S10 pickup 4wheel drive wont shift from 4hi to 2hi

Potential causes to consider:

Transfer Case Actuator (TC Motor): The transfer case actuator, also known as the TC motor, is responsible for engaging and disengaging the transfer case. If it's malfunctioning or stuck, it can lead to issues like the one you're experiencing. Consider checking the actuator for any signs of damage or water ingress.

Transmission in Neutral: When shifting from 2Hi to 4Hi, ensure that your transmission is in neutral. Some vehicles require this step to engage the 4Hi mode successfully. Try shifting to neutral before selecting 4Hi2.

Rolling Movement: Another technique is to roll the vehicle slowly (forward or reverse) while attempting to engage 4Hi. Sometimes, this movement helps the transfer case engage properly. Give it a try and see if it resolves the issue.

Transfer Case Control Module: The transfer case control module (TCCM) manages the transfer case functions. If it's faulty, it might prevent successful mode changes. Diagnosing and testing the TCCM using a high-end scan tool (like a Tech 2 or Snap On) could be necessary.

Indicator Switch on Front Axle: If the 4Lo mode isn't engaging, check the indicator switch on the front axle. Sometimes, manually pushing it in while the connector is plugged in can help troubleshoot the issue.

Remember that diagnosing these issues may require professional assistance or specialized tools.

I'm assuming your refering to a Chevy S10? There should be a small gear shift lever next to the standard gearshifter marked as 2HI, 4HI, Neutral, 4LO. This is your 2wd/4wd shifter. 2HI is for normal everyday driving as it is only powering 2 wheels for better MPG. 4HI is the same gearing as 2HI but powering all four wheels. This is your 'normal' 4WD setting. 4LO is a set of reduction gears designed for serious 'Off Road' conditions and should not be used on public highways. Your top speed will be Very low in this setting and the engine will be maxed out at full revs to get it.
DO NOT leave this lever in the Neutral position without the park brake ON. This setting completely disconnects the transmission from the wheels regardless of the gear you have selected so it will roll even if you select Park.

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.

Which do you have ?
Transfer Case - NVG 236 (Two Speed Automatic)
Transfer Case - NVG 233-NP1 (Two Speed Selectable)
The NV236 transfer case is an automatic transfer case with three drive ranges. Shifting from rear wheel drive to four wheel drive is done automatically when the transfer case shift control module receives wheel rotating slip information from the speed sensors. The transfer case shift control module then engages the transfer case motor/encoder to position the transfer case from rear to four wheel drive. When the transfer case shift control module receives information that the wheel rotation is the same on both axles, the transfer case shift control module sends position information to the motor/encoder to put the transfer case back into rear wheel drive. The three drive ranges are 2HI, 4HI, and 4LO. The 2HI position is for normal driving with the rear wheels pushing the vehicle. The 4HI, position is for driving through sand, snow, mud, gravel, or heavy rain at normal or slightly below normal speeds. The 4HI engages the front axle to allow for four wheel drive. The front axle is pulling the vehicle, and the rear axle is pushing the vehicle. The 4LO range is used for off road driving, heavy snow, deep mud, or shallow fording. The 4LO is similar to the 4HI except that the gearing in the transfer case is set for low speed and high torque. The NV236 is manufactured at Syracuse New York by New Venture Gear a division of New Process Gear for General Motors.
Is the switch light on or flashing ? If you have automatic 4x4 there are several DTC'S that can set >
DTC B2725 ATC Mode Switch Circuit Malfunction DTC C0300 Rear Speed Sensor Malfunction DTC C0305 Front Speed Sensor Malfunction DTC C0308 Motor A/B Circuit Low DTC C0309 Motor A/B Circuit High DTC C0310 Motor A/B Circuit Open DTC C0315 Motor Ground Circuit Open DTC C0323 T-Case Lock Circuit Low DTC C0324 T-Case Lock Circuit High DTC C0327 Encoder Circuit Malfunction DTC C0362 4LO Discrete Output Circuit High DTC C0367 Front Axle Control Circuit High DTC C0374 General System Malfunction DTC C0376 Front/Rear Shaft Speed Mismatch DTC C0387 Unable to Perform Shift DTC C0550 ECU Malfunction DTC C0611 VIN Information Error

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.

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.

something has broke inside the transfer case keeping it from going into any other gear