CAN low voltage too low

Hello All,

2010 Chevrolet Suburban 5.3L
Purchased this truck 'as is', from previous owner. The previous owner has done an AFM/DOD delete cam & lifters kit on the truck,and since putting back together has never idled correctly. All the grounding straps appear to be in place and tight .
When checking with my entry level Autel handheld scanner sometimes all the live data pids will display and about half the time the live data pids,less than half of them will actually display.

So,,, further investigation, done some checking of voltage & resistance at the DLC.
Battery voltage at DLC is 12.6 volts
CAN HIGH is 2.55 volts
CAN LOW is only 2.1 volts --- this seems too low from what I have read on troubleshooting OBDII at DLC.
How could a person start back probing the CAN LOW circuit in this situation,with a multimeter. I have a good wiring GM diagram of this truck but have no idea were to start with multimeter probing.

Thanks

Looks like your CAN-Hi bias is good, but CAN-Lo bias is 4/10ths of a volt low.

1.Open the driver door, wait at least 15 min and ohm pin 6,14 at the DLC, what do you have? Key-off.

2. Do the same test with the battery unplugged, wait 5-10 min, what othe ohms at 6, and 14?

Plug your battery back in.

Do you have both Diagrams for the Hi Speed CAN system?

If so, divide the system into pieces.

For example; ( these will all be Key ON Engine OFF testing, So a jump pack or charger might be required, be carefull the charger doesn't skew your reading during testing, ie elevate it a bit, we're only talking 4/10ths of a volt.)

Start right below at the EBCM, unplug it, is the 4/10th's restored at the DLC?

If not, plug it back in and go to the next module in line which would be, in order, Fuel Pump Module, then Trailer Brake Control Module, then Electronic suspension Control Module ( if you are equipped ).

Did you restore the 4/10 of a volt?

If not, you will rinse and repeat working your way following the three other paths of the bus.

Should you be successfull following this routine, the unplugged module that restores the 4/10's of a volt is the culprit, check pin fitment, check for green cruddies, check module powers and grounds( with at least a 4 amp load; headlight bulb)

Also once the voltage is restored (with the module unplugged) verify that you get no more drop-outs on the live data.

This testing will set all sorts of module codes and Comm codes due to the fact it is Key-on testing. Be prepared for that.

Also, a scope could be required for further direction.

Hi Paul,

Thank You for the detailed workflow you gave. I will try and go through what you have posted here.
I did do the resistance test between ping 6 & 14 and was right at 60 ohms.

Have watched several Scanner Danner videos and has helped get a better understanding of the CAN bus in general.

Edit: Forgot to mention ,when the previous owner done the AFM/DOD delete he did get the DiabloSport Preditor2 as part of his 'deluxe DOD delete package" to delete the DOD/AFM portion,settings in the ECU. . Since checking into this have read several horror stories on these handhelds,and am wondering if this may have something to do with what is happening. The gentleman seemed very thorough at what he done.

Thanks again

Sweet, 60 ohms is a good thing. That means the bus is intact from one end terminating resistor to the other.

So your best bet is to keep unplugging modules on the bus until you can restore the 2.5 volt bias.

Since checking into this have read several horror stories on these handhelds,and am wondering if this may have something to do with what is happening

I won't say it's not possible, but maybe unlikely in your case. I woudn't think a "software" tune could cause 4/10th's of voltage loss.

Time to unplug modules one by one.