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CAN low voltage too low

428 Views 6 Replies 3 Participants Last post by  brcisna

2010 Chevrolet Suburban idles very rough after AFM/DOD delete,, checked compression all 160 psi even across the cylinders

Bought this truck 'as is' knowing of the rough idle situation. The previous owner after doing the cam lifters replace done the AFM/DOD in ECU delete with the supplied DiabloSport Preditor 2. Stock non DOD cam. AFM Racing deluxe kit
After doing some recent checking my Autel entry level handheld scanner would sometimes view the data stream,,and many times would only show half the available pids. Bought a second cheap OBDII ,usb dongle to try as a backup.This reader does the same. Sometimes read data sometimes does not.Both these units work fine on my old 2003 Suburban,
Yesterday done some voltage and resistance checking at the DLC,and with key on battery voltage 12.6 v ,,CAN high shows 2.55 v ,,but CAN low is only showing 2.1v
Anyone have any ideas how to probe backwards on CAN low network. This voltage does diffinately seem too low.
Visually looking all the ground straps motor to chassis looks connected per the GM wiring schematic i was able to conger up.

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The high speed LAN (GMLAN) will read 2.5 volts at network idle; the low speed (Class 2) will read 7 volts at idle.
Thanks for the reply Ray. Everything I found online said with key on CAN High should be slightly over 2.5 volts & CAN Low should be just a few tenths of a volt lower,, 2.45
I will try starting the truck,I see if CAN low is close to the 7 volts. This is new information to me.
Also what is a puzzler the resistance between CAN High and CAN LOW, is what is supposed to be correct of 60 ohms?

Thanks again
There are 2 x 120 ohm terminating resistors on the CAN network so basically they are in parallel. When the network goes to sleep, you can measure the resistance and expect 60 ohms if the module containing the resistors are operational. If one module containing the resistor is dead, you will not get 60 ohms and the CAN signal will be corrupted and your scanner wont be able to link to the network.

Have you looked at updating your scanner firmware? They could be outdated and not able to link to the newer vehicles OBD2 DLC.
Thank You for info @bazar01 ,

I only have an entry level Autel handheld is about 7 years old,,,BUT i did just buy a new OBDII to usb interface that is supposed to be multi protocol compliant. Both these work ok,,on old '03 Suburban but that thing is so old anything will probably work on it.
I did actually do the resistance test across pins 6 &14 at the DLC and have 60 ohms. But the low CAN-Low voltage 2.1 volts is a puzzler. As much monkeying with cars,have never actually dealt with CAN stuff at all.
Havent really had much of chance to put much time into troubleshooting as traveling non stop since dragged the Suburban home.
Need to get more diagnostics equipment to get with it on this project I think,Been wanting pull the trigger on a very good Scanner,,, but it will become a dust collector after i get this rig figured out,like most of my other 'car stuff' tools.

Thanks again
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