Troubleshoot the blinking lights on a 96 Yukon 4 wheel drive select switch

If you are getting a message that says "Service 4 wheel drive" and the switch light blinks after attempting to make a shift then odds are about 60% that you have a faulty encoder sensor that's part of the shift motor on the transfer case or the push button switch on the dash is faulty. If you can get someone with a scan tool to read the service codes then the code will direct you to which one it is. You could also have some other problem but those two items are what I see fail most often.

The reason the light is flashing constantly is because the vehicle is sensing that four wheel drive(4WD) is not working. These 4WD systems use vacuum from the engine to engage/disengage to front axle. The vehicle probably has no vacuum going to the front axle, which would not allow the 4WD shift to complete, which would then leave the light on the dash flashing.
First thing to check is that the vacuum actuator on the firewall is working as it should.

look on the dash for a 4wd button or switch. may be located between the steering wheel and the radio. when you turn the knob to select 4hi, 4lo, 2hi, or auto, the indicator light above the corresponding selection will flash until the shift has completed. if the light stays flashing there is a fault in the 4wd system

Have you had it check for DTC's - diagnostic trouble codes . frustrating because you don't know what your doing. Rookies ! I'd tell you have to test but you don't have tool or knowledge .
What transfer case does your tahoe have ?
Do you know what DTC'S are ? Your vehicle 4X4 can set a bunch .

DTC B0770
DTC B0775
DTC B0780
DTC B0785
DTC B0790
DTC B2725
DTC C0300
DTC C0305
DTC C0306
DTC C0321
DTC C0326
DTC C0327
DTC C0359
DTC C0374
DTC C0378
DTC C0379
DTC C0550
DTC C0558

DTC B0775 Four Wheel Drive High Range (4HI) Indicator Circuit Malfunction

DTC B0780 Four Wheel Drive Low Range (4LO) Indicator Circuit Malfunction
DTC B0785 Two Wheel Drive High Range (2HI) Indicator Circuit Malfunction
DTC B0790 Transfer Case Neutral Indicator Circuit Malfunction

DTC B2725 ATC Mode Switch Circuit Malfunction

DTC C0300 Rear Speed Sensor Malfunction
DTC C0305 Front Speed Sensor Malfunction
Just show a few DTC'S and for everyone of these DTC'S is a diagnostic flow chart . Do you know what that is ? It's fustrating and hard to fix something when you don't have a clue as to what your doing . Do you know how to do basic automotive electrical testing ?

This would sound about what the problem could be , but with out testing id be guessing just like you .

DTC C0306 05: Motor A or B Circuit Short to Battery or Open
DTC C0306 02: Motor A or B Circuit Short to Ground
DTC C0321 01: Transfer Case Lock Short to Battery
DTC C0321 06: Transfer Case Lock Short to Ground or Open

Do yourself a favor and save some aggravation an take it to a qualified repair shop . You do know a computer controls the 4X4 system ? transfer case shift control module .

In the instrument panel fuse box fuse # 8 CTSY LP 10A ,that's for the dome lights, fuse panel is left side of IP. What are you talking about 4 who drive won't light up or work ? 4 wheel drive ? Do you have two speed automatic or two speed selectable ?
Switch Indicator Lamps Do Not Light - One or More ? Check fuse #15 IP fuse box 10A 4WD . This is for two speed auto !

Circuit Description

The indicator lamps circuits consist of 5 individual lamps contained inside the mode/range switch assembly, they are AUTO, 4WD, 2HI, 4HI, 4LO and NEUTRAL. The AUTO, 4WD, 2HI, 4HI, and 4LO lamps are contained inside their respective range button. When the ignition is ON, the transfer case shift control module illuminates the indicator lamp that corresponds to the current ATC gear position by providing a ground through a current limiting driver.
The transfer case shift control module flashes each indicator lamp after a shift has been requested and will continue to flash until the transfer case shift control module can complete the shift. After a shift to AUTO, 4WD, 2HI, 4HI, or 4LO is made, the transfer case shift control module will flash that indicator lamp until the transfer case shift control module receives a high voltage signal on CKT 1695 from the front axle switch.
A solid state circuit located inside the switch supplies ignition voltage to the lamps. The solid state circuit also receives voltage whenever the headlamp switch is in park or headlamp position. When the solid state circuit receives voltage from the headlamp switch it will reduce the voltage that it supplies to the indicator lamps, dimming the lamps when the headlamps are ON.

