Another 4x4 engagement question

I've read bunches about why the 2000 Blazer 4x4 sometimes won't shift into 4WD. Here's my problem: I can't shift into 4 Hi. I've replace the dash switch to no avail. It's the one with 3 buttons. (2 Hi, 4 Hi & 4 Lo) I've checked the vacuum diaphragm under the battery. It gets vacuum when switched to 2 Lo and engages into 2 Lo OK. When the 4 Hi button is pushed, no vacuum. Same with the 2 Hi. I've read about a vacuum switch on the fire wall but can't find one on mine. The vacuum hose to the diaphragm under the battery comes from the back of the block toward the top of the tranny but I lose it from there. The lights on the dash buttons bounce back and forth between 2 Hi and 4 Hi when I am in 2 Hi. Mostly stays in 4 Hi but I know for a fact it's still in 2 Hi. I jacked all 4 wheels off the ground and checked. Sorry for the long description but I wanted to give as much info as I could.:banghead:

Here is a link to get you started.
http://www.automotiveforums.com/vbulletin/showthread.php?t=572914
And always check power and wiring and vacuum lines good.

I think on that one you can ground pin 13 on diagnostic connector and pull codes.
If any codes post back code no.

With the engine off, key on, and you under the truck, have someone push the 4hi or 4lo button. If you don't here the transfer case encoder motor engaging or trying to engage, then your problem is occuring before it gets to the vacuum actuator stage. The three button NP233 transfer case does not have the vacuum switch on the firewall.

MT has the right advice on where to start diagnosing.

James

Sounds to me like an encoder problem.
But, first make sure you check the electrical connections between the TCCM and the transfer case.