NEED HELP Wiring/Class 2 data Woes (2001 Silverado LS RWD)

[jakster918]

So , as the title states I have an 01 Silverado, been through a lot , ultimately, im having issues with the truck staying in limp home mode , but thats a topic for another thread , another day. The issue at hand is that whenever i try and connect a scan tool up the obd2 port , the tool is unable to connect to any modules ....now, im gonna try and keep this as detailed as possible while at the same time staying as concise as possible and hopefully some of the great GMnation minds here can help me get some things ruled out and maybe even zeroed in on what ails this truck. So inwill begin by saying that i have previously installed(not professionally) some aftermarket electrical device that gave since been removed so as not to muddy the waters, everything atm is stock, previously the truck did have functioning remote keyless entry and alarm system. the tools im working with are a DMM, a power probe like tool called a kyzee D10, an odb2 breakout box, a very basic oscilloscope and a vcx nano from vxdiag WITH a copy of tech2win and some resources from here for schematics and a month long subscription to allldata diy for. schematics as well......so here goes , i will recheck again if someone suggests it, but i have previously verified that any related fuses and or relays(cigar, ecm, fuse etc) are intact and working , when i plug in any scan tool the tool will power up but comms will fail, verified on 2 or three different tools, tools are unable to communicate with any module and it will act as though i dont have it connected to the vehicle, verified coonstant power ~12 volts on dlc port pin 16 and verified ground on pins 4 and 5 ......pin 2 data line at the beginning of all this was showing ~.342 volts fluctuating. after much work i was finally able to locate the two splice packs in the driver side dash, the one located behind the interior light dimmer switch as well as the one located near the left side fuse block. well i had tried the method removing the ground combs from the two splice packs and then subsequently removing modules from the network , one by one to see if data was restored but my readings never really changed. Last night however while i was moving the wiring that runs to the interior dimmer switch and the around to deep inside the dash to another mass of wires , i finally was able to get pin 2 to show signs of life , and it was lighting up real nice and hovered around between 3 to 7 volts with heavy fluctuations within that range.....was still unable to acquire any comms through the vcx nano
....it was late , so i decided to tackle the issue next morning ....But the. went i went out next day was frustratingly back to no signs of life on pin2 ....and no amount of wire jostling seemed to bring it back. Please can someone here please suggest some type of method for me to find out if, perhaps my bcm or another module is bad , or if theres a way to go about this is to do one of the methods i previously mentioned, but do it better , please let me know , and lastly , perhaps someone can suggest a systematic way for me to approach locating the fault in my wiring so i can isolate and repair whatever the issue is on the particular circuit so i can get this obd2 port going so i can talk to the truck and hopefully get this limp mode fixed and get her running well again, thanks in advance y'all, have a good one!

 

[mrrsm]

A few suggestions to investigate here...

(1) Establish a Base Line Battery Voltage and its Condition with your DMM. The Class 2 Network Typology is very sensitive to Low Voltage Conditions. You can do this best by hooking up your Break-Out Box and using your DMM to take DC Voltage Readings (KOEO) via your DMM Black Probe at the #4 and #5 Ground positions for the DLC Port...and the Red Probe in the #16 Pin Position.

(2) Re-Visit Will Robinson's eponymous YouTube Video covering his investigation of Class 2 Network FUBARS on a 2006 Trailblazer that is VERY edifying as it follows discreet Step-By-Steps for an IDENTICAL Investigation proving that on the Class 2 Networks....ANY single Failing Module can Take down the ENTIRE Single Wire Network. See the Video Linked Below:

(3) Investigate the Harness Connections at the PCM-ECU for any signs of Rodent Depredation, Damaged or Loose Connectors, Old Add-On Accessory Hook Ups and signs of Shorted or Damaged Wires...RIGHT AT THE SOURCE. PULL THE CONNECTORS AND SPRAY THEM OUT THOROUGHLY WITH CRC ELECTRICAL SPRAY SOLVENT:

Amazon.com : crc electric cleaner

(4) As per Will Robinson's similar experience with NOT Being able to us his Wireless DLC Scan Tool... Try to use an original Wired Scanner that plugs directly into the DLC (or via your Break Out Box Feature) for this investigation in lieu of using the VXDIAG NANO Device.

