Ok, so I have a 2000 Blazer with a three button (233 T/C?) 4WD switch. Over the last few days it has shifted into 4WD HI all by it self about 6 times. I usually don't notice till I make a turn and it binds. One time the 4HI light was on and the 4LO light was blinking. I stop, shift the trans to neutral, push the 2WD button and everything is fine.

Any suggestions appreciated. I have a code reader for the engine, will that pick up any TCCM or other driveline codes?

Cheers,
ski

 

Most likely the vacuum switch on the transfer case is leaking. Vacuum can leak past the O rings in the switch and apply manifold vacuum to the front axle actuator under the battery tray. This will lock the right front axle to the front differential and you're in 4WD HI. Cost of the switch is under $20 at most auto parts stores. Takes about 10 minutes to replace it. The switch is located on the driver side, slightly above and inboard of the encoder motor. It has 3 vacuum ports, visible in this image:

 

what cap said. also something to try, clean the electrical connection at the transfer case with some connection cleaner.
I had a problem with my jimmy where the 4hi light on the dash would come on randomally (i am not sure if 4wd was on or not as I noticed the light) and sometimes it would work when I needed, sometimes not, I cleaned the connection, and it works perfect now.

 

[QUOTE=Captain Hook;598728]Most likely the vacuum switch on the transfer case is leaking. Vacuum can leak past the O rings in the switch and apply manifold vacuum to the front axle actuator under the battery tray. This will lock the right front axle to the front differential and you're in 4WD HI. Cost of the switch is under $20 at most auto parts stores. Takes about 10 minutes to replace it. The switch is located on the driver side, slightly above and inboard of the encoder motor. It has 3 vacuum ports, visible in this image:

Ok, got it on the lift tonight with the part in hand from NAPA. The vacuum lines leaked some water when the connection was removed (3 vacuum lines). So, we removed the switch, lowered the vehicle, started it up and cycled the dash switch to hopefully purge water from the vacuum lines. Then we hooked up the switch to the lines and drove a bit on gravel to see about the 4WD. We could hear the 4WD engaging into lo and hi by the dashboard switch. And it was back to 2WD just fine. So, we thought everything was fixed, but 1/2 mile down the road after replacing the switch the darn thing tried to switch into 4LO all by itself at 40-45mph (it didn't go, but switched to 4HI and the LO light was blinking again). Is there any chance that the vacuum lines could be hooked on improperly (like to the wrong port of the switch? Like is there a position indicator for the 3 vacuum lines?)
I guess I'm going to try to remove the electrical connection to the T/C and clean contacts like the other comment says next. And also check to see if there's any codes again...
Another question, could you link me to a post regarding the vacuum operator underneath the battery tray? Maybe I need to replace that, or blow out the vacuum lines or replace those lines or something...
Thanks for your quick reply to my original post, the picture was really helpful because it's kind of hard to reach when it's in the vehicle. One of the technicians at my shop walked by and he used to work at the parts shop and said that he sold plenty of those switches.
Cheers, ski9600

 

Update, tonight we got out the mightyvac and diagnosed the vacuum system. There's a leak somewhere between the vacuum switch on the transfer case and the actuator under the battery tray. The switch that I purchased was from Napa (Echlin). I think that I have the same problem that I saw someone else had where the nipples on the switch are too small and it's leaking right there. That person had purchased the Dorman brand, and I suspect that the Napa one is the same. My mechanic put it in a loop situation right now so that the front axle is engaged all the time (which is nice since it's been snowing a lot here 93" in January and over 50" so far in February, lol) and told me to order the genuine GM switch and vacuum line from the switch to the actuator under the battery tray.

 

GM switch isn't a bad idea, but the line you can get at any auto parts store a lot cheaper than the dealer. Just bring the switch along and find vacuum line that fits it.

 

Thanks, I'll check with the dealer to see what service options there are. We were thinking that we could get the line as an assembly, but otherwise I'd like to replace the three way fitting at the transfer case switch.

 

The connector at the vacuum switch is keyed so it will only go on the switch one way. Seems like I remember that it has plastic unions, about an inch long, that stick into the rubber connector. Been a while since I had one apart though, might want to check. If that's the case, you can get bulk tubing cheap. Pretty sure it's 7/32" diameter.

 

Yeah, so I talked to my local Chevrolet dealer and gave him the VIN and he came up with the same GM part number that I did and said to just buy a roll of vacuum hose because that's what they do instead of replacing the whole assy (GM doesn't sell it that way, and he can't find the "hose end fitting" that matches the switch). Anyway, we came up with the GM #89059420 for the switch which is available online from a dealer in Houston for about $27 plus shipping or from my dealer for $60 plus local tax. When I cross that part number on napaprolink.com I get three different parts. One of those is an Echlin part which is silver in color (not Dorman?). Sorry to reference Echlin in a bad way above, but the one I bought previously is in the Napa NOE line instead and brass colored like the Dorman ones I see on Amazon (and much cheaper, there's two that are around $19/each). So, the Echlin one is $33 and in stock (ECH FWD18). Has anyone here tried #ECH FWD18 from Napa?
Maybe I'll try that since it's easy to get and ask if they have a vacuum fitting available to fit it.
If they don't have the fitting and it's worn out, I guess we'll just make the 7/32" hose work (might have to trim the sidewalls of the hose to get three of them on the switch).

 

I guess that I never updated this post. Sorry, and thanks for all of the help with my Blazer which is still running great. Anyway, my experience with the vacuum switch was exactly like Captain Hook predicted (Thanks!). I replaced it with the one from Napa, and it was still doing the same thing. Then I replaced it with the Dorman switch and it worked fine.