Transfer Case Shifting Problem

Sorry about the length of this post, but I wanted to provide all the info.

I have a 99 Blazer LT with the 4 mode (2H, 4H, 4L, Auto4) electronic shift transfer case. I mainly use in 2H and a while ago I had a problem where I could shift into 4H, but shortly after the service 4 wheel drive light would come on and I could not shift into another mode. If I restarted the truck, I could shift back to 2H before the service 4 wheel drive light came on. I took the truck to the dealership and was told that the transfer case control module needed to be replaced. Since I didn't need the 4x4 mode, I decided to wait.

With winter on it's way, I need to fix the problem and I decided to see if the same problem is occuring. I shifted to 4H and as expected the service 4 wheel light came on, but now I cannot shift back into 2H, even after restarting or diconnecting/reconnecting the battery. When I shift to 2H, the light flashes, I can hear clicking inside on the passenger side (I assume this is the module), I can hear the shift motor working, but it goes back to 4H. It seems to shift to Auto 4x4 and I have not tried 4L as I do not want it to get stuck there. Also, now when I start the truck the 4H light continuously flashes - if I shift to 2H, that light flashes, but then goes back to the 4H light flashing.

I have noticed a number of previous posts about transfer case shift problems and the starting point that sometimes corrected the problem was the vacuum actuator. I have a few questions:

What is the role of the actuator in shifting between 4X4 modes? I would have thought that the switch, control module and shift motor are the only parts involved in switching transfer case modes.

I noticed that the vacuum line from the actuator goes to a switch mounted on the firewall. It am guessing that when the switch receives a signal, it sends vacuum to the actuator to "turn it on". Is this correct and could the switch be the problem?

Is there any way to test the actuator and switch? I removed the actuator, pushed on the diaphram and air was expelled where the vacuum line connects. Then, with the diaphram pushed in, I blocked the nozzle where the vacuum line connects and the diaphram held its position without any leaks. It seems to me the actuator is OK.

Where is the transfer case control module located on the 99 Blazer LT? Is this a dealer only part or is it available by other auto parts sources?

Is there any way to "manually" shift the transfer case back to 2H as I do not want to drive it on dry pavement in 4H?

Any information, suggestions, etc. would be greatly appreciated.

I've experienced precisely the same problem that you've indicated with my 2000 Blazer LT with the 4 mode (2H, 4H, 4L, Auto4) electronic shift case.

After verifying that the actuator, 4WD front axle solenoid (the one mounted on the firewall) and that the diaphram was good, I swapped out the transfer case control module with a co-workers 2001 LT and found that solved the problem: bad module. To be more precise, there are two double-pole relays on that module and one went bad.
In any case, I purchased and installed a new one and am now shifting on-the-fly with ease!

In order to get the transfer case shift actuator to kick the transfer case back into 2Hi, you can disconnect the front axle 4WD sensor, then start up your Blazer and drive normally. Within 60 seconds the module will determine that it cannot tell whether the front axle is engaged so it will default the transfer case back into 2 WD. That is IF your actuator is functioning properly. However, as you mentioned that your dealer indicated the module needs replacement, so this should work for you.
Once in 2WD mode, reconnecting the 4WD sensor on the front axle will not cause it to switch back into 4WD unless you press either the Auto4WD or 4Hi button.

(It will not go into 4Lo without both the tranfer case and gear in neutral)

The 4WD front axle sensor is located on the front side of the front axle near the centerline of the Blazer, slightly closer to the passenger side.
The connector is a simple two-wire connector that can be snapped off with your fingers.

According to the Helms manuals (which I have) for the Blazer, the transfer case control module runs a diagnostic on itself when you turn on the ignition key.
It checks the RAM and ROM then each relay. It has positive feedback determining whether each relay is in the commanded position. If one of the relays fail (which happened in my case) and it doesn't pass it's own diagnostic test, it then freezes all transfer case shifting for the duration of that ignition cycle (until you turn the key off) and you'll only see the 4WD switch lights flash.

I don't want to stretch this reply out too long, but feel free to E-mail me if you'd like to understand what components perform what functions within the NVG-236/246 NP-8 electronic transfer case system that you have.

The transfer case control module is located behind the plastic panel just to the right of a passengers feet. This panel is held in place by several "snaps" and requires no tools to remove it.
Once you get to the module, you'll need a 10mm socket wrench to remove the 3 mounting nuts. There are two harnesses connected into it which can be snapped loose with a small flat blade screwdriver.

I priced this module at the local dealer and they quoted me a price of $408. I purchased mine at gmpartsdirect.com for $278 (including shipping & handling). It took less than 10 minutes to replace it.

To verify that your control module is the same as I expect, it will have the part number on it of: 15748800
This is the OEM part number but has since been replaced with 12577412.

Hope this information helps.

Regards,
Cujster