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Discussion Starter #1
I have noticed on the forum there is a persistent misunderstanding about the CAN bus, especially in context of flickering headlights. Hopefully I can clear up a few misconceptions about these different terms so people understand what is happening - there's not a lot of black magic going on here.

  1. Flickering headlights have nothing to do with a CAN bus. Zero. It's because of the PWM (pulse-width-modulation) power signal being sent to the lights.
  2. The CAN bus is a networking technology, used only for the various ECUs in the car to communicate with one another.
  3. PWM and CAN bus are not the same thing, nor are they describing a similar electrical activity.
  4. Likewise, for the 'headlight is broken' signal, that also has nothing to do with the CAN bus. It happens because the wiring to the lights allow the TIPM module to tell if voltage is passing through the light or not.
  5. All Jeep JK's have had CAN bus technology, since the beginning of the design.
I think some of this misunderstanding persists because some vendors are incorrectly labeling their anti-flicker harnesses as defeating the CAN bus - wrong. They're doing that either of out their own misunderstanding, or a marketing desire to make that scary CAN bus look like something that is hard to solve.

A lot of people have gotten this right on the forum, so I won't reiterate everything that has been covered before. But, the gist of it in a bit more detail:

The TIPM (totally integrated power module) is the brain of your Jeep. It not only has a sophisticated group of micro controllers within it, but it also acts as a old-fashioned relay / circuit box for the vehicle. It's the one controlling relays and sending power out of the other circuits in the car.

For the lights in particular, the TIPM uses a pulse-width-modulation signal. If you look at this signal with a oscilloscope, you will see a square wave. The end result is that the lights get approximately 8 volts of power sent to them, in pulses, rather than a constant 12 volts. This increases bulb life and reduces fuel consumption (probably on the order of 1-2%). For incandescent bulbs, this is a great trick as the bulb won't start to dim before the next pulse of power is received. For LEDs or other lights, it doesn't work the same, so that's why you get a flicker.

You'll note that I said nothing about CAN bus in the previous 2 paragraphs because it wasn't in play at all. Where the CAN bus comes into play is when the ECUs talk to each other. For example, your Cab Compartment Node (CCN), aka your dash, talks to the TIPM via a CAN bus connection. So does your radio and the Uconnect module. The engine and transmission ECUs talk to each other and the TIPM through a CAN bus as well. There's a bunch more...

The best way to think about a CAN bus is to treat it 100% like a computer network. If you've ever plugged in an Ethernet cable from one computer to another, or to a hub/switch, you are on the right track. CAN bus is a much simpler technology, and only needs two wires plus a shared ground. You can have a few hundred different ECUs on a single CAN bus, all talking happily amongst themselves.

If you go look at the connection pinouts at Mopar Connection Repair Kit you will see pretty clearly if a CAN bus is involved in your circuit or not. Let's compare two examples, both from a 2008 JK. In the first, the headlamp connector, you see a pretty standing wiring arrangement. It doesn't mention a PWM signal, of course, but that's in the service manual. On the second, you actually do see a CAN bus wiring pair mentioned. So there's never a surprise if a CAN bus is involved or not.

I hope this helps clarify things a bit. If anyone has any more questions or would like more details, I'll do my best to explain.
 

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Thanks for the great write up on the CAN BUS, I'm sure that people will definitely find it helpful.
 

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Discussion Starter #3
For a concrete example of what happens, let's go through the flow of what happens when you turn the headlights on. Let's assume this is a manual manipulation of the switch, and not the Auto mode.

  1. You turn the headlamps on using the multi-function switch stalk on the left-side of the steering wheel. This multi-function switch is hard-wired to an electrical module called the Steering Control Module, or SCM.
  2. The SCM senses the switch change, and sends a message to an electrical module called the ElectroMechanical Instrument Cluster, or EMIC. This module is also known as the Cab Compoartment Node, or CCN - either way, this is your instrument cluster. The SCM tells the EMIC about the headlamp switch over something called a LIN data bus, which is a very simple 1-wire serial link bus that runs at a speed of 9.6 Kbps. It is typically used for switches to communicate to an ECU that is monitoring them.
  3. Now that the EMIC knows about the headlamp switch being activated, it sends a headlamp and headlamp beam request message to the Total Integrated Power Module, or TIPM, over the CAN interior bus.
  4. (This text provided directly from the service manual.)The TIPM responds to these messages by providing a pulse-width modulated voltage output to the headlamps through high side drivers on the right and left low and high beam feed circuits to illuminate the selected headlamp filaments.
  5. (This text provided directly from the service manual.)The TIPM also sends the appropriate messages to the EMIC to control the illumination of the high beam indicator.
So as you can see, the digital networking busses in the car (the LIN and CAN busses) are only used to communicate to the ECU about what action to take, and what happened, but the actual voltage control of the headlamps is a simple PWM driver to the headlamps themselves from the TIPM.
 

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I believe the misconception occurs because new cars use this network "BUS" technology where as before in older cars you flipped a switch that went through the fuse hit a relay and provided voltage direct from the battery, no modules to provide this integrated power, so the word CAN-BUS should be swapped with TIPM to make everything sound legit. PWM could not occur in a car without a BUS type system because nothing would regulate the voltage to pulse. Sometimes computers make our lives worse than better. So maybe I'm to blame for the misconceptions of this forum sort of, but I still understood it was the PWM causing the headlights to be terrible.

