Need a little help with CANBus trouble.

[Ham-Truck]

I've traced out a problem with my truck not starting as caused by a problem with CANBus on my truck; in particular CAN_Hi & CAN_Lo are both reading 11.49VDC from V_BATT on the ODB port with engine off. CAN_Hi to SIG_GRND measures as 5.679 kOhms, CAN_Lo to SIG_GRND is 5.634 kOhm, CAN_Hi to CAN_Lo is 121.6 Ohm.
Engine on has CAN_Hi at 1.52VDC & CAN_Lo at 1.06VDC from SIG_GRND.
My main mechanic says that it's likely a fault in either the wiring or in one of the CANBus modules, but where are they all?
I know where the ECU is, & that the Instrument Cluster is another, but where are the rest?

My main mechanic is too busy to look at it anytime this month, and my back-up mechanic tried to sell me a new ECU for $900.

 

[BattleTank]

Are you saying you are seeing batt voltage on CANH/CANL? That's a huge no-no. Power off, you should see nothing after all the modules go to sleep.

If you are measuring OHMS on CANH and CANL, you should be seeing 60 ohms, not 120. It appears you have a dead module somewhere.

Are you noticing anything wrong electronic wise? Radio, windows, cluster etc? All working ok? Trans ok?

 

[BattleTank]

Also, a lot of times, if you pull fault codes and see a lot of U codes that lost communication with someone, that's a good indication too.

 

[davidavidd]

A few years ago something similar happened to me, basically everything is connected to the can bus: A/C, radio, seats, some lights, etc.
Usually above the pedals there is a can bus switch or splice, disconnect everything except the computers required to start the engine, if the engine start, just try connecting one by one until you find the one that "knocks down" the CAN network, in my case it was the seat heater control.

On some trucks there is a reported fault of broken wires next to the steering column and almost all the wires that run through there are vital to the ignition, so once there, take the opportunity to check for open/cutted cables.

 

[j91z28d1]

if you have air ride shocks, the modual for it is in the back and pretty common to get water and stuff in it, taking down the whole network. there's some youtube videos on it.

 

[Ham-Truck]

Are you saying you are seeing batt voltage on CANH/CANL? That's a huge no-no. Power off, you should see nothing after all the modules go to sleep.

If you are measuring OHMS on CANH and CANL, you should be seeing 60 ohms, not 120. It appears you have a dead module somewhere.

Are you noticing anything wrong electronic wise? Radio, windows, cluster etc? All working ok? Trans ok?

A dead module you say...
The only thing I'd previously noted is that rolling down the road (especially when it's wet) sometimes I'll hit a buckle in the road & the gauges in the cluster will zero out & I'll get the "Service StabiliTrak" message on the InfoCenter & the StabiliTrak disabled light will illuminate. A bit later, it'll all come back on.

No, I don't have AirShocks, and this LT model is not equipped with memory seats.
I'll check around the steering column, between the steering rack & pump, and the back of the bellhousing, and the termination resistor over the spare this morning to be sure.

This madness is eating me alive in rental car fees & worse, chewing up my workday when I spend half of it trying to get wheels under me.

 

[Ham-Truck]

I got the computer to talk breifly through the ODB2 port after cycling the ignition off, this tells me that you're on to something @BattleTank

 

[Just Fishing]

When i had lights going crazy, it was a ground.

I even had the truck stall once after a long drive home with both the front and rear ac on full blast.
Hours before that i drove through a mud puddle.

I found the mud coated the grounds on the driver side under the driver seat, and the rear seat.

I cleaned everything, and then removed the grounds to inspect.
They looked great, so i cut the wire insulation back.
I found green corrosion!

I cleaned that up with electrical contact spray and soldering flux, then flowed solder into the joint.

All has been great since then.

 

[petethepug]

What you are describing is the classic scenario of rotten + / - battery cables degrading from the inside out. They look pretty outside, inside they’re resisting and laughing at you chase codes.

Change em out, THEN start following the real codes because the can/bus will have full, uninterrupted 5v to communicate.

