The alarm system of my 1997 BMW 740iL is really annoying. I try to turn it off, follow the instruction of my manual, by press the lock key again after I locked the door. However, the red light of the alarm is still flashing. Do you know how to turn it off?

Question edited for clarity.
Question moved from MISC to BMW 740iL.

Does the new key have a transponder chip in it? Did you check? If it does, you need to reset the alarm and immobilizer, if not you need a proper key and not a Valet Key.

https://www.google.com/search?q=2001+bmw+740il+reset+security+light

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Door Key Switch
The BCM monitors the door key switches in order to disarm the CTD system. When the lock cylinder turns in order to unlock the door, the switch grounds the disarm input to the BCM.
The key lock input to the BCM is probably missing , BCM doesn't see it . So it think's your breaking into the vehicle. Did you try the passenger door to see if it will disable the alarm .
The BCM - body control module ! Electronic -computer !
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Content Theft Deterrent (CTD) Does Not Disarm with Key Lock

The factory service repair manual has a 23 step diagnostic procedure . An you would need a factory scan tool to find the correct fix.
Turn ON the ignition with the engine OFF.
With a scan tool, observe the door key switch parameter in the Body Control Module (BCM) security data list.
Does the scan tool display INACTIVE?

Locking the Vehicle Without Arming the System
Use one of the following procedures in order to lock the vehicle without arming the system:
• Procedure A -- Lock the vehicle using the manual door lock knobs. Close all the doors.
• Procedure B -- Close all the doors. Use the key or the power door lock switch in order to lock the door. The doors must be closed prior to locking the doors for this procedure.
Disarming the System
In the arm initiated mode, the following conditions are true:
• The security telltale is ON.
• The doors are open.
In order to disarm a system in the arm initiated mode, perform one of the following procedures:
• Press the power door lock switch to the UNLOCK position.
• Press the door unlock button on the key fob.
In order to disarm a system in the arm delay or the armed mode, perform one of the following procedures:
• Use the key in order to unlock one of the doors.
• Press the door unlock button on the key fob.
Deactivating the Alarm
In the alarm mode, the following conditions are true:
• The horns sound intermittently.
• The headlights and the park lamps are flashing.
In order to deactivate a system in the alarm mode, perform one of the following procedures:
• Use the key in order to unlock one of the doors.
• Press the door unlock button on the key fob.
• If the vehicle control module (VCM)/powertrain control module (PCM) receives a valid Passlock™ ignition key rotation.

according to Viper tech support (I emailed them your question):

Thank you for contacting Directed support!

You may need to reprogram the remote to your phone. Please use the following instructions to reprogram the remote:

To program the remote to your system please do the following:

1) open the driver door
2) turn the key to the "on" or "run" position
3) locate the valet switch (small black button under driver side of dash) and press once, release, then press again and hold
4) after the siren chirps one time press the lock button on the remote
5) the alarm will give you a unique confirmation tone
6) turn the key off (the same unique tone will be heard again)
Warm regards,

Nigel McGee I Customer Service Rep.
Viper Support

Find the cable under your steering wheel just above the foot break then pull it.

Now im not sure since the manual says your model has the same alarm system as the 328i and on the 328i you can remove the module to disable by doing the following. you can unplug the control module. It is located under the glove box. Climb under the dash on the passenger side and remove a few screws on the trim panel under the glove box. Theres a black control module with a green connector. There should be a lock tab on the connector. press the lock tab and pull out the connector. The alarm should now be disabled. But if it has the special transponder key this may not work.
But if the only reason you want to disable it is because of the motion sensor setting of the alarm for no reason there is a solution for this. To disable the motion sensor, hit the lock button to arm, then hit lock button again within four seconds to disable motion sensor. Red light on rearview mirror should flash four times, then pause, then start flashing on regular intervals. this is most likely the reason if your having the issue where the alarm goes off every 20 minutes.
If you want to completely remove it below is a wiring diagram taken from the manual for you alarm system.
this is from the manual its the alarm wiring diagram.
Battery Constant 12v Positive Wire ( ): Red/White or RedBattery Constant 12v Positive Wire Location: Ignition Switch Harness
Starter Positive Wire ( ): Black/Yellow or Black
Starter Positive Wire Location: Ignition Switch Harness
Ignition Positive Wire ( ): Green
Ignition Positive Wire Location: Ignition Switch Harness
Second Ignition Positive Wire ( ): Purple/White or Purple
Second Ignition Positive Wire Location: Ignition Switch Harness
Parking Light Positive Wire ( ): Gray/Purple and Gray/Yellow. Use Both Wires. This vehicle has a dual parking light system that will require 2 extra relays.
Parking Light Positive Wire Location: At Headlight Switch
Power Door Lock Positive Wire ( ): White/Red
Power Door Lock Positive Wire Location: At Factory Alarm Plug. The alarm plug is a black 12 pin plug located just behind the glove box.
Power Door Unlock Positive Wire ( ): Blue/Red
Power Door Unlock Positive Wire Location: At Factory Alarm Plug. The alarm plug is a black 12 pin plug located just behind the glove box.
Door Trigger Negative Wire (-): Brown/Gray
Door Trigger Negative Wire Location: At Factory Alarm Plug. The alarm plug is a black 12 pin plug located just behind the glove box.
Domelight Supervision Wire: Turns On With Unlock
Domelight Supervision Wire Location: N/A
Horn Negative Wire (-): Brown/Red or Brown/Yellow
Horn Negative Wire Location: At Steering Column Harness
Tachometer Wire Negative Wire (-): Black
Tachometer Wire Negative Wire Location: At Instrument Cluster
Brake Light Positive Wire ( ): Blue/Red or Green/Red
Brake Light Positive Wire Location: At Switch Above Brake Pedal
Factory Alarm Disarm Wire (-): Disarms With Factory Remote Only. Factory Alarm Disarm Wire Location: N/A
Factory Anti-Theft System: Transponder Anti-Theft System, Requires Bypass And Extra Ignition Key. The 1997 model year vehicle may require a bypass module and an extra ignition key, if vehicle is equipped with a special key from BMW.
Factory Anti-Theft System Location: Transponder At Ignition Switch Tumbler
if you require further assistance please let me know and i will help assist you from there. Thanks, Midwest-Tek

