This old 99 LT Blazer, 119000 miles, self-engages the auto 4x4 or 4 Hi. Today I replaced the 4-button switch on the dash. After driving just a few miles it did it again. About 3 years ago I did replace the TCCM due to a similar problem. At over $200 now I hate to just throw money at it. Any other possible solutions would be helpful.

 

First thing I'd check is for a loose connection, or corrosion on the plugs at the TCCM behind the passenger side kick panel. It's quite common for the wires there to become corroded. There's a few threads on here about cleaning the pins and connectors.

 

Been there done all of that. The replacement TCCM had a big glob of silicon grease in it and is still in good condition. I'm thinking could it be a wheel speed sensor???I'll have to dig out the old Clinton's and see if there is wiring diagram for the system.

 

is the Actuator motor actually moving or is the front axle locking in?

 

Yes on both. I mean it locks up tight just like it would if I intentionally pushed either button. Day before yesterday I replaced the transfer case actuator, a real pain. I drove it about 30 minutes and stopped for a couple of minutes at a friends, it did again right on start up. Now what??? Can a code reader pick up a problem like this?

 

Is the 4x4 switch indicating AUTO or 4HI when it does this?
Might be the 4x4 vacuum switch that applies vacuum to the actuator under the battery is switching on its own. Not sure where the switch is on a '99...you will need to research that.

Just a quick thought...

 

Some sensor is telling this system to lock up and it's working perfectly. But I don't want it to engage randomly. The Snap-On scan tool says rear wheel speed sensor shorted and brake light switch is bad. Well, this 4x4 with automatic trans doesn't have rear wheel speed sensors. There is a sensor in the transfer case on the upper right side of the tail housing??? Two years ago, when transmission died, I should have sold this thing for junk. But it's still a nice looking and comfortable little Blazer with lots of miles left in it.

 

you have multiple speed sensors, one on each front wheel, and the transfer case if i remember right has an Input and output shaft sensor.