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My 1998 chev blazer 4WD is not engaging. I can hear the transfer

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My 1998 chev blazer 4WD is not engaging. I can hear the transfer case shift over in 4LO and the front drive shaft engages but the front wheels don't lock up. In 4 HI the lights just flash and it goes back to 2 HI. I had a new engine and transmission put in it 2 years ago and this is the first time I've tried out the 4WD since then.
this is an automatic trans?
Customer: replied 6 years ago.

well this is sounding like you may have 2 different troubles but lets start with the simpler one first ..this is a test to see why the front axle is not engaging

Customer Concern:4WD is inoperative. The transfer case shifts into 4HI and 4LO OK, but the front axle is not engaging.
Tests/Procedures:1. With the transfer case in 4WD, check for vacuum to the front axle servo.

2. If no vacuum, check for vacuum in and out of the transfer case vacuum switch, top of the transfer case.
Potential Causes:Transfer Case Vacuum Switch
Leaking Vacuum Hose
Tech Tips:The front axle servo is under the battery tray.

Customer: replied 6 years ago.

The transfer case will not even switch in to 4HI, I push the 4 HI button from 2 HI and nothing happens.

ok that will either be the module or the motor on the transfer case,here is a test for you to do and let me know what you find.

Customer Concern:Four Wheel Drive (4WD) is not working.
Tests/Procedures:1. Check the ATC fuse, in the under-hood fuse box, to see if it is blown.

2. Check the Red and Black wires from the Transfer Case Control Module (TCCM) to the motor on the transfer case for being shorted.

3. Disconnect the motor, and using a fused jumper wire, manually power and ground the motor with it still hooked to the transfer case, and see if it moves or if it blows the fuse on the jumper leads.
Potential Causes:Open Fuse
Shorted Shift Motor
Shift Motor Wiring - Open or shorted wire to the shift motor.

Customer: replied 6 years ago.

I found the cable that is connected to the front axle servo and the clip wasn't in place to engage the cable so i connected that and tried the 4wd but it still doesn't work. How do you check for vacuum in the front axle servo? Because my problem is that my battery cables are not long enough to pull the battery out of the well and reconnect it so I can mess with this servo. and when I did have the vacuum hose disconnected and then reconnected the battery with the open ended vacuum hose just hanging out i couldn't get my truck to start I'm assuming due to the disconnected vacuum hose.

it will start withthe hose unhooked

start it up and see if it sucks your finger onto the hose when it is in 4wd

Customer: replied 6 years ago.
Okay, I'll give it another shot. You've been a great help thus far!!

thank you

Customer: replied 6 years ago.

Truck started, no suction on the hose!! And the transfer case didn't have any problems shifting into 4HI or 4LO. Now I have a front axle servo on order, should be here in 2 days I'll slap it in and get back to you on if it worked or not.


Thanks a million Sean



if theres no suction on the hose while it is in 4wd then the vac sw on top of the transfer case may be bad
Customer: replied 6 years ago.
On top of the transfer case? How difficult is that to replace?
not too bad , it is a little tight up there but you can reach over the top with a wrench and remove it without dismantaling anything
Customer: replied 6 years ago.

Hey Sean,


I replaced the front axle servo, and the transfer case switch and still don't have vacuum. This might be starting to get out of my expertise. What do you think?




Customer: replied 6 years ago.

Hey Sean,


Nevermind, I do have vacuum....very faint at the front axle servo and a little stronger at the transfer case switch. How strong does it have to be? In my opinion I would think it would be have to be fairly strong in order to pull that cable... I mean this thing is barely tugging when I put it on my lip because I can't hardly feel it on my finger at the front axle servo I had to put it on my lip.



Customer: replied 6 years ago.

Only 2# XXXXX Vacuum... not very good.


This is what I did:


elevated all four wheels of truck off the ground with jack stands, engaged 4wd and checked the vacuum. placed vacuum line back on front axle servo and re-engaged 4LO, transfer case shifted to 4LO, put it in gear, front axle not engaged. placed back in Neutral with truck still running went to front axle servo and manually pulled cable to engage front axle, shifted back into gear and front axle was engaged truck was in 4wd LO. placed back in neutral, shifted to 4HI, lights flashed but went to 2HI, placed in gear and front axle was still engaged and transfer case was in 4HI, after running in gear for a little bit it popped itself out of 4HI. Tried placing in 4HI, 4HI and 4LO lights flashed and went back to 2HI.


before all of this, on my way to the parts store i was cruising at about 45mph and wanted to check the 4HI, I hit the 4HI button and the 4HI and 4LO lights flashed, stayed lit but everything was disengaged I couldn't go. I tried going back to 2HI but nothing happened, I had to bring the truck to a stop, put it in Neutral, place it in 2HI, then put it back in gear.


this is starting to be a real pain and think it's going to cause me to shell out some major cash to fix.


I greatly appreciate your advice thus far,



Chevy expert, ASE Certified Technician
Category: Chevy
Satisfied Customers: 10370
Experience: 30 yrs experience ase cetified tech shop owner
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