Another 4x4 problem

Hi all. This is my first post, so forgive me if I sound naive. I have looked around at other posts, and I'm guessing my first line of defense is the vacuum actuator (is that the same as a vacuum switch?). Here is the problem.

I have a 2000 Blazer LT that seems to be possessed. It drives down the road and then all by itself, all the 4wd lights start flashing and it starts engaging/disengaging as I am driving it. It doesn't make for a very smooth ride. At first it was just annoying, but now it occurs all the time and the "Service 4wd" light comes on everytime the system engages". So, in the mean time, I have unplugged the module entirely, so I don't have this problem anymore while I'm driving. I am getting conflicting reports from a couple mechanics I know - one says start with the vacuum switch, the other thinks it's either electrical or something deeper in the vacuum system.

I have seen posts with similar problems but not identical, and I'm certainly not savvy enough to be able to work on the problem myself.

Any help is appreciated...

I have a similar problem that nobody can seem to figure out either. My service 4wd lite comes on after about 2 miles and at first it used to flash and try to switch by itself but now the lite says 2hi and I noticed by parking and turning it seems that when my service 4wd lite comes on it goes into 4 wd by itself and the lite doesn't specify that it still says 2 hi. I also have conflicting diagnosis. I have read the service manual and tried to use the diag. computer and it says nothing so at this point I think I am going to replace my encoder motor on the transfer case but it is 489.00 from chevrolet. I have been told that if it was a vacume problem that 4wd would not engage but to engage itself is a mystery to those I have spoke with. So if you find out please let me know cause right now I don't have the $489.00 just to guess!

try northernautoparts.com for the encoder motor. They are a hell of a lot cheaper.

Check this link http://www.automotiveforums.com/vbulletin/showthread.php?t=471787

Question: Have you taken the battery cables off during a maintenance service
tune up. The codes have to be re-installed. I had a Alternator replaced 2 years
ago, and they forgot to replace the code when they taken the battery cables
off. Once the codes where installed the problem went away. If the HI LO 4x4
engages when your on the highway driving 55mph and over it could damage
your transmission and drive shaft.

I replaced the battery about 8 months before the problem started and the couple of times that I have pondered the problem I have disconnected the cables myself because the gm service manual says you have to do that to work on it cause it is hot all the time. But I have not heard about replacing the codes. And no I don't drive it since it goes into 4wd by itself about a mile and half down the road so I just parked it in the garage and there it sits until I figure out what to do next! I have been told when I replaced the actuator on the front axle (the first thing I did when the problem came up) but apparently there is a fork inside the front axle that can engage by itself if broke which will put it in 4wd on its own but taking on that task of removing the axles just doesn't appeal to me right now! That would be one of those jobs you have to be in the "mood" to do! Thanks for the suggestion on the codes!

Which transfer case? 233 or 236? they have a tag on the back of them. Check tag and check wiring going to transfer case at same time.
A 233 has 3 shift mode buttons on dash 2W -4W high - 4W low.
You can ground diagnostic connector and get codes on it.
On The 236 transfer case can be checked out with a scanner.
If you can get code no. post back code no.
MT-2500

I have the 236 transfer case. Our scanner doesn't do the transfer case, I know of someone who does have a scanner for it but don't want to drive it there then home again in 4wd. Maybe I should wait untill a good rainy day then it won't be so hard on the car.

Thanks!

When you hook up the scanner it may have some codes from being unhooked. You will need to clear codes and then recheck it or drive it and see what codes shouw up.
If you can get codes post back code no.
Also some scanners will run some diagnostic test on it.
I just got the snappy 5.4 cartiage and it will turn a transfer case any way but lose. All kinds of test for it.
Be careful on driving it hooked up untill you get it fixed.
Good luck