Trouble Shooter

Motor, Nov 2005 by Seyfert, Karl

Vehicle theft deterrent systems have come a long way from the simple kill switch. Fortunately, diagnostic tools and techniques for these sophisticated systems have kept pace.

Passlocked Out

I'm up Iiere in Maine and I'm not flipping burgers-yet! I recently had a problem with a 2,000 Chevrolet K2500 with a 5.7L engine and automatic transmission. The vehicle had about 60,000 miles on it at the time of service. The problem is intermittent and the vehicle can go a week or more with no issues. When the problem does present itself, the engine will repeatedly crank, start, then shut down. Eventually it will start and continue to run if left to sit fora period of time. Spark, fuel pressure, wiring and grounds all test good when the problem occurs. The only DTC tliat consistently stores each time this problem occurs vs P1626 (Theft Deterrent Fuel Enable Signal Not Received).

My scan tool will not perform certain tests as required by tin- diagnostic chart I am following, I did get a fluctuating 0- to 1-volt on/off signal using my DVOM on the serial data line. I assume a scope pattern would be much more accurate, but I do not have one at my disposal at this time.

I replaced the ignition/key alarm switch module on the advice of a 'top-notch dealer tech," although I disagreed with his diagnosis. I had to clear the code after the repair to get the vehicle to start. That repair lasted less than two weeks, and the. problem has returned witti all the .same sijmptoms, as well as the P1626 code.

What is the best tool to use to check signals going into and out of the VTD Passlock module and the VCM? What should the signals look like? Am I correct in stating that the signal arrives at the PCM via the serial data Class 2 bus line? I'd appreciate a breakdown on this system.

Steve Gillard

Sebec, ME

Unlike earlier GM vehicle theft deterrent systems (Passkey I and II), Passlock does not rely on special keys with embedded resistor pellets for its operation. But like those earlier systems, Passlock is designed to prevent vehicle operation if the proper ignition key is not used to start the vehicle.

In normal operation, the key turns the ignition lock cylinder. The Passlock sensor monitors the position of the lock cylinder and relays an analog Passlock signal to the body control module (BCM). The BCM determines the validity of the Passlock signal, then sends a password to the powertrain control module (PCM). When the PCM receives the correct password, it allows the fuel injectors to operate and the vehicle starts normally. Passlock communication between the PCM and BCM takes place over the Class 2 serial data line (CKT 1807), as you correctly surmised.

The Passlock sensor contains two Hall effect sensors. The security sensor is mounted under the tamper sensor. This arrangement enables the tamper sensor to engage first in the event a thief attempts to bypass the Passlock sensor by placing a large magnet around the area of the steering column.

A security resistor inside the Passlock sensor provides further theft protection. Ten different security resistors are available. Each generates a unique Passlock code that must be recognized by the BCM during a starting attempt. If the Passlock sensor, BCM or PCM are replaced, a relearn sequence must be followed to acquaint the new parts with each other and allow normal starting.

Don't feel bad about not having access to a scope, Steve. This is one instance where a scope is not absolutely necessary. But you will need a Tech II or equivalent scanner to conduct the required Passlock diagnostic tests. A DVOM will also be required. Refer to your service information for the specific steps in the diagnosis.

The intermittent no-start or startand-stall symptoms you're seeing on your customer's truck would seem to point to a communication problem either between the Passlock module and the BCM or between the BCM and die PCM. P1626 stores when the Passlock theft deterrent system is enabled and the fuel decision point has been reached (engine cranking). At that point the PCM was unable to establish communication with the BCM and did not receive a valid password before reaching the fuel enable decision point. Once this occurs, the system goes into Tamper mode and starting is prevented for a full 10 minutes. This explains why your customer's vehicle will usually start if it's allowed to sit for a while.

Check the integrity of all VTD wiring and connectors. A poor connection at a harness connector, for example, could temporarily interrupt communication between the components and prevent starting. Look for poor connections between male and female connectors inside harness connectors. Contamination also may enter if a connector seal is damaged or missing, or if the connector itself is damaged.

Also check for poor terminal-towire connections. This includes broken wire strands, poor crimps, poor solder joints, crimping over the wire insulation instead of the wire strands and corrosion in the wire-to-terminal contact area. The wiring may also be rubbed through, causing an intermittent short as the bare spot touches other wires or parts of the vehicle.