Here's some info on how the 4wd system operates on newer Blazers for all those searching for the cause of the flashing dash switch light and no engagement.

4WD System Operation on 98 up (2nd Generation) Blazers/Jimmies

The front drive shaft always turns on “shift on the fly” systems so the drive train is at the same speed in the front as the rear or the whole thing would break when engaging 4Hi or Autotrac when moving. InstaTrac (NV243 case or NP1) has 3 buttons. AutoTrac (NV246 case or NP8) has 4 buttons and clutch packs in the transfer case and is the only system for on pavement driving! Prior were the NV231 (87-93) lever and NP233 (94-97) electric motor. Your VIN # is the only sure way the dealer can get correct parts/listings!
1. Select mode on dash switch
2. Signal goes to the Transfer case control module (TCCM) or computer located in the cab on the passenger side behind the kick panel.
3. If you want 4 Lo the TCCM commands the encoder motor on the transfer case to switch a lower gear in (requires putting the shifter in Neutral and be stopped or only going 2-3mph). Note that if 4 Hi selected no action done in the transfer case. The transfer case is chain coupled and has the low and high gear. You can put all systems into Neutral by pushing the 2Hi and 4Lo button at the same time when the key is in the run position (or running but not off or acc).
4. The left front (drivers side) CV axle is always engaged to the front differential, but the right side is not coupled in 2wd. There is an actuator located under the battery tray that when commanded is vacuum actuated and pulls a cable from it to the right side CV axle coupler engaging that axle. A sensor is on that axle that tells the TCCM the axle position.
5. On 99up there is an additional vacuum switch on the firewall that switches the vacuum on and signals the TCCM also.
6. The dash light on the selector switch flashes while all this is occurring and will not switch out of 2wd unless all parts report correctly – same on the switch to any mode or back to 2wd.

So, there are a lot of possible failure areas with actuators/computer/motor/sensors/vac hoses and a special scanner is required to read the codes when the Service 4WD light comes on. But, a few things can easily be done before going to the dealer.
1. Check all vacuum hoses from the intake to the drivers side reservoir can, then to the extra switch (99up) and down to the under battery actuator. Small hose is 5/32 and larger is 7/32”.
2. See if the actuator pulls the front axle cable to engage it. If not and all hoses are good the diaphragm may be leaking on the actuator. If it engages but the light is still flashing remove the indicator switch on the front axle (only electrical plug in there) and manually push it in with the connector plugged in and see if the light stops, if so replace.
3. Listen to the encoder motor clunk when 4 Lo selected. If not working look at the wiring going to it for any cuts/breaks and repair if any found. Another indicator switch on top of this case. The fluid in the AutoTrac case should be the newer light blue Autotrak –II synthetic P/N 12378508 ($7 qt) not the red Dexron III.
4. If the light keeps flashing after this then disconnect the battery for 30 minutes for a TCCM reset.
5. Still not working – take it to the dealer and stop at the bank on the way for $$$$!

Good job. I added it to the FAQ.

[quote=muddog321]InstaTrac (NV243 case or NP1) has 3 buttons. AutoTrac (NV246 case or NP8) has 4 buttons and clutch packs in the transfer case and is the only system for on pavement driving! Prior were the NV231 (87-93) lever and NP233 (94-97) electric motor. Your VIN # is the only sure way the dealer can get correct parts/listings!
[quote]

Great write up. Just to add the NP 233 was used as early as '92 on these trucks, mostly on the more equipped trucks, eg. the LT's and the SLT's.

Great write up, thank you!

5. Still not working – take it to the dealer and stop at the bank on the way for $$$$!


I think that's where I'm at right now. Except if I stopped at the bank and asked for money they'd laugh me out the door.

Here was my problem.

http://img.photobucket.com/albums/v448/therealsundance/DSCF0049.jpg

On 99up there is an additional vacuum switch on the firewall that switches the vacuum on and signals the TCCM also.