(5) Finally... Download and Install the FREE *GPT4ALL* AI and collect the Llama 3 Large Language Model as shown below that produced some additional suggestions to consider. I have found this Artificial Intelligence can often be Spot On with Automotive Diagnostics (and many other areas, too) as shown in *IT*'s response to your specific problem here:



You can find and Download a FREE Completely PRIVATE, OFF LINE LLM HERE from Nomic called *GPT4ALL*

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[jakster918]

i appreciate the feedback, so i have checked the connections at the ecu but i will look af them again, its wild but i have refrenced that youtube video no less than 10 to 15 times , but i will do a refresher (never hurts) interestingly enough i have been using copilot as of late and used deep seek prior to that as troubleshooting "wingmen" and also to interpret data that i get from dmm, so i very sincerely appreciate the feedback and time u took to answer. i will establish a baseline and recheck ecu connections as well as try to work in the oscilloscope to get some additional data and then i will look at getting a logic analyzer, ty again!!

 

[mrrsm]

Pull ALL the PCM & BCM Connectors and Spray them out with THIS CRC Stuff:

Amazon.com : crc electric cleaner

If you decide to isolate the PCM and BCM for a more discreet investigation, here is another look at what such a Bench Top Rig Looks Like... it may give you a more direct idea of what to shoot for in your Schematic and "Home Made" Custom Bench-Top Harness Layout:

SOLVED! Thread 'How to Make A "GYMKO" Tech 2 Bench-Top Test Harness'

Feb 11, 2018

BIGGER EDIT: 04/24/2019

For THE BEST On Topic Solution ...Go Directly to Post # 42...

BIG EDIT:

This Thread originally was to cover creating a Test Harness to use exclusively for diagnosing and repairing the GM Trailblazer and Envoy, etc Instrument Panel Cluster and Components... but eventually it achieved a life of its own for Testing and Repairing ANY of the Many GM Modules... as long as they are added properly to the "GYMKO" Bench-Top Harness via the precise mirrored PCM and Harness Pin-Outs. More work needs to be done to take each of...

• mrrsm
• Replies: 58
• Forum: Tech 2

 

[Mooseman]

At the main splice pack, remove the comb and jump just one module at a time to the OBD plug for any signs of life starting with the PCM and then the BCM using the VCX and Tech2Win or the oscilloscope for any network traffic. If you get traffic from one, add another module, and another until either all of them are reconnected or one of them brings down the network. It's usually one module that brings down the network or one of their wires is shorted.

 

[jakster918]

good stuff , ty both , i have some oscilloscope leads coming tomorrow and will tackle this anew and let yall know what comes about, thanks so much!

 

[TJBaker57]

good stuff , ty both , i have some oscilloscope leads coming tomorrow and will tackle this anew and let yall know what comes about, thanks so much!

If by any chance you have one of those inexpensive bluetooth or wifi OBDII adapters You can actually watch for messages with a serial terminal app on an android device. Would be seen looking like this...




Not only does this show the traffic but it is fairly simple to deduce what module sent each message.

 

[jakster918]

so your saying get wireles bluetooth dongle for the obd2 port and your saying i could use terminal and then view what traffic is sent/recieved and as a result also what modules are still sending?

 

[TJBaker57]

so your saying get wireles bluetooth dongle for the obd2 port and your saying i could use terminal and then view what traffic is sent/recieved and as a result also what modules are still sending?

Well yes, these dongles get you access to the network..From there a few configuration instructions and the device can be activated in "bus monitoring" mode. In this mode it will show all traffic on the bus without interfering or transmitting anything. Simply eavesdropping.

Far more is possible as well.

I asked the question as many folks have these devices but do not know the potential.

In the sample I posted the third column in green is the hardware address of the sending module. $40 is the BCM, $10 is the PCM $60 is the IPC, $A2 is my liftgate module.

 

[jakster918]

thanks i really appreciate the info

 

[jakster918]

im attaching a photo, i just want to confirm, is THIS sp205? please confirm....

 

[mrrsm]

Check the Wire Color Pin-Out Positions between what you hold there...and this arrangement for the GM SP-205 Splice Pack:

 

[TJBaker57]

tools im working with are a DMM, a power probe like tool called a kyzee D10, an odb2 breakout box

Here's a little video I made years ago that sort of shows what one might expect to see with a DMM on the Class II Serial bus. Yes the voltage pulses can range to like 7 volts but they only exist for like 128 microseconds at the most. A DMM simply cannot react that fast. And the bus has positive voltage only when there is traffic on the bus. In the video the highest voltage seen on the DMM is maybe 2 or 3 volts seen at about the 0:35 second mark when the key is heard turning on.

Here is another short video showing that if the bus gets grounded ALL comms stop. Watch the phone screen showing some brief traffic. It of course stops completely when I ground out the the bus.

I suspect one could measure resistance from the serial data bus to ground to be sure it is NOT grounded somewhere.