[...] Take for example that "Implied Hidden Knowledge" reference made by Eric "O" as to what always needs to be investigated about a
"Three Wire Connector". THIS Article is about "Understanding what 5 Volt Reference Voltage Means" and is a great place to start:
In modern vehicle design, 5-volt reference signals/circuits are a key aspect of vehicle command and control. As summarized by Gary Goms, the 5-volt reference
cloreautomotive.com
Thank you! That's exactly the kind of 'how to think logically about electrical troubleshooting' I need. If we take a step back and 'increase the range of measurement,' so to speak, I can give you a wider idea of what I'm up against:
• This '03 TrailBlazer had been hit by lightning. Repair was covered by insurance, but my daughter cannot find the service records, so I don't know what was diagnosed, repaired, replaced or reprogramed.
• After returning to use, the transmission failed and was replaced. Again, no service records are available.
• 4WD allegedly worked until attempted engagement resulted in loud 'bang'. Visual inspection discovered serious oil pan leak. Vehicle was not repaired and parked for years until divorce necessitated its revival by me. Husband (now ex-,) has tenuous relationship with Truth. He maintains that loud bang *caused* oil pan leak; I believe massive shock to right front axle transferred through suspension/drivetrain into I-Shaft Bearing Housing and broke oil pan. Whatever...
• While rebuilding front suspension and drivetrain, I discovered apparently new A/C Compressor installed but not attached to any electrical wire or refrigerant hose. Daughter knew nothing of this. Ex spews utter nonsense when asked why new compressor installed but not hooked up and wife not informed. Whatever...
• After I repaired the wire harness and got it started, daughter says mileage reading is *way* off; tens of thousands of miles too low. ECU immediately suspect. She assures me reading was correct long after lightning strike repair...
• I've tried an ancient (by today's standards, not '03 standards) ELMScan 5 Compact OBD-II reader running currently discontinued OBDwiz app from ScanTools. It says "No Codes Present."
• Tried VXDiag VCX Nano Tech2 emulator but massively confusing and conflicting instructions caused me to send that pile of nonsense back. I got some info out of it about modules being present - not responding, but I can't trust any of it. Currently looking for an MDI emulator with better instructions. I do have the $40 VIN registered in Techline Connect, so I got that goin' for me, which is nice... ;-)
There's not many miles on it since I put it back on the road. It's running fine and she's as happy as a clam to be mobile again. I'm hoping some error codes will re-establish themselves to give me some clues on where to start, but since there is a parasitic drain on the battery and she is disconnecting it every night, I'm guessing that whatever codes are appearing are getting lost with every disconnect...? Oy, yoy, yoy! Am I having fun yet?!
Anyway, thanks for your help, fellas. Writing this stuff out helps me to get it straight in my head, at least. I'm on my way over there—oh, yeah, I forgot to tell you this thing is 20 miles from my house. It's a 40-mile round trip every time I want to work on it. Today I'm going to try (again) to pin down the parasitic drain...