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.

 

The axle actuator should NOT have vacuum in 2WD. The "clunking" might be from the axle clutch not fully engaging, (parts 9 thru 23) or worn a axle bearing, (part #4) in the image below:




If the axle has been engaged, or partially engaged, for an extended period of time, it will cause premature wear. The vacuum switch could very well be the root cause.

 

Thank you very much for the info.....I have a feeling that diagram is going to be very useful in the near future. Although i'm still unsure at this point.

The vacuum actuator hose had a hole burned in it until i recently replaced it, so i'm thinking that it wasn't under constant vacuum for an extended period of time. I did lift the front wheels off the ground with a jack and by hand I compressed the actuator to engage the front axle. The wheel spun freely by itself before engaging it.....after I engaged it, spinning the front wheel caused the wheel on the opposite side to spin in the opposite direction....which is correct I think.

So I'm unsure what to check next. If anyone has any ideas.....I'm open to suggestions. I ordered a new vacuum actuator switch, and the switch on the front axle just to be sure they are working and to eliminate those as possibilities.

 

Your manual test for engaging the axle was a good one! It appears that the actuator is capable of engaging the axle, (wheels turning opposite direction is normal). Next test is make sure the actuator only has vacuum when 4WD is selected, and no vacuum when 2WD is selected, (engine at idle for both tests).

If there's a problem with a bearing inside the front drive axle, some disassembly will be required to locate it.