My wife's 1999 Blazer 4WD wouldn't engage in to 4WD in the snow. SERVICE 4WD dash light came on immediately and, AUTO 4WD button lite continued to flash ,4WD didn't engage. Thinking it was just a simple problem. I did some research on this site and now I'm stumped. All fuses Ok, all buttons light correctly.
Next I applied vacuum to the front axle actuator under the battery, it locked the axel, and the axel lock switch closed (zero ohms) as it should. I measured the resistance of the coil of the electric vacuum switch mounted on the firewall it was very low. Preplaced it with a new one(Dorman #600-104) that has a coil resistance of 68 ohms.
Don't fix same results as original problem.
Next I disconnected the vacuum switch and turned on the ign to run, and the dash SERVICE 4WD lite did not come on, so I bypassed the vacuum lines(connected together as if the switch would have if it was working) started the Blazer ,I heard the axle lock, pushed 4WD H button and it flashed went into 4WD-H just perfect. I drove it a little on my driveway ice it defiantly was in 4WD, pushed 2WD button, went back to 2WD as it should. I'm assuming that the TCCM and all the transfer case components must be OK if it worked with vacuum switch bypassed
Next I checked the voltage at the connector ,and switched Ign on, it read 6.8v, then pressed 4WD button reading went to 11.7v. Brown wire positive, Tan/Black-stripe wire negative.
Can anyone tell me where the control voltage to the vacuum switch comes from? I have found many schematics on 1999 Blazer 4WD's and none of them show the circuit(or wiring) to the actuator vacuum switch
Is it correct to have 6v applied to it with Ign in run without any 4WD buttons pressed?
Why would the SEVICE 4WD light cone on with switch connected and not be on when I disconnect it?
Is 68 ohms the correct coil resistance for a good switch, is my new one bad?

 

I'm having the same problem. I don't think my encoder motor is working. Are there 7 or 8 pins on the encoder motor connection? I have 3 4x4 blazers in the driveway now. Barring taking down the skid plates and looking on another one you have probed the pins.

 

The NV236 Wiring & Diagnostic Information thread contains the wiring diagram showing the front axle clutch solenoid wiring. The ground is supplied by the TCCM and the power comes through SP200 (splice pack #200). SP200 is actually in the wiring harness below the glove box that gets its power from the 4WD fuse.

*Moving to 2nd Gen Tech* Please create a new introduction thread if you'd like, but choose the appropriate section for requests for technical help.

 

Thanks that the best info I found. I will try to locate the problem and report back.

 

The other day I checked the switch/solenoid operation and wiring by un-plugging the TCCM and grounding F4 with key to run, switch/solenoid worked fine. Checked E10,E1 & E11, for 12V, also checked for good ground on E16. Concluded it must be a bad TCCM, ordered replacement one.
Replaced the TCCM everything worked great, so much for a simple cheap problem. Those TCCM's are treated like gold by the parts stores.
Thanks to swartlkk for the good schematics.