[Chevy-SS]

This doesn't personally affect me, but I had to comment on how well done and thorough the solution was documented. Bravo!

John, I ditto these remarks ^^^^^^^^^^^^. Nicely done, and thanks so much for posting all the detailed info and pics. Excellent work! This should be a "go to" thread for anyone with Passlock issues. I'm gonna copy your posts and pics and save them.

So, just to be clear, are you 100% fixed now?? :?: :?: :?:

 

[Sparky672]

So, just to be clear, are you 100% fixed now?? :?: :?: :?:

Good question!

One thing I know for sure is that my issues were getting progressively worse and exponentially more frequent... to the point where the Security light was on all the time for the last several weeks. Now that the bypass resistor has been replaced and re-learned and wires cleaned up, the lamp is off and everything seems to be working properly.

Only time will tell.

 

[Sparky672]

Since the research is spread out through this thread, here's an index...

1. General background info about Passlock II and the various other versions of GM anti-theft systems

2. Additional background info about Passlock II and why "trying different keys" would be pointless

3. What I found installed in my truck and how it likely failed - includes bypass instructions

4. GM schematics for my 2000 Silverado including Passlock II bypass modification

 

[Sparky672]

I wanted to delve into some more details of my experience that might help others. The following is based on personal observations combined with some educated speculation, so I do not guarantee my assumptions.

The BCM is looking at voltage, not resistor value. If the voltage coming from the Passlock sensor or the bypass resistor is not the same value as stored in the BCM, there will be a Passlock error and trouble codes set in the computer. These are the codes I was seeing:

P1631 - Anti-Theft Device - PCM/IPC Password Error
B2960 - Security System sensor data incorrect but valid
B2961 - Key in ignition circuit malfunction

Failure to Start

Over the first 18 months, I had maybe only three random "Security" failures that resulted in the truck not starting. I didn't initially realize this was a Passlock issue because the Security light intermittently flashes along with the battery light and other dash messages. After repeatedly trying to start or waiting, the engine eventually starts up after approximately 10 minutes. Some online documentation states that there is a 10-minute Passlock relearn procedure. However, in my experience, it does not matter if the key is left on, left off, or repeated start attempts... it just starts back up after 10 minutes. This sounds more like the 10-minute Passlock lockout than a re-learn. Passlock II has a 10-minute lockout timer according to some online sources. I think my bypass resistor was just starting to flake out and if the Passlock signal voltage momentarily glitched on start, it would go into the 10-minute lockout and set a computer code.

Security Light while Driving

Occasionally over this same 18-month period, the Security light would display randomly while driving. If there is any Passlock failure while driving, the engine continues to operate as normal and computer trouble codes are set. In my case, simply turning off the engine and restarting would clear the light, which indicates that this too was triggered by a brief glitch in the Passlock signal voltage, otherwise a hard failure of the circuit would have left the Security light on.

--------

mid-December 2020

Until last December, I had not had to do any Passlock re-learns. Just the occasional wait 10 minutes to start or the Security light while driving. Then the bypass resistor connection started to breakdown further and one day it took more than 45 minutes of trying and waiting before I could get it started up. I'm guessing the Passlock voltage signal was deviating farther and the voltage glitches more persistent and frequent. I was able to get it home but could not start it at all the next day at all. At this point, I did my first 30-minute re-learn procedure, which got it running again. Apparently the resistor connection had failed further and this new Passlock signal voltage value needed to be set in the computer.

Afterwards for a couple months, I had quite a few more instances of Security light while driving and failure to start with a 10-minute lockout. The resistor lead was hanging by a thread, so I'm sure the voltage was glitching even more.

mid-February 2021

Security light came on while driving and this was not a glitch. There was further breakdown of this bypass resistor connection and I was forced to do a 30-minute re-learn in a dark parking garage. (There is no way to disable the headlights during this procedure so just hope the battery is in good shape. My advice would be to keep a flashlight handy and shine it on the ambient light sensor to force the headlights off.) After the re-learn, the Security light went off and everything seemed good for 45 minutes, then the Security light came on again while driving and stayed on permanently for the next four weeks.

There was such a Passlock signal voltage deviation that the system went into a software bypass mode. During a hard Passlock failure while driving, the Security light stays on all the time, and Passlock goes into this mode that lets you start the engine something like a couple hundred times, presumably enough until you can get to a dealer for service.

mid-March 2021

This is when I took everything apart and discovered the Passlock bypass resistor, although it was not visually obvious that it was barely hanging by its last thread, it was very sloppy workmanship. I cleared out the trouble codes, which cleared out this Passlock software bypass mode, therefore engine would not start. After yet another 30-minute re-learn, it started right up and the Security light stayed off. After 24 hours of driving with the dashboard still torn apart, I had planned on cleaning up the wiring, and that is when the resistor literally broke apart in my hands.

I believe my whole problem was caused by this very poorly installed bypass resistor that had a connection that slowly broke down over a period of years. It may have been nicked by the installer's wire cutter, twisted too tight and close to the resistor, overheated during soldering, corroded from excess soldering flux, cracked from daily vibration, or some combination of these.

 

[RedSLEd]

Great write-up. Thanks for taking the time and effort.

 

[Sparky672]

So, just to be clear, are you 100% fixed now?? :?: :?: :?:

Yes. It's been just over 60 days since I posted my findings and solution. Still holding. The problem seems to be gone for good.

 

[Chevy-SS]

Woohoo!!!!!!!!!!! All fixed... and you posted some of the best info on the old interweb. Nice work.

 

[rmunnal13]

What did you do to fix the issue? My 2000 chevy is doing the exact same thing I mean exactly the same... security light pops on sometimes like a minute after I put in drive or 30 minutes after I leave and it seems to be when I'm in a rush or something it won't start buy crank over!

 

[RedSLEd]

@rmunnal13 Did you read thru the thread?