Most likely cause is the encoder motor on the transfer case.

Dave,
The problem your are speaking about could be a few things. All things that are somewhat common to go bad on Chevys/GMCs. Each one of the cars I worked on had a different problem. I have changed the switches, the 4wd solenoid, and TCM on different trucks. IMO the TCM location in the passenger kick panel is a poor location allowing water and other passenger debris to get into it. Once water gets in that area you can imagine already what occurs. One of the cars the TCM bracket was rusted by the ground and eventually the weak ground fried it. The weird thing is the TCM would allow it to shift sometimes and not other times or partially engaging causing the wonderful light to blink rapidly. What I have found of some time with these trucks is pushing the button to change the truck to two wheel drive and driving a few feet in reverse helps ease the change.

So you can start with a new switch that pops in and out of the dash in a few mins. Can get one on ebay for under 25 dollars. Although, I personally would take off the kick panel and take a look at the TCM. If you see any rust around it, my money would be on that.

Good luck

The New Venture Gear Model NV233 is an electronically controlled optional transfer case for use on four wheel drive T trucks. The driver selects 2HI, 4HI, or 4LO by pushing one of three illuminating shift select buttons mounted on the instrument panel. The shift select buttons display the transfer case mode and range, self-test, diagnostic trouble codes, and electronic shift mechanical engaging problems.
During normal driving the transfer case is in the 2HI mode. When the transfer case is in the 2HI mode. When the transfer case is in 2HI both the 4HI and 4LO switch circuits are open, and both lights are OFF. When the four-wheel drive shift select button has been pressed, the four-wheel drive indicator lamp under the sift select button turns ON when the front axle has engaged. The shift select button will blink while the front axle is engaging. This is a normal condition.
The shifting of the transfer case is performed by a motor/encoder that receives drive signals from the transfer case control module (TCCM). When the driver selects one of the transfer case shift select buttons, request signals are sent to the TCCM. If the correct input signals exists, the TCCM will send drive voltages to the motor/encoder. The motor/encoder will position the transfer case to the ordered shift position.

Transfer Case Control Module Description
The TCCM performs the following functions:
• Receives input signals
• Processes the signal information
• Develops output signals
• Sends the output signal in order to control the shifting of the transfer case
The TCCM receives input signals from the following sources:
• The transfer case shift select buttons
• The NSBU switch on the vehicles with automatic transmissions
• The clutch position switch on the vehicles with manual transmissions
• The vehicle sped sensor calibrator module
• The encoder/motor range and the mode feedback signals
• The diagnostic enable
• The front axle mechanical status signal
In order to ensure the electronic shift system is operating properly, the transfer case control module (TCCM) continually performs diagnostics checks on itself and other parts of the electronic shift system when the ignition switch is in the run position. The following are different types of system checks that the TCCM continually performs.
Data Memory Retention Check
The TCCM checks the stored diagnostic trouble code memory to see if the memory has changed since the ignition switch was last positioned to OFF. This self test checks in order to see if the RAM standby power supply has been interrupted. If the TCCM detects a RAM standby power supply interruption, (the loss of stored diagnostic trouble code) the TCCM stores a DTC of 1 indicating RAM standby power failure.
This condition occurs when the TCCM is disconnected from the wiring harness or battery power is removed. The DTC of 1 automatically clears from the TCCM after cycling the ignition switch ON and OFF five times.

Before replacing parts you should test the system !
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 (40 and 1640), and Ignition 3 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. Refer to Transfer Case Shift Control Module Replacement .
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 .
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. Refer to Transfer Case Shift Control Module Replacement .

Is the 4WD fuse # 15 -10 amp located in the I/P fuse box good ?

Try commanding the drive ranges while in netural cycling them. This may normalize the controller that senses the position from the encoder motor assembly. There maybe a procedure for that in a service manual.

You should be able to put in 4x4 while in drive less than 35mph. Mine had a switch to turn to engage or dis-engage.