I have been reading a lot of different troubleshooting threads on the 4WD transfer case issues that blazers seem to be having. Though they've been kind of helpful, they haven't really had any that described what I am experiencing.

I have a 2000 Blazer S10 LT - When i press 4Lo (4 button selector by the way), 4lo definitely engages cuz she gives a good "CLUNK!" and when I drop it in gear, there is definitely a ton of torque there. Light is flashing on 4Lo. Now, when I press 4hi, nothing happens but the light on it flashing and does not go back to 2Hi nor 4Lo but if I switch it from 4Lo, the 4Lo stays solid while 4Hi flashes indefinitely. With ignition turned off but in the accessory on position and I press 4hi, i hear what I assume is the TCCM make a click and I hear something underneath the truck make almost a humming noise and then it stops but does not engage 4hi. (this is in park or neutral)

Any specific ideas? I am getting under there this weekend, weather permitting, and going to look at hoses as well as remove the battery tray and take a look at the lines and canister under there as I have read about. What are the chances of the Front Drive Axle Vacuum Actuator Solenoid Valve going out? I can't imagine thats failed if 4Lo works.

If anyone can steer me in the right direction, it would be so appreciated. We haven't really had our good first snow here in Grand Rapids but its coming soon i'd like to be ready for it. The past 2 years I've mounted the winch to the rear end and helped people out of ditches on the highway, side streets as well as out of snowbanks and would like to continue to do so.

 

Is your 'Service 4wd' light on? If so, the TCCM has stored at least one code and is trying to tell you what the problem is. If the light is on, getting the TCCM scanned will give direction to your diagnosis. The crappy part is that this requires an enhanced scan tool (ex GM Tech2) to access the TCCM codes.

You could pull the switch, tccm, and encoder motor connections and test the wiring out for excessive resistance and just to check for corrosion in any of the plugs. Corrosion seems to be a problem with these, especially at the TCCM. The wiring information is found in the Tech Article (DIY) section (link to the index post in my signature below).

 

The Service 4WD light has not come on, no. So far i have checked vacuum hoses and found some by the ABS that are becoming eroded. All other vacuum driven systems are operating normally so i dunno. I am going to check the wiring and hoses running to the transfer case and transmission today if the rain lets up.

 

The vacuum system on the NV236 equipped trucks doesn't run to the transfer case. The vacuum supply for the front axle engagement runs from the engine to the check valve 'T' assembly, possibly through another 'T' and then to the electronic solenoid on the firewall. You could also check/test the solenoid for proper operation when you attempt to shift. The transfer case won't lock into 4HI if it doesn't see front axle engagement through the electronic switch located on the front axle near where the cable enters the axle.

 

vacuume problem it seems.

are you 100% the front wheels engage? when u put it in 4LOW you hear the "CLUNK" coming from the transfercase, which is normal, but this does not mean your front wheels are engages.

you hear TCCM clicking - good.
you hear the endcoder motor rotating - good. (that humming sound under the truck)

lights on dash flashing - engagement is not complete. I had a very similar issue. turned out to be a broken vacume hose under the hood that engages the front axel.

have a look here, could be a very cheap fix (mine was $6 lol)

https://blazerforum.com/forum/2nd-ge...-module-63553/

in that thread/pic is where my problem was, but this problem can happen anywhere on any of the vacume hoses that make the 4x4 work. I'd go over all of them, or just replace them right off the bat.

 

x2 Your front axle isn't engaging (flashing light indicates problem) all you are hearing is the transfer case shifting to low range. Not an uncommon problem, we passed on two such Blazers (who's owners said the 4wd worked) when looking for one for my daughter.

 

swartlkk - the Service 4WD light has not come on at all :-/ Sunday I am going to bring her to my buddy's garage and get under there and take a look at all the electrical connections and clean em up and check vacuum hoses and see if anything has deteriorated. I wouldn't be surprised considering its age.

Tinted - 4Lo is fully engaged. I misstated earlier when saying 4lo flashes. it doesn't. it stays solid. front wheels lock and when she's in gear, you definitely feel the torque pull the truck when you drive with it engaged (slowly lol, i'm not stupid enough to drive very fast in 4lo) definitely a difference between that and 2Hi.

Will keep ya posted. Hopefully this will be a cheap fix lol. Thanks for the replies guys! I don't post here often but in the 3 years i've had the truck, you've had the answers I needed

 

So you've checked and ensured that the front wheels are engaged when in 4LO?

 

your front wheels are not engaged. what you hear and feel when you put it in 4lo is the front drive shaft being "engaged". it will give you every impression of the front wheels are being engaged (i also thought the same when this happened to me), but in fact they are not engaged. when the transfercase switches to 4lo you will still "feel the torque pulling the truck" even without the front wheels engaged. and there will be a huge diff between 2HI and 4lo........even without front wheels actually getting any power.

u have either a bad vacume hose or the air balloon thingy under the battery tray is broken. those are the best case scenarios.

 

Tinted - 4lo IS engaged. As i stated - when in gear i can feel the extra torque over 2hi. That and when turning the wheel on dry pavement and moving it does the typical tire slipping because theyre locked to the rear
rotations. Plus i jacked it up and attempted to turn the front wheels by hand before actual driving test

No big. Gonna get under there tomorrow and take a look. Will probably buy a bunch of hose and replace all i can get to. Better to replace all i can now instead of run into this again (if it is a hose) halfway thru winter and have to replace another hose. Again, thanks for the ideas guys