Re:2011 Jeep Liberty can c help

Hello everybody-
Have a 2011 Jeep Liberty that's giving me a headache.. Was towed to shop as a no start. Off course it fired right up when I went to it. Drove around for 3 days no issue. On my last drive- when I approached it, I noticed the key fob wasn't working. Unlocked door with key. Alarm went off and that was it. With key on- fans...windows..radio.. lights all work but the car is a no crank. Dash lights doing weird things.... prndl flashing at random. Plugged in scantool- truck auto id's through tipm but I have a no comm with anything on can c. Measured resistance on 6 and 14 and I have 60ohms for the "diag network" When I measure resistance at the pcm- I have 118 ohms on can "c". I start unplugging things and realized that when I have the wcm and 4wd modules unplugged- I can talk with abs, pcm, steering angle and srs modules without issue. I plugged everything back in. Closed it up and it sat for 2 days until I had a minute to get back into it. When I approached it- the key fob worked and the truck started up. Scanned it for codes- and the only module that had a code in was the pcm claiming comm issues with TIPM. Truck continued to behave. Customer took it back and 3 weeks later- as expected- it was towed in with same issue. Don't have very much can experience- but I feel as though I have a module that has a failed terminating resistor on can c. I had another Liberty in (knowing this one will be back) and I quickly checked can c resistance and it was 60 ohms. Looking at shopkey wiring diagrams- I cant see where it shows what module these are located in? How do you know? Any help with any of this would be much appreciated. Thanks in advance!

The CAN network, generally, has two 120 Ω. One at each end of the network. With two 120 Ω resistors placed in parallel, total network resistance is 60 Ω. Since your resistances check out, I would check for good POWERs and GROUNDs on all modules. Do you have a labscope to look at the CAN Bus signal?

Just last week, I had a Pontiac G6 that went to the local Brake/Muffler shop for an intermittent no start. They threw a starter at it and turned it into a never start, No Communication with "Error" displayed in the instrument panel. My resistances checked out fine, but the signal looked horrible. To make a long story short, I found 8 volts on my Transmission Control module GROUND. This bad ground was causing the TCM to corrupt the network. After fixing the engine ground, communication was restored and the signal looked beautiful.

fudgie wrote: When I measure resistance at the pcm- I have 118 ohms on can "c".!

Did you backprobe the PCM or stab the wires to take that measurement? If you unplugged the connector and front probed it then one of the 60 ohm terminating resistors could be in the PCM. Fairly common module for one of the terminating resistors to be in.

Thanks all for the responses. I measured resistance in 2 places. One was at the pcm (unplugged) and to confirm- measured again at the 4wd module unplugged. Both results were the same.

Well ya need to do what chad says. Scope the lines. Or just start unplugging modules one at a time.

Matt T wrote: one of the 60 ohm terminating resistors could be in the PCM

I'm sure you meant one of the 120 Ω terminating resistors.

Chad wrote:

Matt T wrote: one of the 60 ohm terminating resistors could be in the PCM

I'm sure you meant one of the 120 Ω terminating resistors.

:oops:

Quick update. Had about 30 seconds between jobs today to look at this.... with all modules plugged in using piercing probes inside the truck, battery unhooked- I have 119 ohms on can c. Attached is a quick pic of the waveform. Its ugly. As Chad suggested- I think its time to start checking grounds on every module. Would still love to know where these resistors are located. Not sure why they are not shown in any diagram I have yet to look at in shopkey.

It sounds like you are missing one of the modules that has a terminating resistor. Some service info digging on the can system might help find which modules have the resisters so you can start your tests there.

Nissan Technician

Quoting from your original post,

Measured resistance on 6 and 14 and I have 60ohms for the "diag network"

Quoting from you last post,

...with all modules plugged in using piercing probes inside the truck, battery unhooked- I have 119 ohms on can c.

What changed? 60 Ω is a good measurement. 119 Ω (120 Ω), as Tutti57 pointed out, indicates a missing Termination Resistor.

According to SI, Termination Resistors are located in the Power Control Module (PCM) and the Wireless Control Module (WCM).

Hey chad-
Pins 6 and 14 are the diag network . The can c network is my issue. Thanks for info. So- if I measure resistance on wcm unplugged I should have 120 ohms and 120ohms at the pcm?