Here is an uopdate on 98 NP233 transfer cases:

Info on the 3 port vac switch I bought a Motormite #49315 for $15 at Advance and its on the Help part isle (not listed online so don't try and look it up) lists for 1983 -2000 but think 98 was last year in Blazers. Two ways to look at it so I'll explain looking down on the plastic 3 port switch with the locator raised tab at the bottom, the port to the right is the Vac port from the engine, one to left of tab is a vent port, and the other is the vac to actuator port:
Pic of ports O to front act port
vent port O . O vac port

So when the spring loaded switch ball is down (2wd) the vac from the engine goes no where or is off (other 2 ports are open and connected together). When 4Hi is pressed an arm in the transer case pushes the ball up and the vac is applied to the front actuator port and front axle coupler engages. When 2wd pressed again the vac is removed from the front act port and it vents thru the vent port releasing the actuator. This vent port is T to a rear transfer case vent port also and the line usually runs up the trans dipstick with a small vent on it. The 3 vac lines coming in to the switch are in a formed rubber fitting that keys to the tab on the switch top so hard to get wrong. >>>>Remember that if you look into that rubber connector ports are reversed from my above description<<<<.

This was a NP 233 transfer case 3 button and for all note that the front drive shaft does NOT turn at all in 2wd - I tested and watched.

Excellent Tech Article, and timely advice on the vacuum solenoid!

[QUOTE=blazes9395
Great write up. Just to add the NP 233 was used as early as '92 on these trucks, mostly on the more equipped trucks, eg. the LT's and the SLT's.[/QUOTE]

The NP 233 was also used as late as 2003.

I have a 2001 S10 crew cab with the NP 233 transfer case (It's really muddy right now so I didn't look) and I have a 2003 S10 blazer with the NP 233 transfer case. The blazer is still clean, I looked to be sure. Both have NP1 in the RPO Codes.

James

Another bit of information some might find necessary:

Document ID# 1350749
2002 Chevrolet S10 Pickup - 4WD

HVAC Mode Switch Inoperative or Stuck in Vent Mode, Oil in HVAC Control Head and/or Vacuum Lines (Replace Transfer Case Vacuum Switch and Contaminated HVAC Components) #03-01-38-009 - (Jun 25, 2003)
HVAC Mode Switch Inoperative or Stuck in Vent Mode, Oil in HVAC Control Head and/or Vacuum Lines (Replace Transfer Case Vacuum Switch and Contaminated HVAC Components)
2001-2003 Chevrolet Blazer, S10
2001-2003 GMC Sonoma
2001 GMC Envoy, Jimmy
2001-2003 GMC Jimmy (Export Canada Only)
with 4WD and Two-Speed Electric Shift Transfer Case (RPO NP1)

Condition
Some customers may comment that the HVAC mode switch may be inoperative or stuck in the vent mode. After some diagnosis, oil in the HVAC control head and/or vacuum lines may be found.

Cause
When the transmission or transfer case heat up, the oil may expand and find its way into the transfer case vacuum switch. This oil expulsion may cause the vacuum switch seal to fail and allow transfer case oil to be drawn into the HVAC vacuum lines.

Correction
Replace the transfer case vacuum switch. Refer to the Transfer Case Vacuum Switch Replacement procedure in the Transfer Case sub-section of the Service Manual. Also, replace all contaminated parts in the HVAC system.

Parts Information
Part Number Description
15664811 Switch -Transfer Case Vacuum
Parts are currently available from GMSPO.

Warranty Information
For vehicles repaired under warranty, use:
Labor Operation Description Labor Time
K4120 Switch, Vacuum Control Transfer Case - Replace 0.6 hr
Add Diagnosis Time 0-0.3 hr
Add With Skid Plate 0.2 hr
Any additional time for replacement of HVAC components must be submitted as Other Labor Hours and requires appropriate authorization.

Since that document was released, the part has been redesigned. The old part was 15664811 and had a green stripe on it. The new part is 89059420 and has a blue stripe on it. You can find more information <a href="http://www.s-10crewcab.com/phpBB2/viewtopic.php?t=3197">here</a>

James

What happened to the edit button?

Try this:
http://www.s-10crewcab.com/phpBB2/viewtopic.php?t=3197

I have read the How-to section above but I want to know anyone elses experiences.

