OK - I'm going to assume that 2Hi is normally lit steady for you all the time. When you select another mode the light for whatever button you select flashes for a while, but then the controls pop back to having the 2Hi button illuminated steady. If this is incorrect please stop me now and describe indicator light operation further since a lot of the info below depends on the control button lights operating exactly as I described.

Here is a lot of factory info on your 4WD system, including detailed description of operation and all wiring diagrams.
https://blazerforum.com/forum/2nd-ge...-blazer-89044/

Pull the TCCM out of the right kick panel but leave it connected to the wiring harness.

Take a look at the first wiring diagram and locate the wiring between the encoder motor and the TCCM in the diagram. This is where we will do the testing (at the TCCM). There are two connectors and one has some larger diameter wires (This is connector C2). The other one with only small wires is connector C1.

Using the multimeter, first test for power from the ATC fuse to pin D of connector C2 (orange wire) by probing into the back side of the connector or wire while it is connected. You should have 12V. If not you probably have a blown ATC fuse in the underhood relay center (and you should strongly suspect the encoder motor). If you have 12V, unlock and pull the connector C2 out of the TCCM and visually inspect the terminals for the red, black, and orange wires for corrosion. If corroded, clean and see if that fixes your system. But if still doesn't work go on to further testing

The way this system works is when moving to another mode, first the TCCM must unlock the brake on encoder motor by grounding pin E1 on Connector C1 (tan wire circuit 1569). If it doesn't unlock, the shift will fail and controls will go back to illuminating the button for the previous mode.

First test will be to test the unlock control of the TCCM. 12V Should not be present on terminal E1 of connector C1 when you have selected a mode change (or are in auto4WD). So probe the back side of this terminal for voltage (with TCCM still connected) and watch it when you select 4Hi. If the voltage is present and then goes away when you select 4Hi, the circuit and TCCM is working correctly for that circuit. If not working correctly, TCCM is most likely bad. If no voltage at all, there is a problem with wiring or the encoder motor itself. If working correctly, then go on.

Second test will be to check the power from the TCCM to turn the actual encoder motor to switch between modes. Probe across the big red wire (connector C2 pin B 1553) and big black wire (connetor C2 pin A 1552) with the voltmeter. When you select 4Hi, you should see either a positive or negative voltage across those circuits when it tries to turn the transfer case to the new position while the light is flashing. If you get voltage on these wires but transfer case is not switching modes, the encoder motor is probably bad. If you get no voltage, then TCCM is probably bad.

Remember this is very abbreviated testing as this system is designed to be diagnosed with a scanner that can sense what positions are commanded and what positions are actually seen, along with use of a variety of TCCM trouble codes and diagnostic charts based on them. So it is not fool proof definitive testing, but will probably work for you if you want to do it yourself without a capable scanner.

Bad encoder motors and blown ATC fuses because of bad encoder motors are common. A shorted encoder motor can take out the TCCM. Corroded TCCM terminals are common. Controls are commonly replaced and almost never the problem. In the link there are instructions for testing the controls (buttons) with an ohm meter, but really doubt if that will be bad. Strategy here is to try to isolate a likely problem in TCCM or Encoder motor or Switch and replace that part first. Better than just the parts cannon.

Yes, you can have a shorted encoder motor ruin the TCCM by drawing too much current, and both can need replaced (was the case with mine).

So do some testing and come back with the results. Good luck and best wishes for a successful repair.

Les

 

Thanks Les, I will know in a day or so. And thanks to who also replied.

 

To start with I did test the vacuum diaphragm under the battery tray, at the firewall - test OK. I also tried a different shift motor & no luck. So, yes we are back to the tccm as mentioned above. Will a scanner at a for real transmission shop tell me anything about the tccm ? + I also noticed there was no difference when I switched from defrost to vent heat to floor heat ect. Back to tccm ? Also I did check the fuse in the side dash cover, any other fuses to check before I move on to what I was told - tccm

 

Yes, transmission shop should be able to diagnose using their scanner and factory diagnostic charts.

If you have vacuum leaks causing front axle to not lock, then that in itself could potentially be a problem with no 4wd.

But if you still want my help please advise results from specific testing I outlined in post #11 (all of it in order). Thanks!