Programming
When
Key batteries completely discharged. Key batteries not replaced within 60 seconds. Key added or replaced. DO NOT press remote control buttons during battery replacement.
How
Unlock vehicle. Enter vehicle. Close driver's door. Switch ignition ON and OFF. Remove ignition key. Press and hold unlock button Fig. 1 [1]. Press lock button Fig. 1 [2] briefly 3 times within 10 seconds keeping unlock button [1] depressed. Release unlock button Fig. 1 [1]. Some models: LED Fig. 1 [3] flashes slowly for 10 seconds. Some models: Central locking system locks and unlocks rapidly to indicate correct programming. Some models: Repeat above procedure if central locking system does not work and LED does not flash or LED flashes continuously

To change batteries remove panel back of key then screws on plastic bar
Batteries are available from your local motor factor shop
You might not need new batteries try re programing as described above

Synchronization
When
Key buttons operated repeatedly while vehicle out of range and system becomes inoperative.
How
Carry out the following within 1 minute: Press key lock or unlock button for approximately 1 second. Manually lock or unlock vehicle. System should now lock or unlock vehicle. If not, carry out programming procedure.
Programming
When
Key added or replaced. System malfunction.
How
Switch ignition ON. Insert key to be programmed into driver's door lock. Lock vehicle manually. Press lock or unlock button. Wait at least 1 second. Press lock or unlock button again. Vehicle horn sounds to indicate correct programming. Remote controls can also be programmed using diagnostic equipment.
NOTE: A maximum of 4 keys can be programmed

I replaced the water pump on my 1997 740i without a manual & so can you! Go to e38.org for instructions with pictures. Read up & get prepared with the parts & tools that you will need. Not a very difficult job, but take your time to do it properly. Good luck!

Here are the instructions to view, change alarm time and turn off alarm:

From normal time display:
Press MODE once to display stop watch.
Press MODE again to change alarm time.
Press MODE again to change time.

From normal time display:
Press RESET once to display current alarm time.
Press and hold RESET then press START to turn on and off alarm (watch for alarm symbol on top left)
Press and hold RESET then press MODE to turn on daily alarm vs one time (watch day marker lines at the bottom)
Hope this helps.

SYSTEM OPERATION

System warning lamp located on top of the instrument panel
Press UNLOCK button once , turns on interior lamps and deactivates alarm (if fitted)
Press UNLOCK button twice w/in 5 seconds unlocks all doors, turns on interior lamp and deactivates alarm (if fitted)
To activate alarm system (if fitted):
Switch ignition OFF
Open driver's door
Press Lock button on remote
Close Driver door
warning lamp will flash when system is activated
Press remote power sliding door button will open sliding door (if fitted) only if unlocked first
Pressing remote panic button:
*Sounds horn intermittently
*Flashes exterior lamps
*Turns on interior lamps
**Pressing panic for the second time OR switching ignition ON deactivates alarm.


PROGRAMMING
Remobe key from ignition switch
Remove the "BCM PRGRM" fuse, from the fuse box
Close all doors
Turn ignition switch to the ACC position
With in 1 SECOND , turn the ignition switch to OFF then back to 'ACC'
(really, with in one second)
Open and close the driver's door. A chime will sound, indicating programming mode activated
Press and HOLD LOCK and UNLOCK SIMULTANEOUSLY untill a chime sounds 2 times ( about 15 seconds)
Repeat this exact procedure for all remotes to be programmed
Remove key from ignition
Refit 'BCM PRGRM' fuse.

4 REMOTES MAXIMUM CAN BE PROGRAMMED