have a similar issue w/ my 93 4x4.
Goes into 4w easy enough,
but getting it back out requires time & copious use of 4 syllable words.
Here's some info on how the 4wd system operates on newer Blazers for all those searching for the cause of the flashing dash switch light and no engagement.
4WD System Operation on 98 up (2nd Generation) Blazers/Jimmies
The front drive shaft always turns on “shift on the fly” systems so the drive train is at the same speed in the front as the rear or the whole thing would break when engaging 4Hi or Autotrac when moving. InstaTrac (NV243 case or NP1) has 3 buttons. AutoTrac (NV246 case or NP8) has 4 buttons and clutch packs in the transfer case and is the only system for on pavement driving! Prior were the NV231 (87-93) lever and NP233 (94-97) electric motor. Your VIN # is the only sure way the dealer can get correct parts/listings!
1. Select mode on dash switch
2. Signal goes to the Transfer case control module (TCCM) or computer located in the cab on the passenger side behind the kick panel.
3. If you want 4 Lo the TCCM commands the encoder motor on the transfer case to switch a lower gear in (requires putting the shifter in Neutral and be stopped or only going 2-3mph). Note that if 4 Hi selected no action done in the transfer case. The transfer case is chain coupled and has the low and high gear. You can put all systems into Neutral by pushing the 2Hi and 4Lo button at the same time when the key is in the run position (or running but not off or acc).
4. The left front (drivers side) CV axle is always engaged to the front differential, but the right side is not coupled in 2wd. There is an actuator located under the battery tray that when commanded is vacuum actuated and pulls a cable from it to the right side CV axle coupler engaging that axle. A sensor is on that axle that tells the TCCM the axle position.
5. On 99up there is an additional vacuum switch on the firewall that switches the vacuum on and signals the TCCM also.
6. The dash light on the selector switch flashes while all this is occurring and will not switch out of 2wd unless all parts report correctly – same on the switch to any mode or back to 2wd.
So, there are a lot of possible failure areas with actuators/computer/motor/sensors/vac hoses and a special scanner is required to read the codes when the Service 4WD light comes on. But, a few things can easily be done before going to the dealer.
1. Check all vacuum hoses from the intake to the drivers side reservoir can, then to the extra switch (99up) and down to the under battery actuator. Small hose is 5/32 and larger is 7/32”.
2. See if the actuator pulls the front axle cable to engage it. If not and all hoses are good the diaphragm may be leaking on the actuator. If it engages but the light is still flashing remove the indicator switch on the front axle (only electrical plug in there) and manually push it in with the connector plugged in and see if the light stops, if so replace.
3. Listen to the encoder motor clunk when 4 Lo selected. If not working look at the wiring going to it for any cuts/breaks and repair if any found. Another indicator switch on top of this case. The fluid in the AutoTrac case should be the newer light blue Autotrak –II synthetic P/N 12378508 ($7 qt) not the red Dexron III.
4. If the light keeps flashing after this then disconnect the battery for 30 minutes for a TCCM reset.
Here is information on NP233 transfer cases 1983-2003
Info on the 3 port vacuum switch for 1983 -2003 on S10/Sonoma (Part Number Description 89059420 Switch -Transfer Case Vacuum, available from GMSPO) but I think 98 was last year in Jimmy/Blazers. Two ways to look at it so I'll explain looking down on the plastic 3 port switch with the locator raised tab at the bottom, the port to the right is the Vacuum port from the engine, one to left of tab is a vent port, and the other is the vacuum to actuator port:
Pic of ports O to front act port
vent port O . O vacuum port
So when the spring loaded switch ball is down (2wd) the vacuum from the engine goes no where or is off (other 2 ports are open and connected together). When 4Hi is pressed an arm in the transfer case pushes the ball up and the vacuum is applied to the front actuator port and front axle coupler engages.
When 2wd pressed again the vacuum is removed from the front act port and it vents thru the vent port releasing the actuator. This vent port is T to a rear transfer case vent port also and the line usually runs up the transmission dipstick with a small vent on it.
The 3rd vacuum lines coming in to the switch are in a formed rubber fitting that keys to the tab on the switch top so hard to get wrong. >>>>Remember that if you look into that rubber connector ports are reversed from my above description<<<<.
This is info for a NP 233 transfer case 3 button and for all note that the front drive shaft does NOT turn at all in 2wd.
Also inspect the front axle switch (4wd sensor) for damage. The lost of that signal from a faulty connection will cause the TCCM to command the transfer case to switch back to 2wd.
Hope this helps and class over
Molson02536
