Another three button 4wd issue.
 

I bought a 98 Blazer 4wd LT this summer, the 4wd didn't work.
I read all the posts and then some... I ended up replacing the vacuum
actuator, cable and switch (at the end of the cable)

The 4wd worked all summer and fall, testing it now and then
to be sure, before it got too cold to work on it.

I've used it several times this winter without fail, I could easily
shift on the fly into 4hi/2hi. Now it stopped working :-(

Here's the deal, while driving, pressing the 4hi button does
nothing. IE: no clicking from the 4wd computer. Now if I put
the transmission in neutral, pressing the 4hi button does make
the 4wd computer click a few times, but returns to 2hi mode.

I have a good understanding of this system and how it works,
but why is the three button switches being ignored by the
4wd computer when in park/drive ?

When you command the 4WD system from the switch, the TCCM, (Transfer Case Control Module) requests data from the other network modules, (PCM, BCM, ABS, etc). Each module must send the requested data, and the TCCM must receive it. The TCCM then determines if it will allow a mode or range shift. If the data is not received, is incorrect, incomplete, or prohibits a mode or range shift, the TCCM will not perform it. If the check engine light or ABS lights are illuminated, the TCCM might not allow a shift. You will need to access each module and check for DTC's. This will require a "high end" scanner. You can also command the encoder to shift if the scanner is bi-directional. Without a capable scanner, you can check the basics: vacuum lines, make sure manifold vacuum is present at the vacuum switch, make sure the vacuum switch works, electrical connections, fuses, etc.

If the TCCM commands the encoder motor to shift, and the TCCM does not receive the correct positioning data from the encoder, the TCCM will revert back to the previous mode or range.

The PCM collects data that tells it what gear the transmission is in. If the correct data is not making it to the TCCM, it will not command the encoder motor to shift the transfer case.

Hope this helps.

No type of DTC codes were detected according to my Innova 3100 ABS
scanner.

I did jumper pins 13 & 5 on the OBD2 connector and the leds on the
shift buttons blink once every three seconds. As I understand, this
indicates RAM standby power lost. I did remove the TCCM connector
to look for corrosion on the pins (none found) so I guess this would
cause the error.

FWIW: I tried shifting into 4hi and it works fine now :-) Still doesn't
explain whats going on. Only time will tell.

I still suggest to everyone to remove the 4wd switch assembly from the dash. Carefully separate the halves and reflow every solder connection especially the connector pins. I found very little solder on these pins so I added a tiny bit.

This solved all the switching in/out of 4wd problems my '02 was having. Did this over a year ago and all still 100%.

Got my 2000 Blazer in '09. Since I've had it about 70% of the time I've to pop the 4x4 switch out of the dash then angle it so it messes with the wires then hit the 4 high button... Been like that for four years never had any problems with it not engaging. Since I'm about done revamping it for a 3,500 mile trip down the Alaskan-Canadian Highway to the states guess I'll try soldering the contacts like Lanny says...