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1/528 | Motor Age June 2005 www.motorage.com

COMMUBREAK

PASSKEYNO-START DIAGNOSTICS


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When the engine in a GMvehicle wont start, bettercheck the vehicles Passkeysystem. Are you ready to

tackle a vehicle thats beentold not to start?BY JACQUES GORDON,TECHNICAL EDITOR

Since the mid-80s, General Motors (GM) hasused three different theft deterrent systems, butthey are all passive, meaning its activation is auto-matic. Its operated by communication between acontrol unit and the ignition key through the igni-

tion switch. If the switch is vandalized, if its operated withthe wrong key or if the Class 2 communication link is dis-rupted, the engine wont start.

Any time the theft-deterrent system causes an unwantedno-start, chances are its a simple communication break-down. To find it, you need to understand how the systemdetects theft and how it prevents engine starting. What youwill find here is information about GMs Passkey I andPasskey II theft deterrent systems, along with a few tips pro-vided by our Troubleshooting Editor Roy Ripple.

Weve concentrated on these systems because Roy tells us

these are the systems that generate the most help requests.He also says the problem is usually just a faulty connection,

botched accessory installation or even just a worn out keyor ignition lock cylinder. These can be easy to troubleshoot,because all you need is a wiring diagram, a DVOM andknowledge of how the system is supposed to work.

OPERATING STRATEGY

The Vehicle Anti Theft System (VATS) was first installedon the 1985 Corvette. The Personal Anti-theft SecuritySystem (PASSkey) replaced VATS in 1988, and while thereare some differences, VATS and Passkey I and II all work the same way. There is no radio communication involved;the system merely looks for the ignition keys unique elec-tronic signature. Passkey III and the newer Passlock sys-tems were introduced in 1998,but the earlier systemswerent completely phased out until 2003. The easiest wayto tell the difference is by the appearance of the key.

The older Passkey systems all have a pellet imbedded inthe ignition key that communicates with the TheftDeterrent Module (TDM). On VATS and Passkey I, theTDM is hidden deep in the dashboard.On Passkey II, theTDM is built into the Body Control Module (BCM). Thekeys visible pellet is a resistor, and you can read its value bytouching meter probes to the contacts on either side of thepellet. With the key in the ignition, the pellet touches con-tacts in the lock cylinder. These contacts are wired to theTDM, which reads the value of the resistor to identify thekey. If the resistance is correct, the TDM sends a pulse-width modulated signal to the PCM, which then enables thefuel injectors. The TDM also operates the Theft DeterrentRelay (TDR) that allows power to reach the starter solenoid

when the key is turned to the START position.If the TDM doesnt recognize the key pellet, it decides a

NICATIONDOWN

Photo Illustration: Michael Stassus/Getty Images www.motorage.com June 2005Motor Age | 29


3/530 | Motor Age June 2005 www.motorage.com Illustration source: General Motors

theft attempt is in progress and it will go into anti-theftmode. In this mode, the TDM will:

Turn on the Security warning light on the instru-ment panel.

Prevent starter operation by not turning on theTheft Deterrent Relay.

Send a signal to the PCM telling it to disable thefuel injectors.

Disable the key recognition circuit for three minutes.This last point is important to people who own more

than one GM vehicle. With multiple GM keys on thesame key ring, accidentally inserting the wrong key inthe ignition will prevent start-up, and the TDM waitsthree minutes before it will read the key circuit again.Also notice that the PCM is looking for a specific goor no-go signal from the TDM before it will decidewhether or not to operate the injectors. The absence of a go signal is not a valid no-go signal, and vice versa.

If the TDM decides a theft attempt is in progresswhen the engine is already running, it will not stop theengine, but it will turn on the Security light and enteranti-theft mode at the next key cycle.

