Hi guys, Im new to the forum; however, I have used it as a resource for many problems and it has always been very helpful. So Im hoping that someone can answer my question about not being able to engage my 4x4. I have searched the forum over and over and haven't been able to find a scenario similar to mine.
The blazer is a 2001 with the 4 push button 4x4 (NP8 transfer case). When I select 4 hi, the 4 hi button flashes and returns to 2 hi. While the 4hi button is flashing, I can hear the vacuum switch on the firewall click and the vacuum actuator under the battery depresses and actuates, but then releases the vacuum and returns to it's original (disengaged 4x4) state. The "Service 4WD" light is constantly on. I have checked the vacuum actuator under the battery, it holds vacuum. I have checked the cable to make sure it isn't sticking and it seems to move freely but I greased it up anyway. I have checked the ATC fuse under the dash. Would this be an issue with the position sensor on the transfer case? or some loss of vacuum in one of the lines? Any help is much appreciated, it is really snowing here and Im stuck in 2WD.

 

I wouldn't say it's electrical because it does try to engage but loses it. I'd lean toward a vacuum issue. It tries, starts to engage then falls off says vacuum to me.
Run the engine, grab a can of brake cleaner & spray it over the vacuum lines. When/if the engine revs a bit or stumbles - that's where the issue is. You may not even be able to see it, but the vacuum should draw the brake cleaner in & make the engine react.
The vacuum actuator could be old/weak as well.
I love brake cleaner, if I have another son that'll be his name - it's so useful! lol

 

Rottidog, that is a really good idea! I would have never thought of that. Im gonna give that a try first thing in the morning! I should make clear that the actuator depresses and then releases the vacuum over a matter of seconds. Actually the entire sequence that I mentioned above occurs over a matter of maybe 5-10 seconds.

 

Yeah that sounds like a vacuum issue or the diaphram in the actuator is toast. Same difference really but you can't see or test the diaphram the same way you can a vacuum hose.

 

get the 4x4 codes pulled

you need someone with an advance scan tool to do this

 

WHAT? No pics or video John? lol you're slipping!

If he has a CEL he didn't mention it & yes we know that there can be a DTC stored but not yet displayed.

Don't worry, John (gio or geo as he has been called) & I seem to follow each others posts for some reason that I am not acquainted with yet.

 

Heres a pic for ya



Just messing with ya Happy birthday buddy

 

LMAO I was having a slug of my beer when I "read" that part. My bad ... A hole hehehe

& ty for the b day wishes!

Wait a sec, if the service 4WD light is on then it is a vacuum issue with either a line leading to it or internal in the actuator. I say that because of the way it starts to work & then takes a few seconds to die off, doesn't STOP immediately.

 

So I take it you guys know eachother? haha. Well unfortunately I don't have an advanced scan tool. I began to try the jumper method at the data link so I could read the flashes from the dash switch, but I realized that the NP8 transfer case equipped vehicles don't have a dedicated terminal to read codes from the TCCM. So I guess I will be chasing vacuum lines until I find a leak or until I can get the codes read on monday; whichever comes first! haha. So the fact that the "Service 4WD" light is on indicates that it is a vacuum issue?

 

Ok guys, I have a bit of an update. If I bypass the vacuum switch on the firewall, the 4x4 engages, the front wheels lock up and remained locked. So the system is holding vacuum. If i push 4hi from inside the truck though, I still get the same scenario, the 4hi button flashes and then returns to light solid on 2hi. This entire time "Service 4WD" stays lit. Im leaning towards the position sensor on the transfer case but Im still unsure. Is any of this pointing to a failure point that Im neglecting? Any input is really appreciated.