My son's 2000 Blazer with 4 button 4 wheel engages on it's own while driving. Right now we have the fuse for the TCCM out and the power to the vacuum switch on the firewall disconnected to prevent this.
With winter coming we'd like to get this fixed.
I've done reading on here and read this link (https://blazerforum.com/forum/tech-a...help-7180) and many others. I suspect I know the problem is going to be either the TCCM or the encoder motor. How do we determine which one is causing the problem?

 

Just a quick suggestion, but my 1997 does the same thing and it is a 3 button switch and a friend of mine which is a service manager @ our local chevy dealership said it was the acuator on the transfer case. He said that it wasnt closing all the way and vacum is leaking and causing it to change while driving. Hope this helps.

 

Ok. I probably didn't give enought history in the original post. I was trying to not make it too long.
When the problem first started we disconnected the power connector to the vacuum switch on the firewall. This didn't prevent it from trying to engage while driving, that is why we pulled the fuse for the tccm out of the fuse box.
This is what leads me to believe that it might be either the encoder motor or the tccm.
So I guess the question really is: how do I determine whether it is the tccm or the encoder motor? Or could it be something else?

 

Have you disconnected the TCCM/Dash-switch connections & inspected them for corrosion? If not, try that first. Faulty connections can cause false resistance readings to the TCCM making it think that a button on the switch is pressed when it is not.

Have you tested the switch resistances at both the switch itself and the connector at the TCCM? Again, if the resistances are off, the TCCM could sense a button push when it actually isn't being pressed.

Most phantom NV236 transfer case shifts are due to a fault in the dash switch to transfer case wiring.

 

Thanks.
We did inspect the tccm connections, they appeared to be clean.
We didn't look at the switches in the dash. We'll do that this weekend.
Is there any trick to getting them out?
Or does the whole dashboard have to come apart?

 

Just pops out.

 

if there is corrosion on the connection is there a safe way to clean it off and not jack up the connection ??

 

A small brass pipe cleaner (1/16") and electrical contact cleaner can be used.

If a pipe cleaner that small is not available, then you may have to remove the terminals from the connector using the appropriate terminal disconnect tool and open them up to manually clean them.

When you put it back together, apply dielectric grease liberally to the connector.