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Dan
Dan, ASE Certified Technician
Category: Chevy
Satisfied Customers: 2270
Experience:  20 Year Factory Trained Current GM Technician
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1998 s10 blazer 4.3l 4x4 Locked in 4 hi. selecting 2 wheel

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1998 s10 blazer 4.3l 4x4 Locked in 4 hi. selecting 2 wheel or 4 lo gets a click from the tccm, the light momentarily flashes in the range selected but no actuation. Replaced 3 port vacuum switch ( cheap to replace ). There is trans fluid in the vacuum lines to the 3 port switch. The small actuator button on the switch was stuck all the way in. Also engine roughs very rough, horrible economy, like a vacuum leak..
Hello JACUSTOMER-xyh5xi10-,

Have you checked the fluid in the transfer case. See if it is over filled. Let me know if it is.

As for it not going back into 2wd. You can try disconnecting both battery cables and touching both disconnected ends together for a few seconds. hook them back up.( make sure the battery cables are clean and tight. Start the vehicle and put the transmission in neutral and foot off the brake and try pushing the 2wd or 4 hi button. If that don't work. Locate the TCCM behind the panel on the passenger floor board ( to the right of where the passenger feet goes) disconnect the connector to it and reconnect Make sure the connector has no corrosion on it. And then try switching it into 2wd.
Let me know what happens
Customer: replied 7 years ago.

Fluid level is correct. Have disconnected both tccm and battery, no change.

Evap emissions code on ecm.

I also noticed heater/defrost only blows air through defrost. AC works fine through vents. Any chance of a vacuum leak somewhere?

WHen the switch on that transfer case goes bad and causes the fluid to suck up the vacuum line. The fluid gets into the vacuum controls for the HVAC and causes the diaphrams to go bad in the HVAC actuators. When this happens, I replace the switch on the transfer case, The vacuum canister, the HVAC control head, all 4 of the vacuum actuators, the front axle actuator and then flush out the vacuum lines.
As for the system not switching from 4low to 2 hi, is an electrical issue, The vacuum control the front axle to enguage.
You have either a encoder motor (actuator on the transfer case) bad or a TCCM (transfer case module) bad or a wiring issue.
You can check the codes in the system. Here is how to do that.
You can pull the encoder motor off and manually put it in 2hi or 4hi and take apart the encoder motor to get the gears to line it up to the transfer case.
Is this the first time you put it in 4low or have you had it into 4 low before?
Customer: replied 7 years ago.

I can't get it into 4 lo.

It is stuck in 4 hi. If I can get it into 2 hi I'll be happy. At least I'll be able to drive it until I get time to start repairing it. How do I disengage the front axle?

Customer: replied 7 years ago.
How do I get the front axle to disengage?
The front axle is enguaged from the transfer case being enguaged or the front axle actuator being stuck on. If the transfer case is in 4 wd,, then disconnecting the vacuum line on the switch on the transfer case and plugging the vacuum source. If the actuator under the battery is stuck, you can remove it and or the cable that goes to the front diff.
Dan and other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
Thank You for your help. I'll do what you recommend. What a pain. Expensive too!