Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

Body Accessory Systems Operation 343
Figure 15-41. System functioning relies on strategically
located switches installed in the door jambs, the
door lock cylinders, and the trunk lock cylinder. (GM
Service and Parts Operations)
Figure 15-43. The UTD disarm switch is part of the
LH door lock cylinder. (GM Service and Parts Operations)
Figure 15-44. Delphi (Delco) VATS system. (GM
Service and Parts Operations)
Figure 15-42. Tamper switches are installed in all
door locks and the trunk lid lock. (GM Service and Parts
Operations)
Delphi (Delco) VATS/PASS-Key
II System
The Delco (Delphi) vehicle antitheft system
(VATS), introduced as standard equipment on the
1986 Corvette (Figure 15-44) functions as an
ignition-disable system. It is not designed to prevent
a forced entry, but to protect the steering column
lock if an intruder breaks into the vehicle.
When used on Corvettes with the UTD system, the
combination is called the forced entry alarm system
(FES). When used on other GM vehicles, VATS is
called PASS-Key II. The system (Figure 15-45)
consists of the following components:
• Resistor ignition key
• Steering column lock cylinder with resistorsensing
contact
• VATS or PASS-Key II decoder module
• Starter enable relay
• PCM
• Wiring harness
A small resistor pellet embedded in the ignition
key contains one of 15 different resistance values.
The key is coded with a number that indicates
which resistor pellet it contains. Resistor pellet
resistance values vary according to key code and
model year. To operate the lock, the key must
have the proper mechanical code (1 of 2,000); to
close the starter circuit, it must also have the correct
electrical code (1 of 15).
Inserting the key in the ignition lock cylinder
brings the resistor pellet in contact with the resistor
sensing contact. Rotating the lock applies battery
power to the decoder module (Figure 15-45). The
sensing contact sends the resistance value of the key
pellet to the decoder module, where it is compared
to a fixed resistance value stored in memory. If the
resistor code and the fixed value are the same, the
decoder module energizes the starter enable relay,
which closes the circuit to the starter solenoid and
allows the engine to crank. At the same time, the
module sends a pulse-width modulated (PWM)
cranking fuel-enable signal to the PCM.