4wd system operation 98up Blazers

[NOVA71]

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.

[brian111]

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.

[brian111]

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

[OBCKilroy]

Here's a puzzler...
All lights on my 4wd switch are lit and will not turn off (Auto, 2HI, red Neutral led, 4hi and 4lo) and the blazer starts out very sluggishly. Service 4wd light is NOT on.
The switch was replaced about 10K miles back.


Any ideas? nothing in the manual refers to the lights all staying on at startup, only if they are flashing.

[david102172]

Originally Posted by muddog321

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 $$$$!

It seems like you know a lot about this system, I have a 2000 jimmy with a 4 wheel drive issue, the vacuum and the actuator works fine,I was under the truck with the cable assembly pulled off and had a friend push the 4 wheel drive button and the cable pulled so I guess that it works, I don't know what step to take next in diagnosing the problem. There is a sensor and I guess that sends the signal to the electric 4 wheel drive motor, If I jump the two wires on that sensor will that send the signal to the motor to shift the transfer case? I am new at these forums and am not sure on where to ask questions I hope this is the right spot.

[LabRat116]

I just bought a 99 Blazer yesterday.

The Lights in the 4x4 Switch are NOT lit up. The 4x4 function does NOT work. There is no noise from the Encoder/Motor at all.

The Switch lights DO light up when the truck is turned on then go out after the diagnostic's test.

After driving a short while, the Service 4x4 light illuminates.

What do I need to do next ?

[snorander]

I have 2001 Jimmy and it has started shifting into 4WDHI on its own. It will go back into 2wd if I push the button a few times. some times I'll see it out of the corner of my eye flashing back between 2wd & 4wdhi by it self. I have a spare switch and tried my spare, but it did the same thing. I didn't notice anyone in this thread with the same problems. Any suggestion on where to start looking to fix it? Anyone else have this problem? How did you fix it?

Thanks!

-Steve

[LabRat116]

Originally Posted by snorander

I have 2001 Jimmy and it has started shifting into 4WDHI on its own. It will go back into 2wd if I push the button a few times. some times I'll see it out of the corner of my eye flashing back between 2wd & 4wdhi by it self. I have a spare switch and tried my spare, but it did the same thing. I didn't notice anyone in this thread with the same problems. Any suggestion on where to start looking to fix it? Anyone else have this problem? How did you fix it?

Thanks!

-Steve

Replace the TCCM.

Check out www.Car-Part.com for used ones. That's what I ended up doing.

Also, your replacement switch very well could be BAD as well. The dash switches are incredibly flimsy and break EASILY. They are cheap enough to buy new and replace.

Check out www.RockAuto.com for New switches at cheap prices.

Let us know how it works out.

[snorander]

Thanks will do!

[snorander]

Crap those are expensive! any chance it could be the vacuum switch on the fire wall?

[snorander]

I looked on www.car-part.com and couldn't find TCCM or transfer case controll module listed any where...

[LabRat116]

Originally Posted by snorander

I looked on www.car-part.com and couldn't find TCCM or transfer case controll module listed any where...

There is no exact heading for TCCMs.

They are listed under
Chassis Control Box(not Engine)
Chevy Blazer S10/Jimmy S15/Typhoon

You'll need to specify if you have a 3 button dash switch or a 4 button dash switch.

[LabRat116]

Originally Posted by snorander

Crap those are expensive! any chance it could be the vacuum switch on the fire wall?

Nope. It's an electrical problem.

Is your's throwing a Service 4WD light ????

If so, go to a tranny shop and have them pull the Codes for you.

[snorander]

No the service "Service 4WD" light is not on.

[laxman21]

Make sure you get the new updated one if you go used. Part #12577412


Find a u pull it yard. I pick one up today for $10. Go\tta love the clunkers program for Cheap parts.


Saw another clunker sitting there with the 4 button T case, but it wasn't out in the yard yet. Gonna go back and get it for a spare.