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You don't need a reader to get the tccm code. TCCM codes are, 1, 2, 3, 4

Pin 13 is the pin use use to ground to check codes.

If you did indeed ground #13 the 4wd light will blink

Code "1" is "RAM standby power" and is considered normal, may be caused by clearing the codes.
Code "2" is "Encoder fault"
Code "3" is "TCCM motor circuit"
Code "4" is "RAM/ROM failure"

Ground pin 13 at the data connector under the dash. The lights on the switch should flash. You are only going to have a choice of 4 codes as mentioned above. Here is the procedure to further clarify.

Diagnostic Trouble Code (DTC) Displaying:
The transfer case shift control module is equipped with the ability to store diagnostic information, which is useful to a technician in the event of module, component or wiring failures. This information can be retrieved from the transfer case shift control module by way of flash codes, which are displayed on the three transfer case shift control switch buttons. These buttons are located on the instrument panel to the right of the steering wheel Before obtaining DTCs, perform the following transfer case control module (TCCM) Self-Test.

TCCM Self-Test
Observe the transfer case shift control switch indicators while turning the ignition switch to RUN. A properly operating system will flash all indicators and then will return to the current gear. If the system is operating normally, then proceed to Displaying Codes.
If the TCCM failed the Self-Test, then test the following circuits.
• Battery Positive Voltage, and Ignition Voltage for battery voltage
• Ground Circuit for ground
• Diagnostic enable circuit for an open or high resistance
• Transfer case shift control switch connector

If the above circuits are normal but the TCCM still fails the Self-Test, replace the TCCM.

DTC Displaying
Turn OFF the ignition for 10 seconds.
Connect PIN 13 on the data link cable (DLC) to a ground. The DLC is located in the cab under the instrument panel on the driver's side.
Turn ON the ignition, with the engine OFF.
Observe the transfer case shift control switch select buttons for flashing codes. Refer to Diagnostic Trouble Code (DTC) List above.

Important: If the transfer case shift control switch buttons all blink only once and then stop, no fault codes are stored in the transfer case shift control module.

• When one DTC is stored in memory, that code will blink that number of times followed by a 3 second pause and then repeat.
• If more than one code is stored, the first code will blink, followed by a 3 second pause and then the second code will blink.
• This sequence will continue until PIN 13 on the DLC is no longer grounded.
• If the TCCM will not communicate the presence or absence of DTCs, test the above circuits in Self-Test Step 2. If the circuits are complete, then replace the TCCM.

Subject: Service 4WD Indicator Illuminated, DTC B2725 Set (Replace Transfer Case Shift Control Switch) #02-04-21-004 - (04/15/2002)

Models: 1999-2002 Chevrolet and GMC C/K Pickup and Utility Models (Silverado, Sierra, Avalanche, Suburban, Tahoe, Yukon, Yukon XL) with Four Wheel Drive and NVG 236/246 Transfer Case (RPO NP8)

Condition
Some customers may comment on the Service 4WD indicator illuminating. Upon investigation, the technician may find DTC B2725 set.

Cause
The condition may be due to a intermittent fault in the transfer case shift control switch.

Correction
Important: The transfer case shift control module should not need replacement. DO NOT replace P/N 15749703 unless diagnosis points to the module as the malfunction.

Replace the transfer case shift control switch using published Service Information procedures.

Parts Information
Part Number
Description

15709327
Transfer Case Shift Control Switch


Here is a diagnostic chart that might also be of some help to you.

Document ID# 525340
2000 Chevrolet Chevy K Silverado - 4WD

DTC B2725 ATC Mode Switch Circuit Malfunction

Circuit Description
The range/mode switch circuit consists of four normally open switches. The transfer case shift control module supplies a regulated 8 volt, DC to the switch through the 8 V Reg circuit. The current travel through a 9.09 K ohms resistor located inside the switch. It returns current to the transfer case shift control module through the switch signal circuit.

The transfer case shift control module constantly monitors this signal voltage to determine the condition of the mode switch circuit. If no buttons are pressed, and the transfer case shift control module detects a voltage level outside the possible range (approx.-0.5 to 1.0 volts) for longer than 5 minutes, the transfer case shift control module will set this DTC or if a button is held down or sticks for a period longer than 5 minutes.

When each of the switches is depressed they will complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch input to determine whether the 4HI, AUTO 4WD, 2HI, and 4LO button selections are made by the driver. Neutral may be obtained if the following conditions are met: The engine is running.
The automatic transmission is in neutral (or the clutch pedal is depressed on a manual transmission application).
The vehicle speed is below 3 MPH.
The transfer case is in the 2HI mode.
Once these conditions have been met, pressing and holding both the 2HI and 4LO buttons for 10 seconds will shift the transfer case into neutral, turning on the red neutral indicator lamp.