I have a 98' Blazer with the 3 Button selector. When I click either button all I have is a click or clunk and the light blinks, it doesn't engage. I took the battery cover off and started it then removed the battery, looked at the vaccum switch under the battery plate, had a buddy press the buttons but nothing moved there.

I just had the intake gasket fixed and they said there were no vaccum leaks. So What could it be?

The mech. under the kick panel on the passenger side, that vaccum thing under the battery or the transfer case encoder, is there anything else im missing?

Thanks for the help in advance,
Justin Tidwell

there is not much to say in addition to this perfect article, exept that on 99 Blazers with 4 Button switch is an NP236 Transfer Case used.
But again, perfect, this article saved me a lot of cash.
For example a Transfer Case over here in Germany cost 3850,00 Euro, which is aproxx 5000 $ (sick), a Transfer Case motor aproxx 1300 $

I'm having the EXACT same symptoms that Skymac is describing, on the same truck (98 Blazer). I'm fairly convinced that the TCCM is the issue because I can here a click coming from it, and there is no movement, sound, or any signs of life past it along the line. This afternoon I completly removed the TCCM and I'm hoping to find somewhere to test it. Anywhere (other than dealers with inconvient hours) that might be able to tell me something? AutoZone, Advance, Napa. . .any ideas?
Looking around on the net it seems that a lot of these trucks with electronic 4WD have issues. GM never did a recall on this did they?

OK, A question about all this, if the tccm is indicating 4 wheel low all the time and therefore the trans is shifting into 4th gear and trying to lock up the torq converter by 15 mph, therefore causeing the engine to lug down(like putting a manual in 5th gear at to low of speed) but the actual shifting of the transfere case buy way of the buttons on the dash work fine would this then only be the tccm or would the trans case switch still be a likley problem? I can watch all this happen on the scanner (snap-on modis).:screwy: I talked to a dealer tech who said the problem is the tccm and don't bother testing anymore as they, and this problem, is commin here in minnesota.

Good Morning,

Just purchased a 2001 Blazer S10, 41,500 miles. Been reviewing your forum and it looks like this site is going to be invaluable to me in the months to come.
Thanks to all for posting this information.

One question, at what mileage do ball joints start to go ?

Thank you,
Jack

In my experience, ball joints start to fail on average at about 70,000 to 80,000 miles, but thats here in Minnesota with rough roads all winter long, and pot holes up the ying-yang all summer! The better drivers get about 120,000 or more out of them. Be sure to keep an eye on the control arm bushings as well, not sure if they have improved them over the older models, but they are a common failure as well.

Quick question.....My 4hi and lo actually engage, but, the light still blinks in both positions. I have confirmed that they engage and work well. The only other caviat to that is that if in Auto4wd, it will switch back to 2wd. But, again, switching to 4hi engages 4wd and it stays there. It just continues to blink. Could this be a bad switch that needs to be replaced with one from motormite? Thanks

Hello, I have an 04 Blazer with NP8. There is something wrong with the transfer case shifting (every Blazer I get has something wrong with that system!). I've narrowed the problem down to either the TTCM or the Encoder Motor assy. However, the Encoder Motor works it just doen't seem to shift the TC all the way into gears. As soon as I press the Auto 4WD from 2Hi the Service 4WD light comes on but the system seems to work. However SOMETIMES after changing out of 2Hi and back the drivetrain will be in Neutral or pop into Neutral (happened while driving). I suspect the encoder motor's POSITION SENSOR is the problem. Does anyone have the pinout of the position sensor so I can check it with a meter for proper operation before I drop $$ for a new Encoder Motor Assy? Does anyone know where I can buy the Encoder's Position sensor seperately from the Encoder Motor?

Thanks,
Leif

Is my blaze throwing a code if the service 4wd light is not on?
It wont go into 4wd the light just flashes.
How much will it cost avg to hook it up to a scanner at a shop? $100.00?
Anything I can try myself ,before I go that route?
Thanks!
CS

i have a 99 with the 4 button switch, my 4x4 wont engage, and the lights wont come on on a brand new switch, could all this be because my abs light is on. oh and my service 4x4 light is not on

Here was my problem.

http://img.photobucket.com/albums/v448/therealsundance/DSCF0049.jpg

On 99up there is an additional vacuum switch on the firewall that switches the vacuum on and signals the TCCM also.hey bud were abouts is it located what side firewall???

