So about a month ago my brother and I did a little maintenance on my 2002 Blazer 4dr LT with 24k miles. Changed out the serpentine belt and hooked up my subs. He tested the 4wd and everything worked it took it a couple of seconds for the shift to complete as indicated by the lights on the 3 button switch blinking a 3-4 times. I drove the vehicle home about 100 miles and noticed that it was idling low when I stopped in my driveway. I immediately thought vacuum leak as it was a battle that I had tackled with my previous vehicle. Come to find out that the line going to the vacuum reservoir was disconnected because as it turns out my reservoir isn't there. So I plugged it with a screw and taped it off. Fast forward to today, I saw a gravel parking lot decided to test the four wheel drive. Hit the button, heard the TCCM click and heard an electronic noise of the electronic motor but not a thunk like normal with the transfer case and according to the switch on the dash the shift completed instantly but it doesn't drive like it's engaged or like it did before. Could this be because I plugged the vacuum line or does it seem like something else. For you other NP233 owners about how long does the shift take, mine seems to fast in comparison to how it was when it last worked.

 

So I shifted the Blazer into 4-hi crawled under and tried to twist the drive shaft. It didn't bulge so Im no longer thinking its an issue with my transfer case. I'm thinking its something in my front axle not engaging. Is there any vacuum that runs to the front diff or is it just that cable that connects to the diaphram under the battery? From what I saw it was just looked like that cable with an electrical connector plugged into the side of the cable where it connects to the front diff.

 

Sure sounds like your front axle isn't engaging. The vacuum actuator under the battery pulls on the cable that runs down to the front diff. The electrical connector you see at the front diff is to a switch that tells the TCCM that the front axle is engaged (or should).

Can you pull the cable at least 1/2" and lock it in place using a vice grip? Then with the transfer case in 4HI or 4LO and one front wheel off the ground, can you turn the wheel?

 

When me and my bro did the maintenance we checked the vacuum diaphram under the battery tray and from visual inspection it looked okay. Wasn't torn or looking to be suffering from the rot. So at this point if I were to do what you say and the wheel doesn't turn, it would be safe to assume the front axle is locking together and working? I don't have a service 4 wheel drive light but from what I have read it isn't uncommon for something to go wrong in the front diff and not set off any alarms. Thanks for the help swartlkk, you seem to be the resident expert for everything mechanical with these trucks on the forum . I'll try to check it out in the next couple of days and let you know if things are working out.

 

There are a few things you need to do in order to properly diagnose a problem with the front axle engagement.

#1 on my list of things to do is to use a vacuum gauge to read the vacuum level to the actuator under the battery when in 4HI or 4LO. It should be at or close to engine manifold vacuum. If it isn't, the actuator won't operate properly and you have a problem somewhere between the engine & the actuator (lines and/or the mechanical vacuum switch on the t-case - NV233). If you have insufficient vacuum pressure at the actuator, move back to the switch on the transfer case and test there.

You mentioned plugging off a vacuum line because you do not have a vacuum reservoir. Did you look for the res inside the front fender between the brake booster & the ABS control module? If not, check there. It is a black plastic housing that is actually INSIDE the fender with just a flat surface exposed between the two previously mentioned devices.

The test I proposed that involved manually locking in the front axle engagement cable is to see if the inner workings of the front axle are working correctly. If it engages you should not be able to turn the wheel that is off the ground. Alternatively, you can manually lock in the cable and, with the transfer case in 2HI, try to spin the front driveshaft with all 4 wheels on the ground. If the front driveshaft spins, then the front axle is not engaging and the problem lies either with the cable itself, or inside the front axle.

 

I've allready tried finding that damn vacuum reservior and it's definitely missing. So basically what your saying is when the 4wheel drive is engaged and the front driveshaft won't spin by hand it could still either be a failure with the Tcase or front axle because if either engages like its supposed to it will prevent the front drive shaft from spinning.

 

So I did what you suggested and the front axle is definately locked in. So I'm guessing my 4 wheel drive is working. Thanks for the suggestions Swartlkk, you should probably post up the information you gave me under the Four Wheel Drive no worky DIY article. It would probably help other people in diagnosing their issue or in my case my non-issue lol.