Security relay flashing. Why doesn't it start?

Your vehicle does not have an impact activated fuel shut off switch like the ford vehicles. Power is provided to the fuel pump by the fuel pump relay. Power is provided to the fuel pump relay by the Main relay. Both relays are controlled by the Engine computer or PCM. The PCM can disable the fuel system by not energizing the fuel pump relay if the immobilizer system has been triggered by the use of a non-authorized ignition key being used to start the vehicle. This should also trigger the security light to be flashing on the instrument cluster as well. Look for that rapidly flashing security light to see if this is the problem.

it sounds like security start relay malfunction ,check and or change out relay.

Not sure of the year of your Tahoe but I can tell you the standard procedure to program your key for say like a 1999. First make sure your battery has a full charge. Next if you have any security related trouble codes stored in your computer, you will need to clear the codes either by clearing them with a scanner or disconnecting the battery for at least 10 minutes to clear the codes. The first step is to turn the key and just let it turn over quick but not enough to where the vehicle starts and shuts down again. Just kind of tap the key as if starting it so the starter turns the engine for a second. When you do that just release the key so it stays in the "Run" position. Lights will be on in the instrument cluster including the security light. The security light may flash on and off. Leave the key in the run position for 10 minutes. After 10 minutes the security light usually goes out but not in all cases. After 10 minutes in the run position, turn the key to the off position but do not remove the key. Wait 10 seconds and do the first step again (Tap again let key stay in run position for 10 minutes). You will need to do this 3 times totally about 30 minutes. On the 3rd time when the 10 minutes is up, turn off key, do not remove the key and then start the vehicle. This procedure should work to progam your key. If it doesn\'t you may have another problem with the security system. Let me know how it turns out and I hope there isn\'t another problem and the computer just lost the transponder code. there is a security relay/module that sometimes goes bad in those Tahoes but try programming the keys first. If that doesn\'t work and you need further help as far as that relay/module & to let me know how it turned out,

Usually the security system either works or it doesn't. Is the security light on or flashing when you have trouble starting the engine ?
It could be a fuel pump or pump relay problem.

You could have a bad relay in the start circuit. The relay is there because of the security system. If someone tries to jump the car to steal it, the alarm will disconnect the relay and the car will not start. I believe it is in the relay box in front of the air cleaner

Read the following paragraph from GM, this will give you some insight on the passlock system on your vehicle.

The electrical switching portion of the ignition lock cylinder assembly is separate from the key and lock cylinder. The electrical switch portion and the key and lock cylinder synchronize and work in conjunction through the action of the mechanical assembly between the two components.

PASSLOCK(TM) SENSOR
The Passlock(TM) sensor contains 2 hall effect sensors. The tamper hall effect sensor is on the top. The security hall effect sensor is under the tamper hall effect sensor. Both of the hall effect sensors monitor the magnet of the lock cylinder through an opening. The tamper hall effect sensor is physically placed on top of the security hall effect sensor. This arrangement enables the tamper hall effect sensor to engage first if an intruder attempts to bypass the Passlock(TM) sensor by placing a large magnet around that area of the steering column. There is a tamper resistor inside the Passlock(TM) sensor in order to help prevent tamper to the system. Passlock(TM) equipped vehicles have a selection of 10 different security resistors ranging from 0.5K ohms up to 13.6K ohms . Installing one of the security resistors inside the Passlock(TM) sensor will generate a unique Passlock(TM) code. All 10 combinations of the Passlock(TM) sensor have the same part number.

BODY CONTROL MODULE
The body control module (BCM) contains the theft deterrent system logic. The BCM reads the Passlock(TM) data from the Passlock(TM) sensor. If the Passlock(TM) data is correct, the BCM will send a class 2 message to the PCM to enable the fuel injectors and crank relay.

During the tamper mode the vehicle may start. The vehicle will then stall. If the BCM receives the wrong Passlock(TM) data, the VTD system will immediately go into the tamper mode. The tamper mode will lock-out the vehicle fuel injectors and crank relay for 10 minutes . The SECURITY indicator will flash while the VTD is in the tamper mode.

If the Passlock(TM) sensor sends a correct password to the BCM when the ignition is in the ON position, the BCM will send a fuel enable and crank relay enable signal to the PCM.

SECURITY INDICATOR
The SECURITY indicator appears on the message center inside the instrument cluster. If the SECURITY indicator flashes or if the SECURITY indicator lights continuously during vehicle operation, refer to the system diagnosis.

POWERTRAIN CONTROL MODULE
The PCM communicates with the BCM over the Class 2 serial data line. When the BCM determines an incorrect password, it sends a Class 2 serial data password to the PCM in order to disable the crank relay and fuel injection system. If the BCM receives the expected voltage from the Passlock(TM) sensor, the BCM sends a class 2 serial data password to the PCM in order to enable the crank relay and fuel injection system. The PCM then allows the vehicle to start normally.

FUEL LOCKOUT CYCLE
The Passlock(TM) system has a lockout cycle of approximately 10 minutes . Once the vehicle is in the lockout cycle, the vehicle remains in the lockout cycle for 10 minutes , even if the ignition switch is turned from the ON position to the OFF position.

The Passlock(TM) system uses the lockout cycle in order to synchronize all of the Passlock(TM) components when any Passlock(TM) related part is replaced. The Passlock(TM) system requires 3 consecutive lockout cycles in order to complete the 30 minute learn procedure for a changed component.

CHANGING THE PASSLOCK(TM) COMPONENTS
The design of the Passlock(TM) system is to prevent theft even if the various theft deterrent parts change. The parts that can no longer be changed without the possibility of going into a tamper mode are:

* The Passlock(TM) sensor
* The BCM
* The PCM

If you replace any of these parts the vehicle may not start for 10 minutes . This is the long tamper mode. If this occurs, the system must go through a long tamper mode cycle. During this time the SECURITY indicator will be flashing for the full 10 minutes and DTC B3031 will be set. The BCM and the PCM require the full 10 minutes in order to complete a learn cycle. The ignition switch must remain in the ON position until the SECURITY indicator stops flashing. You will need to repeat the cycle if the ignition switch does not remain in the ON position. When replacing any of the above parts, perform the Programming Theft Deterrent System Components procedure.

The design of the Passlock(TM) system is to prevent the vehicle operation if the proper ignition key is not used in order to start the vehicle. The mechanical key, in normal operation, will turn the ignition lock cylinder. The Passlock(TM) sensor monitoring the position of the lock cylinder will relay the Passlock(TM) data to the body control module (BCM). The BCM will determine the validity of the Passlock(TM) data. The BCM will send a password to the powertrain control module (PCM). When the PCM receives the correct password the PCM allows the crank relay and fuel injectors to operate normally. The Passlock(TM) system requires the PCM and the BCM to communicate the various functions in order to operate. These functions transmit over the class 2 serial data line. For an explanation of the class 2 serial data description and operation refer to Data Link Communications Description and Operation in Data Link Communications.

The security system does not disable the starter, it only disables fuel to the engine. In the fuse block under the hood, locate the STARTER relay. Place your hand on this relay and see if it clicks while someone else turns the key to start...if the relay is ok then the starter itself is faulty. Good luck, i hope this helps.

ecm more likely

not sure what you mean this car doesn't have an (antitheft relay) it has a BCM that receives resistance data from ignition switch and when the data matches programming then car will start. if the data is changed or not reading then the car will not start and security light will flash BCM will shut down starter and fuel delivery. if the security light flashes when you attempt to start the car. the security sensor in the ignition switch will need to be replaced and programmed to the BCM. this is very common problem to these cars.

sounds like you need a new ignition switch the pass lock is screwing up common.