Hello, new to the forums and first post. I have a 2003 ZR2 blazer. I’ve gone through and replaced a lot of bad switches, etc. I replaced the front axles right after purchasing it. The 4wd has worked great up until recently. Reading through the posts I fond some trouble areas that are common. I’ve checked the vacuum actuators for the front axle, the cable and everything is working. I checked the TCCM and sure enough it was corroded, I removed it to clean it up and there was one pin that was broke or lose so I got a new TCCM. I cleaned up the plugs as good as I could and put it back together. I also replaced the actuator motor on the the transfer case. When I start it it goes through the self checks and goes back to 4hi. If I try to change it to 4lo or 2wd I can here the encoder motor and I can hear the TCCM clicking. This is a 4 button system , sorry forgot to mention that earlier. If I pull the vacuum line to the vacuum thing under the battery it does go into 2wd. I have looked underneath and believe it does have the 3 post vacuum switch. It seems to me that if this switch is stuck open it would create the issue I’m having , your thoughts ? I have not cleared any codes that could be set in the TCCM yet. Would or could the TCCM be creating this problem ? Thank you for reading this post, wanted to try to give as much information as possible . Thank you for your replies as well

 

Hello again, I just found the 4x4 no worky section lol definitely sounds like a vacuum switch. Still welcome any more input anyone might have

 

Try switching the hoses on the electrically actuated vacuum valve for the front axle that is mounted on the firewall near the distributor. I suspect it is trapping vacuum from switched hoses, because the valve only vents one side to atmosphere. If the hoses are switched on the ports, the vacuum is trapped and the front axle will stay engaged unless the hose to the servo is unplugged.

 

Hello and thank you for the reply. I did try switching the vacuum lines as you said , though I only ever had one off because it was dry rotted and I replaced it. Out of curiosity I tried switching them and still no difference. I have a constant vacuum going to the vacuum diaphragm under the battery so the front axle is constantly engaged. I pull the bottom line on the firewall switch and the front axle does disengage so I know all the components on that part of the system are working correctly. I do not have the vacuum switch on the transfer case, the 3 post one so at s loss here. I have a friend with a scanner but have been unable to reach her so guess I’ll have to find a shop to scan it

 

What did you decide was wrong, my 2002 Zr2 is having a very similar issue and it's driving me nuts.