Conditions for Setting the DTC
The system constantly monitors the voltage on the switch signal circuit.
If the system detects a voltage level outside the possible range produced when no buttons are depressed, (approx.-0.5 to 1.0 volts), the DTC is logged.
Action Taken When the DTC Sets
All shifting will be disabled.
The SERVICE indicator (4WD/AWD) lamp will be latched on for the remainder of the current ignition cycle.
Conditions for Clearing the DTC
The transfer case shift control module will clear the current DTC if the condition for setting the DTC no longer exists.
A history DTC will clear after 100 consecutive ignition cycles without a fault present.
History DTCs can be cleared using a scan tool.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.

Tests for proper operation of the transfer case mode select switch.

Tests for proper voltage supply to the transfer case mode select switch.

Tests the transfer case mode select switch for proper resistance values in all switch states.

Tests the switch signal circuit for a short to voltage.

Tests the switch signal circuit for an open or short to ground.

Tests the switch signal circuit for a short to the 8 V Reg circuit.

Determines whether the 8 V Reg circuit is greater than or less than the specified voltage.

Tests the 8 V Reg circuit for a short to voltage.

Tests the 8 V Reg circuit for an open or short to ground.

DTC B2725 Mode Switch Circuit Malfunction Step
Action
Value(s)
Yes
No

1) Was the Transfer Case Diagnostic System Check performed?
--
Go to Step 2
Go to Diagnostic System Check

2) Important: Neutral Range will only be obtained by pressing and holding both the 2 HI and the 4 LO buttons simultaneously for 10 seconds. Refer to Circuit Description.

Install a Scan Tool.
Start the engine.
Place the transmission into NEUTRAL (or clutch pedal depressed on the manual transmission applications).
Set the park brake.
With a scan tool, observe the Commanded Mode Indicator data parameter in the Transfer Case data list.
Depress each of the Mode/Range buttons.
Does the scan tool indicate that the Commanded Mode Indicator data parameter is indicating the correct state?
--
Go to Testing for Intermittent and Poor Connections
Go to Step 3

3) Turn the ignition to OFF.
Disconnect the selector mode switch.
Turn the ignition to ON, with the engine OFF.
Measure the voltage from 8 V Reg Circuit to a good ground.
Does the voltage measure within the specified range?
7-9 V
Go to Step 4
Go to Step 8

4) Connect a DMM between the Selector Mode Switch Signal Circuit and the 8 V Reg circuit at the switch.
Measure the resistance through the selector mode switch while depressing the range buttons and comparing the values.
Does the DMM indicate all resistance values within the specified ranges?
Normal 8.63-9.54 K ohms Auto 4 WD 61.7-68.1 ohms 4 HI 656-670 ohms 2 HI 1.50-1.53 K ohms 4 LO 2.32-2.37 K ohmsNeutral 1.015-1.035 Kohms
Go to Step 5
Go to Step 11

5) Turn the ignition OFF.
Disconnect the transfer case shift control module.
Turn the ignition ON, with the engine OFF.
Test the switch signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Was the condition found and corrected?
--
Go to Step 13
Go to Step 6

6) Test the switch signal circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Was the condition found and corrected?
--
Go to Step 13
Go to Step 7

7) Test the switch signal circuit for a short to the 8 V Reg circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Was the condition found and corrected?
--
Go to Step 13
Go to Step 12

8) Was the voltage measured in Step 3 greater than the specified value?
9 V
Go to Step 9
Go to Step 10

9) Turn OFF the ignition.
Disconnect the transfer case shift control module.
Turn ON the ignition, with the engine OFF.
Test the 8 V Reg circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Was the condition found and corrected?
--
Go to Step 13
Go to Step 12

10) Turn OFF the ignition.
Disconnect the transfer case shift control module.
Turn ON the ignition, with the engine OFF.
Test the 8 V Reg circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Was the condition found and corrected?
--
Go to Step 13
Go to Step 12

11) Replace the selector mode switch. Refer to Transfer Case Shift Control Switch Replacement.

Is the repair complete?
--
Go to Step 13
--

12) Replace the transfer case shift control module. Refer to Transfer Case Shift Control Module Replacement.

Is the repair complete?
--
Go to Step 13
--

13) Use the scan tool in order to clear the DTCs.
Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
Does the DTC reset?
--
Go to Step 2
System OK

Document ID# 525340
2000 Chevrolet Chevy K Silverado - 4WD

На ТАХЕ у меня ( давным-давно ) загорелось SERVICE 4WD
оказался просто перетёрт провод возле раздатки,
говорили болезнь. Заизолировали и всё
Каждый случай индивидуальный