I have a 95 blazer that I recently noticed a problem with. When I first got the truck and I attempted to engauge 4wd the light would usually flash for a sec then stay lit letting me know it was in which I thought/think is normal. Now it doesn't do that at all. As soon as I press which ever selection it just stays lit and on top of that when I'm done with 4wd hi and I want to switch back to 2wd I'll put it in neutral or park press 2wd, The light will switch from 4whi to 2wd but the truck will still be in 4wd. In order to get it out of 2wd I have to either put it in park and shut the engine off and then start it back up again or while I'm driving usually at about 20mph the front end will thump loudly and it'll feel like I ran over something and that is the 2wd disengauging. Any ideas? What is the propper way with this year truck to go in and out of 2wd, 4whi, and 4wlo?

 

You can shift into or out of 4 HI and 2 HI on the fly...up to 55 or 60 MPH. You have to be in Neutral either stopped or below 5 MPH to shift into or out of 4LO. Now, your problem is usually caused by one of two things:
1. You have different sized tires on the truck..Either front-to-rear or side-to-side, or a low tire.
2. The vacuum switch on the top of the transfer case is sticking.

You should already know if the tires have been messed with so I will assume you have a bad switch. They are about $30.00 at any dealer,sorry, I don't know how much from aftermarket. Like I said they are mounted on top of the driver's side of the t-case, above the encoder motor. It has a 3 port vacuum manifold attatched to the top of it and make sure you orient the thing right when you reinstall the manifold to the new switch, it has a locator hole on one side of it that corresponds to a tab on the switch for proper indexing. You will only need one tool to change it, a 7/8" wrench.

 

Awsome. Thanks for that info. I got another question about this. Under the hood there is a small selanoid that controls the vacume to the diaphram underneath the battery which pulls a cable conected to the front dif. Now I thought that was responsible for engauging the 4wd. How does that selanoid your describing on the transfercase tie into that?

Also I'm certainly not questioning the tires being a possibility because I know that is a common cause but HOW is it? When I press the button for 2wd if it were operating normally does it only switch it when I'm moving or can it switch into 2wd while I'm parked because if it can I wouldn't think any tires being different sizes or pressures would have any difference on the operation. Lastly do you know the correct name of that vacuum switch?

 

That solenoid you're talking about is real close to the battery right? and has a vacuum T in it? and a harness that is routed back the passenger's inner fender accross the firewall and connected to the main harness over by the brake booster? That entire setup was part of a recall done on your truck...#09041 IIRC...lol It is not a solenoid, but a switch that is vacuum controlled...it lets the EBCM (electronic Brake Control Module) know that you have selected 4WD...I can't remember why exactly, but it will not effect your 4WD operation.

The switch on the transfer case is called a t-case vacuum switch as far as I know, and it supplies vacuum to that hose that is connected to the recall switch I mentioned in the last paragraph. (actually it just switches engine vacuum to the front axle actuator(the diaphram under your battery tray that pulls the cable) when 4WD is selected.

As for the tires...I have been specializing in electrical/electronics and HVAC for 6 years now, so I have replaced much of my memory of driveline dynamics with modern electrical system function and such.....lol. What I do remember isthat I had an s10 4x4 come in with a complaint of a loud pop type noise that shook the entire truck violently when you shifted out of 4wd and there was a long disengagement time. I remember it did it every time and it was about fifteen seconds after you pushed the 2HI button....that thing just went BANG when the front axle finally disengaged. The truck had 235/75/15 tires on the back, and 235/70/15 tires on the front. I switched the tires off of another truck (a blazer) and the problems were both gone...completly fixed it! So I then installed the mix-matched tires on the blazer and drove it...the problems were just as evident on the blazer. I sold the guy four new tires and fixed both his problems. That's why I always look at the tires first whenever I get a 4WD engagement concern. Sorry I cant explain exactly why the tires need to be the same size, something to do with the fact that a smaller tire turns faster than a taller tire does and the axles aren't in-phase whith each other so there is a clash that can occur.

Yes you can shift in and out of 2wd and 4HI sitting still, and it will only completely engage/disengage the front axle if the splines just happen to be perfectly lined up. And sitting still I'm sure you don't have the concern about felling like you ran over something...right?

 

To be perfectly honest the more I thought about it I can't remember if I have that selanoid/switch or not. It may have been a different truck I was working on and asosiated it with mine because it was similar. Maybe I just have a vacuum hose going straight to the diaphram under the battery.

Thats right. When I shift back into 2wd while in park or neutral the light goes from the 4hi button to the 2wd button right away but it wont actually be in 2wd unless I either shut off the engine and restart it or drive for like half a min then it will pop out and feel like I ran over a grown man.

I happen to be at autozone yesterday pickin up oil and a filter cause of their $11.99 sale and I asked about that switch. They had one for $15 so I just bought it and right on the back of the package for the switch it says if defective the 2wd light will be on even when in 4wd. I paid close attention yesterday to the transfercase switching motor and it is definately working correctly enguaging and disenguaging when I switch between 2wd and 4hi. I definately think you were right about that switch. It makes pefect sense to me. I wanted to put it in yesterday but we got about a foot of snow here and my apartment complex hasn't plowed out our parking spaces yet.

My tires are all the same size. They're all diferent brands and I haven't perfectly adjusted the tire pressure in a while but none of them appear low enough to change the total diameter off ya know?

 

Yeah, if you don't have a flat you probably just need the switch. But seriously, a low tire will cause this concern...you better make sure they're all close to the same pressure. When you change the switch put your truck in 4HI before you do and when you take it out you will probably see the detent ball on the switch is sticking in. Don't forget to post up when you change it.

 

Thats a good idea although when I take the switch out I could probably check it by pushing the ball in and seeing if it sticks in then also. Now that I remember its been doing this for the whole time I've had the truck so I'm pretty sure it HAS to be that switch. I know I haven't had low tires all 2 years I've had the truck. I'll definately repost when I get a chance to put the switch in. Thats the least I could do for a site thats helped me out quite a bit. Thanks alot man. I apreciate you takin the time to help out a fellow tech. Believe it or not I'm an auto tech also. I recently started workin for a full service shop but for the last 7 years before that I was working at Midas just doing brakes, exh, susp. etc. Don't yet have a lot of experience with much else yet. I'm gettin there though. I feel alot better having a descent understanding of how the 4wd works and doesn't.

 

My Blazer does the exact same thing but turbo expressed the problem MUCH better than I did in my post.

If the problem does lie with the switch, is it safe to drive the truck in 4HI until I get the problem fixed or is it damaging to drive it any further in this condititon,

Thanks for youir help

 

You shouldn't drive excessively on dry pavement in any 4WD mode. It's a $15.00 part...I'd get it fixed today. That's just me though....I know not everyone's a technician.....lol

 

so I changed that vacuum switch today while I had the truck up doing an oil change. I set my tire pressure too. took it for a ride switching it from 4hi to 2wd and the problems still there. so I say well let me look to make sure all the tires are the same size cause thats definately how its acting. ya I got one 235/70R15 on the right rear and 3 other 235/75R15. I have no idea how or when I put that on there but my guess is that was my spare when I bought the truck used and cause it was in better shape than one of the others I swapped it out. I'm gonna fix that the next slow work day I get and I'll repost. I'm sure thats it though. Thanks for the help just the same.