'98 Tahoe Notorious Security Issue

[Romeroman64]

Hi Everyone!

So I have an odd issue with the notoroius anti-theft system on a '98 Tahoe.

Here's a rundown of everything;
1. Fuel Pump replaced a second time after a shade tree mechanic misdiagnosed the crank/no start issue and did an improper pump replacement (disconnected filler hoses, lines, electrical, etc.)
2. VCM replaced after diagnosing internal shorts at the TPS, ECM, & IAT ground circuits. YES those grounds are located at and controlled by the VCM.
3. Replaced ignition lock cylinder/key due to damage from an attempted theft.
4. Replaced VAT sensor/harness at the steering column due to damage of said theft.
5. Performed the 30 min password relearn procedure multiple times with NO Success. Discovered the VAT module cannot be relearned to a new resistor value once it has been previously programmed to another.
6. Replaced VAT module and sensor/harness from a salvage donor that have already been mated together.
7. Performed the 30 min password relearn multiple times with no success.
8. Scanned the systems with a high end scanner and found DTCs stored in the VAT Module (C0503/C0472)
9. Repaired the issues related to said DTCs, cleared codes, & verified the repairs.
10. Performed 30 min password relearn with no success.
11. Performed the bypass procedure by reading and replacing the resistor at the ignition switch. I followed this tutorial

12. Performed 30 min password relearn/SUCCESS!!...I think
13. STILL has crank/no start issue. VCM has DTC P1631 "Password Incorrect"/ VCM still not activating fuel pump & injectors.
14. Verified fuel pressure (62 PSI), verified spark, verified compression, verified ignition timing. Vehicle DOES run when manually spraying fuel into the intake.
15. Verified continuity on circuit 1807 data bus between VAT and VCM. Continuity is good (0.9 ohm)

At this point I have no clue what could be going on. The scanner says VAT has valid code, indicating password relearn was successful, yet VCM still reading incorrect password. Anyone have any ideas? Thank you.

 

[strutaeng]

You can't just get VATS deleted altogether from the ECU (or VCM?)

That's probably the best thing to do on these systems. Otherwise it will fail again and leave you stranded at the worst time in the worst location.

 

[Romeroman64]

You can't just get VATS deleted altogether from the ECU (or VCM?)

That's probably the best thing to do on these systems. Otherwise it will fail again and leave you stranded at the worst time in the worst location.

That would be great, if I knew how. Perhaps you could point me in the right direction?
(According to the service manual, ECU is the appointed acronym for the diesel engines. VCM is the appointed acronym for gas engines. I'm guessing so the engineers & manufacturing plants don't mix up the parts. I dunno.)

 

[strutaeng]

You can send off the VCM to a place that does these. Look here: https://www.gmt400.com/threads/need-vats-removed-on-my-98-silverado.40849/

Not sure where you are, but maybe try finding a local person that does tuning. Most will use HP Tuners software and it should not cost that much money. I believe 1996 is the year that is not compatible with HP Tuners (?), but later years are. I'm going off memory. Check on the HP Tuners website for compatability.

 

[Romeroman64]

You can send off the VCM to a place that does these. Look here: https://www.gmt400.com/threads/need-vats-removed-on-my-98-silverado.40849/

Not sure where you are, but maybe try finding a local person that does tuning. Most will use HP Tuners software and it should not cost that much money. I believe 1996 is the year that is not compatible with HP Tuners (?), but later years is. I'm going off memory.

I will definitely look into that. Thank you.

To cure my OCD, I would still like to know what the heck is going on here that I have a valid password but the VCM is not accepting it.

 

[east302]

Have you run through the P1631 diagnostic in the manual? Here it is for 1998. It may be the 30-minute relearn that you mentioned, in which case the manual appears to just go in a circle if that doesn’t work. It does have some cautions about maintaining battery voltage during the process and following certain time limits.

The manual can be downloaded here:

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

 

[Romeroman64]

Yes. I ran through the diagnostic steps. As you said, it just runs me in circles. The relearn has worked, as you can see by the Valid Code 9 and monitor state parameters on the scanner. It's the actual VCM that doesn't want to accept the password.

Aside from all that. I have opted to delete the anti-theft program from the VCM. I found a place in my city that does it. I am just worried that if I delete the program, will the VCM still function normally and activate the fuel pump & injectors? Or will deleting the program somehow render the ECM useless?

 

[east302]

Here’s a recent thread on a related forum where some of the tuner names/products were mentioned.

Newbie with a quick PassLock question

Longtime lurker here, who has learned a lot by lurking. Thank you all for that. I have also posted in the Introduce Yourself location. I have a ’99 4x4 Suburban with a 5.7. I was having PassLock issues that I believe were brought about by a bad battery. It seems to be working fine now, but I...

www.gmt400.com

My impression is that deleting the program caused no side effects. Knock on wood, I’ve had no issues with it in mine.