If the starter is operated without the key in the lock cylinder, such as with a remote starter switch, the TDM

will interpret that as a theft attempt. Likewise, if there isa short to power or ground in the key recognition cir-

cuit, it is interpreted as a theft attempt. This can happenif the wiring in the steering column is damaged duringtheft, repair or installation of electronic accessories.

Either way, if the correct key is not in the ignitionswitch when the key recognition circuit fails, the TDMwill turn on the Security light and enter anti-theftmode. If the correct key is in the cylinder and that cir-cuit fails while the engine is running, this is considered amalfunction, not a theft attempt. The Securitylightmay turn on to warn of a system malfunction, but even if it doesnt, a key recognition circuit failure with the prop-er key in the lock will cause the TDM to enter what GMcalls a fail-enable mode. Essentially, the theft-deterrentsystem goes offline and the engine will start and runwith any key that turns the lock.

KEYS

In the original VATS system, the keys resistor pelletand the TDM were programmed together at the factory.In the Passkey systems, the TDM will memorize theresistance of the first key inserted into the lock. Thereare 15 different resistance ranges or codes, and all areavailable as key blanks from GM dealerships. Withineach range, the actual resistance may be more than 100

ohms different from the resistors nominal code value,and even though a TDM will memorize the resistance of

THE PASSKEY I SYSTEMA LOOK INSIDE

P A S

S K E Y N O - S T A R T D I A G N O S T I C S

Here is the Passkey Isystem with a separateTheft Deterrent Module.The Special EquipmentOption (SEO) uses

jumpers to bypass the keyrecognition circuit in theIgnition Lock Cylinder andthe Security light in theinstrument panel.


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the first key inserted into the lock, it will recognize a keypellet in the same resistance range. Whether the TDM isa separate unit or built into the BCM, that key resistancerange memory cannot be reprogrammed. The resistancerange of that first key is what it will look for every time.

There are three ways to get that resistance range num-ber. One is to look at the cars original invoice. Anotheris to insert the key into GMs special Passkey trou-bleshooting tool, the J35628A Interrogator Tester,which will display the keys range number directly. If neither of these options are available, you can read thekey pellets resistance with a DVOM.With that reading,the dealer parts department can look up the resistanceon a chart and supply the correct blank.

TESTING

A Tech II or Mastertech scan tool can read all the cir-cuits in the Passkey system.The most common failure issimple wear and tear of the ignition key itself or the lock cylinder contacts. Its a good idea to have all copies of the key available for testing, because if the vehicle isdriven with the same ignition key most of the time, therepair may be a simple matter of replacing a key that hasworn out resistor pellet contacts.

If the system doesnt recognize any of the keys, and if your scan tool cant display Passkey codes, youll need

to undressthe steering column and find the ignitionlock cylinder connector to test the key recognition cir-cuit. If someone has already been into the steering col-umn before, check the lock cylinder wiring carefully fordamage. Roy advises that the second most commonPasskey problem is pinched wires in the key recognitioncircuit, almost always caused by reassembling the steer-ing column improperly after some other repair.

With the lock cylinder connector unplugged, measureresistance in the circuit with a key inserted and in theOFF position. It should match the resistance of the keypellet itself. Try wiggling the wires and the key in thecylinder. If resistance changes, the wiring is damaged orthe key or lock cylinder contacts are worn and shouldbe replaced. A heavy load swinging on the key ring hasbeen known to cause problems with these contacts. Beaware that the TDM sends a 5-volt reference throughthis circuit at all times, so its important to unplug theconnector before testing the lock cylinder circuit.

LOST KEYS

If you have none of the original keys for the car, thereare two options. One is to use the Interrogator Testertool, which plugs into the key recognition circuit

between the lock cylinder and the TDM and simulatesthe key pellets resistance. Using any key that will turn in

32 | Motor Age June 2005 www.motorage.com Illustration source: General Motors

THE PASSKEY II SYSTEMA LOOK INSIDE

P A S S K E Y N O - S T A R T D I A G

N O S T I C S

Passkey II is integrated into the BCM.


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