 

[Ham-Truck]

What you are describing is the classic scenario of rotten + / - battery cables degrading from the inside out. They look pretty outside, inside they’re resisting and laughing at you chase codes.

Change em out, THEN start following the real codes because the can/bus will have full, uninterrupted 5v to communicate.

I would, but I just replaced them all with brand new 2 ga made in USA wire only 6 months ago. I crimped them. wrapped them in 2 layers of wire loom after putting adhesive heat shrink tubing on the ends.
I did both battery hots (daul battery mod), the isolator to the alternator, the grounds to the chassis from each battery, & the wire to the starter, I even put in a new one bonding the engine to the vehicle.

 

[BattleTank]

Have we gotten codes out of it yet?

 

[Ham-Truck]

Found it!!!
The ABS module was cactus. The fuel pump control module connects to the ECU through it.

What's the spread on the ABS module being out causing the vehicle not to crank?

(Background: the start relay only fires when the ECU is commanded to by a diagnostic test, it will not fire the relay when the key is cycled, and it does properly report the key's status.

 

[Ham-Truck]

Have we gotten codes out of it yet?

Only codes are fuel pump system & EVAP related, see above for the fuel pump & the EVAP is a sticky vent valve.

 

[BattleTank]

Think about the CAN network like your 2 wire phone network in your house. Back in the old days(90's) and we had dial up network and you were trying to watch Mary Jane Rotten Crotch do weird things and your mom downstairs picked up the phone to call someone and interrupted your 'video'. Same thing applies here, if a module takes a dump, it will interfere with other modules trying to chat because they are all daisy chained via the 2 wire 'phone line'.

 

[j91z28d1]

Found it!!!
The ABS module was cactus. The fuel pump control module connects to the ECU through it.

What's the spread on the ABS module being out causing the vehicle not to crank?

(Background: the start relay only fires when the ECU is commanded to by a diagnostic test, it will not fire the relay when the key is cycled, and it does properly report the key's status.

if you have the abs modual pin out. you can unplug it, jump the can in and out pins to complete the circuit and see if your can network comes back it. normal truck should start without the abs module plugged in.

it so, it could be your abs module, or you could also find a bad pin or connection at if while doing it.

 

[BattleTank]

^ What he said!

 

[Ham-Truck]

When i had lights going crazy, it was a ground.

I even had the truck stall once after a long drive home with both the front and rear ac on full blast.
Hours before that i drove through a mud puddle.

I found the mud coated the grounds on the driver side under the driver seat, and the rear seat.

I cleaned everything, and then removed the grounds to inspect.
They looked great, so i cut the wire insulation back.
I found green corrosion!

I cleaned that up with electrical contact spray and soldering flux, then flowed solder into the joint.

All has been great since then.

Thanks, but it's not lights going crazy, it's the ECU not allowing the vehicle to crank. It registers the key turning to Start, and will fire the starter only when commanded to by a scan tool..

 

[j91z28d1]

Thanks, but it's not lights going crazy, it's the ECU not allowing the vehicle to crank. It registers the key turning to Start, and will fire the starter only when commanded to by a scan tool..

on mine that's scary cause everything is involved with enabling power when key is on, but a normal truck, what all can inhibit crank if it's seeing key requests. Immobilizer? neutral safety switch? is your little PRND indicator lit up? I seem to remember that means something in the tranny gear switch won't tell the ecm what gear it's in. so it doesn't start. some new stuff won't start without your foot on the rake but I don't think these do that.


yeah, I don't know. sounds theft related.

 

[Just Fishing]

Just saying, bad grounds make things go fkn nuts.

 

[solli5pack]

So is the CAN down or not? If it is that's why the truck is not starting and you're gonna need wiring diagrams. First step is to check ohm's with a volt meter at the Obd port to see if the CAN is intact. I'm not sure what the correct ohmage should be and what two ports you need to check but you can find it out pretty easily with some research. Pine Hollow Diagnostics on YouTube has some pretty good videos on CAN troubleshooting.