That switch is located above the distributor on the center (engine side) firewall.

That switch is located above the distributor on the center (engine side) firewall.call me a dope but i dont see that switch,do all 99 blazers have this mine is an electric shift tc.i followed the vac line from the actuator(under the battery tray and it leads all the way to the tranny where it does hook up to a vac switch of some kind.any thoughts?

The actuator is under the battery tray and is NOT the same as the vacuum switch located on the center of the firewall above. The vacuum switch is pretty small and has one electrical plug with two wires and two vacuum lines going to it.

The actuator is under the battery tray and is NOT the same as the vacuum switch located on the center of the firewall above. The vacuum switch is pretty small and has one electrical plug with two wires and two vacuum lines going to it.yes i understansd the difference between the actuator (under the battery)and the vaccuum switch u say is located center of the firewall near the distributor,problem is that i just dont see it.do u need to remove the distributor cap to see the vac switch?//?perhaps a pic might help.usually im pretty mechanically inclines but shit i just dont see it lol.

Here was my problem.

http://img.photobucket.com/albums/v448/therealsundance/DSCF0049.jpg

On 99up there is an additional vacuum switch on the firewall that switches the vacuum on and signals the TCCM also.


Do you have any additional info on the part (i.e. part number, where available). I have the same problem want to get fixed ASAP. Any help greatly apprecaited.

12577412 is the part number for the Transfer Case Control Module (TCCM). I changed that and all my problems were fixed. Same part failed on two different blazers of mine- 1st a 2000 and then a 2004.

Only one problem with the technical description.
"Note that if 4 Hi selected no action done in the transfer case."
Not true. The front drive shaft is always connected to the front left tire. Not the x-fer case. When 4-Hi is selected the transfer case is engaged and there is a ball plunger actuated vacuum switch that sends vacuum to the diff actuator. That diff actuator (located under the battery) is what draws the collar inside the front axle to couple the differental to the front right tire.

This is a common misconecption and misrepresentation in the GM tech documents.

Still not seeing this vacuum switch that is on the 99s and up, Anyone have a picture? Above my distributor cap there is nothing but red paint and a fancy sticker from the General.

Ok, I got a 2000 with the 4 button switch. Mine will not shift into 4hi at all and will shift into auto 4wd most of the time (sometimes it wont go in that either). Then once it's in auto 4wd and I hit the 2hi to take it out, nothing happens. But if I hit the 4hi button, the 2hi button will light up and it will go into 2hi. Sometimes it takes several attempts of pushing either the 2hi or the 4hi button for it to go into 2hi. Usually it's the 4hi button being pressed that makes it shift back into 2hi. Any ideas???

Why couldn't they have just stuck with the good old manual shift lever on the floor? Sometimes I really miss my old 85 Blazer.

I am having some of the same problems as everyone else has posted about not going into 4 hi, goes into 4 low no problem, took to dealer, they diagnosed as being encoder motor & dash switch, had a local mechanic replace encoder motor & dash switch for about $500. Drove it home & still had same problem, replaced switch located on differential for about $15, still not fixed, took battery out & examined actuator, actuator appears to be working w/ no problems & holds vacuum, vacuum switch located on firewall seems to be doing it's thing also. My big concern is that it appears that the cable is getting hung up on something & isn't able to be pulled far enough to engage, when pulled as far as it will go, it only pulls the actuator diaphram about half way down. I have tried spraying wd40 down the cable, however it appears that it is not in the cable, but something in the front diff. I am not familiar w/ how the cable connects in the diff or what exactly it does, or what it could possibly be hanging up on, & if it is, how to take it apart & fix it. I would certainly appreciate any help or advice.

update.... turns out 4 high now works after discovering broken hose @ vacuum smitch on firewall & reconnecting it, however selector switch light still flashes in 4 high. I have already replaced switch on front diff & have disconnected the battery for over an hour to reset computer & am unsure of where to look next to solve the problem of flashing selector switch