I was testing the auto transfercase today, 99 blazer 4.3, nv236transfer case. I started in 2hi, slowed to 2-3 mph,put it in neutral,I shifted to 4-hi, then i stoped in neutral,Iturns along with the rearwent to 4-low, worked good, I went back to 2-hi then to auto. that is where it stayed! I put the car on stands,started it, and the front axle turns along with the rear axle. the light is lit on the "AUTO" button, And the "SERVICE 4x4 Dash light is on all the time . Pushing any of the buttons dosen't change anything. Do i need to get it scanned? any help would be great! Mike

 

More info! I'm working on this blazer for my Daughter, Today, I was checking everything out, the AUTO 4X4 light was still lit, I removed the battery ,and the holder, the actuator for the front end was clean, no rust, or cros. I hooked a vacume pump to the hose that goes to the actuater, it worked fine ! when i started the truck, the service 4x4 light was out, the 2wd light was on! every thing seemed OK! I went for a test ridi, 2-hito 4-hi shifted fine stopped, went to 4-low, all good. went back to 2-hi perfect! Went to AUTo 4 hi Good!! Tried to go back to 2-hi---- Stayed in AUTO!! Took the vacume hose off the switch that goes to the front actuater,with engine,and ign off, started the engine, All of the lights for the auto transfer lit up, as if there were no problem!! Somthing in the AUTO mode Isn't letting the transfercase to go to 2-hi, or any other mode, until you disconect the vacume hose at the fire wall that goes to the accuator for the ft axle!! After that, every thing is OK!! 4x4 light on dash is out, All lightson the select switch light when You turn the key to start, ALL IS GOOD!!! But don't go to AUTO MODE!! I know this is a lot of info, but there has to be an answer!! Not giving up!! Mike

 

So removing the vacuum line to the switch in the transfer case resets the switch in the dash? That is an odd problem... I would start by replacing the switch in the transfer case as it may be hanging open until the vacuum supply is disconnected...

 

it's the vacum line that goes to the accuater for the front diff. There is a small switch just above the intake manifold on the firewall,it has two wires going to it. vacume from the manifold is the source, the switch then sends vac to the front diff acuator. what's funny is every thing works great except it won't come out of "auto", until i dicconect the vac line from the switch on the firewall! A friend is going to scan it today, I'll post the results. Mike

 

Odd. I wonder if the switch isn't venting properly...

 

Got a scan on the transfercase today,code co305 came up( front drive shaft speed sensor malfunction) Looked like the front sensor on the transfer case was bad. WRONG!! put a new sensor in at $64.00, same problem!! I have a short, or open in the wires going from the transfer case to the atc module. these systems are really complex, when they work, LIFE IS GOOD!!! I'll find the problem, I'm a retired mech, when i find a problem I can't fix, I'll quit! hasen't happend yet! Mike

 

I just created a new DIY post on the C0305 with the book explaination and diagnostic steps in case you have access to a scan tool to run through the functional checks - C0305 Explained. It is possible that it is a short in the wiring for the front prop shaft speed sensor...