simpler communications network). The Cluster then communicates that position to the TIPM over the CAN link. The TIPM then decodes the switch state from the message and makes a decision about what to do. The TIPM then controls the headlights via driver circuits (transistors for the most part). These circuits have the ability to monitor current and protect the circuit by disabling the drivers. This is why there is no fuse for the headlights or fog lights. This is also why trying to run a higher wattage bulb does not work (without a relay), the system detects too much current and disables the circuit.

In the end, all your inputs and outputs are controlled by a computer module and the module uses the CAN link to share that information. Why make it so complicated?
How would You get a Message from point A to point B then???? Call it by cell phone?

There are a lot of reasons to start with it allows for multiple modules to share inputs without having to run multiple wires (and without having to have multiple inputs - this makes controllers cheaper)
Like which ones share? the Air Bag Module and the The Headlights all sharing a Module? No.....The Human Brain though, is all in one Place, well, most peoples Brain is.....

, more precise control, better diagnosis and better diagnostic control via a service tool for troubleshooting. Is all this necessary for a wrangler?

Do you think Air Bags are the same as when they were First installed in Vehicles?? No....They have the ability to Multifunction but if we bogged the Module down with having to THINK, WEIGHT the Situation, check the seat position and weight, oh and go read the Speedometer and Steering wheel angle...blah blah and after you get that done you lazy Module, Deploy that bag as best as you can without Killing a Human.....Not perfect, but some Help from that Bomb hitting the hardest it can ....The Air bags need that Help don't they???? or Turn yours off if it's a waste of Network

Probably not but I am sure it allows for a lot of cross platform carry over and reduces engineering/manufacturing costs.

Different modules broadcast different periodic messages. The reason you get into trouble removing something from the link is because there are other modules expecting that message and diagnosing it. And in the case of removing the cluster, the TIPM doesn't know when to turn the headlights on.

How do you think it works when you get a light that gas is low, bulb is out, which bulb is out, turn signal out, oil is hot, trans it hot, brake lines are gone...LOL...without a Network of some kind in Life, how would you know what is going on? Network is all around you....

I realize I am over simplifying it, but the principle operation of the CAN link is pretty basic. It's the interaction between modules that gets you in trouble. Though there are ways around this with the right tools and know how.

Simplifying Is A UNDER statement. No network, no job, no music, no tv etc etc

I hope this helps somewhat. Feel free to add to this, point out corrections or ask questions.
You haven't helped except confuse the Interested that are far more Confused now.
 
#40 ·
It just kills me....one of First Statements is it doesn't know anything or do anything on it's own...LOL...
Internet, Network Router does what? Brings things/computers together with the Internet. It did a job, But not sure we'd want it to think on it own and let everybody on the block in on the network. It has a job, a Job we tell it to do.

CanBus is the same It takes a Program/software does what we tell it to.

You might want yours to do things on it's own but Not me...Run you into that tree and forget to deploy Air bags as it Cranks up the Radio...lol
 
#42 ·
Chrysler has started using a Controller Area Network ( CAN ) Bus for intercommunication between most every system in at least 6 vehicles. These vehicles are: 2005+ Chrysler 300, Dodge Magnum, Dodge Charger ( All LX line vehicles ), 2004+ Dodge Dakota, 2006+ Dodge Ram, and the 2005+ Jeep Grand Cherokee.

There are 3 separate CAN buses in these vehicles.

-[CAN-C] Engine / Critical systems CAN Bus ( Engine, Transmission, AntiLock breaks, ESP, etc.. )

-[CAN-B] Interior Comfort CAN Bus ( Climate Control, Car Audio system, Dash cluster, EVIC, etc.. )

-[diagnostic CAN-C] Diagnostic CAN Bus ( Has the capability of pulling service codes from the other two buses, but not directly connected to either )
 
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