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2023-11-06
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Jeep JK CAN Bus and Flashing Sway Bar Indicator

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Tagged As: DIY, How To, JK, Jeep, Repair, and Wrangler

Introduction

Your Jeep won't start. Your dashboard lights are going completely bonkers. You're already late for work. WTF is wrong with your Jeep today?

If you follow the various Jeep forums, most of the recommendations come back to a loose battery terminal. And in what seems to be an overwhelming number of cases, really tightening those flimsy JK clamps down seem to resolve the matter. But sometimes, the culprit is a little more sinister and hard to track down.

Sway Bar Failure

Regarding your Jeep's ability to ford water, Jeep claims:

With Trail Rated® capability on your side, you can traverse bodies of water that other vehicles wouldn’t dare attempt. Electrical connections and body openings are sealed, and the air intake is positioned higher to help protect your vehicle when driving through overflowing creeks, streams and deceivingly deep rainstorm puddles.

Page 472 of the Owner's Manual clearly states:

This vehicle is capable of crossing through water at a depth of 30 inches (76 cm) at speeds no greater than 5 mph (8 km/h).

Neat - but what does that have to do with a sway bar?

The Rubicon features an Electronic Sway Bar Disconnect feature on the dashboard, allowing us to lazily remain in the vehicle while off-roading and enabling increased articulation. Unfortunately, the electronics for this component are notorious for not being waterproof and is definitely below the stated 30" of safe water-fording ability. Even without water crossings, it's only a matter of time before this component becomes water logged from rain or car washes and is a well documented complaint of owners across various forums.

The sway bar harness is located under the bumper, just in front of the tie rod (about 16" off the ground). Removing the harness to check for moisture is pretty easy. There is a gray latch that lifts and rotates across the harness body. Then just jiggle the connector until it comes free. I recommend situating yourself to the side while doing this to prevent water from pouring onto your face. NOTE: Unfortunately there is also an access hole from inside the connector to the actuator and sway bar linkage - which depending on how much water has gotten in may lead to component rust, too.

This quick YouTube clip from Doyles Automotive LLC easily shows the water damage you're likely to find inside the module.


When this part fails, the "Sway Bar" light on the dashboard will flash in yellow. Initially, it's somewhat easy to ignore for most owners as its not that often you'll be in 4LO going for maximum flex. After all, the sway bar is connected by default so if it were having a fault, it is presumably irrelevant for daily driving safety and can be fixed later. When mine began flashing, I simply told myself I would have this addressed the next time I went for services.

What is the CAN Bus

Direct from Wikipedia:

A Controller Area Network (CAN bus) is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. It is a message-based protocol, designed originally for multiplex electrical wiring within automobiles to save on copper, but it can also be used in many other contexts. For each device, the data in a frame is transmitted serially but in such a way that if more than one device transmits at the same time, the highest priority device can continue while the others back off. Frames are received by all devices, including by the transmitting device.

There's a great video and write-up on the CAN bus from CSS Electronics, a company that specializes in products related to ... the CAN bus.


In a nutshell, before the CAN bus, the engine was entirely mechanical in its operation and any indications of problems [maybe] were provided directly from a sensor to a dashboard light. As ECUs entered the scene for improved emission and engine control, it also became easier to manage all of the vehicle sensors through that central computer. However, that would still be a lot of independent wiring so the CAN bus allowed for all of those signals to be multiplexed digitally across a common set of wires. In general, this greatly reduces the amount of wiring looms necessary under the head as they can now be shared.

The Problem

On the Jeep JK, one of the CAN bus lines specifically handles signals from:

  • ANTILOCK BRAKE MODULE
  • POWERTRAIN CONTROL MODULE
  • ENGINE CONTROL MODULE (DIESEL ONLY)
  • TRANSMISSION CONTROL MODULE
  • STEERING ANGLE SENSOR
  • AUTOMATIC SWAY BAR SYSTEM MODULE
  • WIRELESS CONTROL MODULE
  • OCCUPANT RESTRAINT CONTROLLER MODULE
  • OCCUPANT CLASSIFICATION MODULE
  • TOTALLY INTEGRATED POWER MODULE

This information was derived from a technician service manual apparently hosted on Amazon AWS that Google was able to index. At least this portion of the manual is specifically related to diagnosing problems with "U0002–CAN C BUS OFF PERFORMANCE."

Initially when the sway bar system fails, it simply triggers a dashboard light because the module is operating incorrectly (due to being filled with water). It took mine about a month after it first flashed at me to escalate to the next level. When it really fails the module begins interfering with the CAN bus operation. Essentially, it is either generating spurious signals or voltage fluctuations onto the shared wire which cause "collisions" that prevent good signals from other components from communicating. TotalPhase has a decent overview of message collisions and frame formats and this discussion on StackExchance has solid explanations of the electrical signaling on the bus.

You can see the result of all these miscommunicating parts with a simple OBDII reader which will report a multitude of codes. The important, and real, code is U0002 which relates to the performance of the CAN bus. Effectively, the Jeep's ECU was reporting that it could not communicate with the plethora of devices connected on those wires.

The result of these signal failures immediately result in all of the dashboard lights and indicators going haywire in addition to particularly dangerous phenomena related to the transmission. On my Jeep that included:

  • Transmission reporting P, R, N and D all engaged simultaneously.
  • Inability of transmission to shift out of 1st
  • Check Engine Light
  • ABS Lights
  • Traction Control Lights
  • Tachometer bouncing between real RPM to 0
  • Temperature Gauge spiking
  • Display reading "HOT OIL"
  • Engine Fan running at insane speed

Everything eventually culminated with an inability to even start the Jeep. At first I thought it was related to the faulty electrical components perhaps misbehaving and continuing to draw power all night draining my battery. However, that checked out fine and eventually, I isolated the problem back to the sway bar interfering with the CAN bus and specifically that the Jeep did not believe it was in Park which prevented it from starting.

The Fix

Unplug it.

Disconnecting the harness will prevent the failing swaybar's electronics from continuing to interfere with the CAN bus. It can no longer introduce errant signals, tie up the signaling lines, or introduce voltage corruptions onto the bus that prevent the Jeep's working components from communicating to the ECM. If you have a hope of eventually fixing the parts or are covered by a warranty, at least wrap the exposed harness plug and connector securely in plastic to prevent further damage and the wiring to prevent it from interfering mechanically with the suspension.

To truly address the issue, you're really facing about three choices:

  • You can pay more than $2000 to have an out-of-stock component completely replaced (and likely face the issue again anyway).
  • You can remove the electronic sway bar and replace it with a variety of after-market options like Currie AntiRock, JKS Disconnects, or TeraFlex Sway Bar System to name a few.
  • You can remove the electronic actuator and retrofit it with a manual option like the EVO No Limits Manual Disconnect or the JKS Cable Conversion.

It's pretty unlikely Jeep is actually going to fix this problem, especially since the JK line was already retired six years ago. Out of the 2.1 million JK Wranglers produced by Jeep, perhaps only 18% of them were Rubicons. Although the electronic sway bar was definitely susceptible to the water issue, by the time the failure occurred, the vehicles were likely already out of warranty, time to upgrade the component anyway, or the owners simply didn't care anymore. Thus, although annoying, it is easy to understand the business decision to ignore correcting the matter altogether.




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