I have a 99 Blazer ZR2. It has the 4 button switch for the 4wd functions- Auto 4wd, 2 Hi, 4 Hi, and 4 Lo. I just replaced the electronic shift motor that is on the transfer case due to salt corrosion and it seems to be functioning correctly now. No blinking lights and it does shift to all the positions with no problems, except that when in 4wd I hear a constant clunking noise that seems to come from the front end.

I have inspected the vacuum lines and replaced the one that runs to the vacuum actuator underneath the battery....it had a hole burned in it, and was replaced before the electronic shift motor was installed. However, I noticed that the vacuum actuator has constant vacuum to it as soon as the engine starts, no matter what position the dash switch is in it always has constant vacuum.

From what i've read on these forums that means the vacuum switch is stuck in the open position. If that is so....shouldn't the 4wd drive still work when i switch it in to 4wd? Instead I hear a clunking noise that seems to come from the front end as if it isn't quite engaging all the way. The noise stops when in 2 Hi.

Any direction or info anyone could give me on what to check from here would be greatly appreciated.