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Antitheft system a demon

| Friday, Jan. 11, 2013, 8:58 p.m.

Q I own a 1998 Pontiac Grand Am SE. She has 242,000 miles on her. Recently the theft light comes on, blinks and then stays on. Sometimes the car will die starting up, then it may start up and run fine. Sometimes the light will stay on, and other times the light will go out.

I was told that this was a safety feature GM installed, although a cheap one, to prevent stolen vehicles. I was wondering if this is something I can fix or disable myself. I have an aftermarket alarm system that has been installed on this vehicle since 2000. That has keyless entry, which I use regularly.

I was told by one person that buying a new battery for my clicker would solve the problem. Another person told me I have a bad fuse that needs to be replaced, and another said I could disable the system by cutting a yellow wire running from my ignition switch. Can you help me? I depend on my vehicle for my job and drive some distance every day. What can I do?

— Tom Houdeshell

A Your Grand Am employs GM's Passlock antitheft system, which is known for causing intermittent starting headaches and big repair bills to fix it. A sensor in the steering column determines the ignition lock cylinder has successfully rotated, which requires the correct key. This creates an electrical output to the antitheft brains in the instrument panel cluster, which then sends an “OK” handshake to the powertrain control module. That allows normal function of the fuel injectors. Should an ignition switch input occur without the accompanying sensor signal — perhaps if a hammer, pliers, and screwdriver are used to rotate the lock cylinder — no IPC/PCM handshake occurs. The fuel injectors are cut off, causing the engine to start then stall, and the theft deterrent light is illuminated.

In many cases, it seems the system forgets the lock cylinder sensor's code and a relearn procedure restores operation. This is performed by leaving the ignition switch in the run position with the engine stationary for 10 minutes or until the indicator light shuts off. Try multiple times as needed. In the event of a starts-stalls episode, this offers the best chance of driving again.

Diagnosis of a repetitive problem such as yours is best done by a pro, using a Tech-2 scan tool. Rather cryptic information gleaned from the body control module's data list, along with any diagnostic trouble codes — these won't illuminate the check engine light — can narrow the search to the lock sensor, instrument panel cluster, PCM or wiring between.

Some folks report success bypassing the system by tampering with the sensor circuit. A resistor is spliced into the circuit — don't simply cut the yellow wire. A more sure way of putting this demonic system in its place is to bypass it completely with a counterfeit PCM handshake. I have not personally installed or validated such a product but it's worth looking at.

Your aftermarket alarm system makes me nervous, but doesn't appear to be involved in this issue as the blinking indicator confirms a Passlock fault.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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