A wiring harness gets rid of the PWM because its pulling power direct from the battery and the harnesses that LED lighting people sell just use a capacitor to help store the voltage to correct hold voltage that tricks the PWM.
 

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This is awesome, thank you. Definitely sticky material.
 

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For the lights in particular, the TIPM uses a pulse-width-modulation signal. If you look at this signal with a oscilloscope, you will see a square wave. The end result is that the lights get approximately 8 volts of power sent to them, in pulses, rather than a constant 12 volts. This increases bulb life and reduces fuel consumption (probably on the order of 1-2%). For incandescent bulbs, this is a great trick as the bulb won't start to dim before the next pulse of power is received. For LEDs or other lights, it doesn't work the same, so that's why you get a flicker.
Question: How does this reduce fuel consumption?

.
 

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The theory would be less power draw, the alternator does not need to work as hard. The reality because of the computer systems we now have a standard 160 amp alternator inside of the old 35/60 amp alternator.
 

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Discussion Starter #9
Yeah light bulbs running can actually be a bigger draw on fuel than you'd expect. I remember reading some GM data years ago that DRL's were 2-3% economy hit. This is old, old data (pre-2000), so who knows what the numbers are today.
 

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Discussion Starter #12
I believe the misconception occurs because new cars use this network "BUS" technology where as before in older cars you flipped a switch that went through the fuse hit a relay and provided voltage direct from the battery, no modules to provide this integrated power, so the word CAN-BUS should be swapped with TIPM to make everything sound legit. PWM could not occur in a car without a BUS type system because nothing would regulate the voltage to pulse. Sometimes computers make our lives worse than better. So maybe I'm to blame for the misconceptions of this forum sort of, but I still understood it was the PWM causing the headlights to be terrible.

A wiring harness gets rid of the PWM because its pulling power direct from the battery and the harnesses that LED lighting people sell just use a capacitor to help store the voltage to correct hold voltage that tricks the PWM.
Yeah you are mostly right - you're not likely to see PWM without a micro-controller somewhere in the mix, but it's not a mandatory thing. The reality is, that's what's been going on in automotive since the 1980s so this isn't a new thing, really. Memories of how bad cars really were back then when all of this was new...

The engineering decision to go with PWM for the headlights seems weird to me, so I wish we had some real data from the engineers to share and speculate about. I mean, the TJ didn't do that, correct?

The harnesses that use direct battery feed and use the main headlight driver just as a signal switch is totally the right way to go as a work-around. It's clean and simple. The CAN bus controller project I'm working on is another way, but it's more complex/powerful so we'll see if that ever comes to fruition or not.
 

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The harnesses that use direct battery feed and use the main headlight driver just as a signal switch is totally the right way to go as a work-around. It's clean and simple. The CAN bus controller project I'm working on is another way, but it's more complex/powerful so we'll see if that ever comes to fruition or not.
Excellent info. Thank you, Dr. DC!
 

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Yeah light bulbs running can actually be a bigger draw on fuel than you'd expect. I remember reading some GM data years ago that DRL's were 2-3% economy hit. This is old, old data (pre-2000), so who knows what the numbers are today.
If any truth to the above would LED headlights improve Fuel Economy since their draw is so low?
 

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Discussion Starter #15
If any truth to the above would LED headlights improve Fuel Economy since their draw is so low?
You know, I would say yes, but can you perceptibly measure a 1-3% gain either way? You'd go from say 17 mpg to 17.5 mpg in that case... I wouldn't notice it.

I think it matters more across the fleet of vehicles when they're doing the CAFE stuff, but otherwise?
 

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Thanks for the information! Just a quick question, is that true of all lights in the vehicle? Suppose I put an led 3rd brake light, same thing?
 

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Wow! Somebody's making it rain knowledge up in here. Sometimes I miss the good ole days of my '68 Chevelle. Thanks for explaining this to us dummies.
 

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Thanks for the information! Just a quick question, is that true of all lights in the vehicle? Suppose I put an led 3rd brake light, same thing?
I think this is where it gets confusing for most people. Some light sockets don't have any issue with a flicker from LED bulbs, and some don't have an issue with the LED bulbs itself, but the lack of a higher voltage causes it to believe there is a bulb out and cause other issues.

For example, the dome lights in a 4 door JK from front to back are -
map lights on the mirror
b-pillar lights over the front seats
rear cargo lights over the rear seats

The mirror lights are already LED, the front seats lights are two pods with a 194 sized bulb in each facing sideways, and the rear cargo is a festoon bulb.

You can change out the front bulbs with any 194 LED and it works fine. No flicker, no "CANbus" issues.

If you change out the rear cargo light with a basic (not CANbus rated) LED bulb you'll find everything works but the map lights will stay on after the doors are closed. In doing my LED project, I found the map lights will stay on constant when the Jeep is running, and will time out after about 15 minutes when turned off and parked. Very similar to what happens if the door isn't closed tight and the lights stay on for about 15 minutes before timing out.

To fix this issue with the cargo dome light I ran a tiny 12v standard bulb inline on the hot wire to provide the feedback for the brain to figure out there's a bulb there and it's ok. Now whether that brain is the TIPM or the CANbus, that's where most just call it a CANbus issue and leave it at that. That includes myself, as I understand what's happening but I could really care less as the I know the fix too.

The biggest problem I see is that back when cars began having CANbus systems and bulb issues came up - the label for compatible bulbs was "CANbus compatible" or "CANbus error free", so this pushed the reason for bulb issues to being a CANbus issue.

Canbus Error Free LED bulbs Frequently asked questions
 

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