2000 Blazer
Issue stated, I engaged to 4WD Hi for use in the snow, was working fine, parked for a couple hours, started the car and press the 2Hi button and the 4Low button started flashing and it shifted into 4Low. Now the buttons are completely unresponsive, they do not flash when you press them.

Troubleshooting done:

Tried methods of shift out per the owners manual and tips on the net.
Unplugged the battery for 1 hour.
Replace the 4 button switch on the dash board.
Replaced all the vacuum lines.
Checked the vacuum actuator, diaphragm is good and holds a vacuum.
Replaced the encoder motor.
Checked the pins and connector on the TCCM, one was slightly corroded, I cleaned it.
I can hear the relays in the TCCM when I turn the vehicle on.

Now I know when I installed the new encoder motor I had to manually turn the cam on transfer case to 2Hi, yet when I turned the vehicle on it shifted back into 4Low.

I am thinking its the TCCM, but before I drop more money into it I would run it past all of you.

 

Well, the TCCM is the only thing that hasn't been replaced yet...

I would measure the resistance on the wires from the TCCM to the dash switch. If, for some reason, the resistance of the wiring has increased to the same value as 4LO, it could cause the unintended shift to 4LO. The NV236 Wiring Information thread will give you the pertinent values and wire colors. You want to test the resistance between pins F14 & F16 at the TCCM.

 

Tested the leads on the switch, resistances were correct
Tested F14 & F16, resistances were correct
Tested Voltage at the switch: 8.2v which should be ok

This was great advise, I really thought you were right on the money, but still no fix.

I have an OBDII reader, there was only 1 semi-related code which was transmission slippage, I cleared the code so I dont have the number anymore, but I think that is due to another off topic issue where the tranny shifts hard when the vehicle has been running for a couple hours on a long trip.

Anyways, I read turning the key 5 or 6 time and waiting till the lights stop flash will sometimes throw a code. Didn't happen.

Right now if I turn the key without starting the engine the 4Low light will flash endlessly until it is started and then will go solid and stop flashing.

Are there any sensors, scenarios, or conditions that would trigger 4low? For example the front axle may be stuck/locked enagaged or being reported so by a bad sensor.

 

There is nothing that should cause the shift other than the switch. From the sounds of it, there is something internally wrong with the TCCM. If you haven't tried to get the TCCM scanned to pull codes, that would be my next step. It cannot be done with a generic scan tool though...

 

This actually happened to me the summer before last. I put it in 4hi and it went into 4lo by itself but said it was in 2hi. It turned out to be the motor that shifts the transfercase.

 

Well, I ordered a used TCCM off of Ebay for $80, installed it today, my issue has been fixed.

Thanks everyone for your great advise.

 

Glad you were able to get it fixed! Thank you for the follow up!