i'm experiencing what appears to be a problem with my '97 blazer's 4wd system. when in 2-high and decelerating from highway speeds (65+ mph) it will sometimes begin to wail and drag like it's in 4wd (think of an air raid siren, the sound is very similiar and very loud). at that point if i keep it moving above a few miles per hour, the 4wd seems to stay engaged. if i slow down far enough that the truck almost stops, the noise and drag disappear and everything seems normal.

also, when purposely switching to 4wd it sometimes will not disengage until i shut off the engine.


edit:
just a side note: i think it's going into 4-low on the highway. i haven't played around too much, but the sound 4-high makes is very similar but not nearly as loud as what happens when it randomly engages.

 

if anyone has any ideas at all, please share. i hate to bump this thread to the top again, but i do 200+ miles per week of highway driving.

 

here's an easier question: if i yank fuse #19 will that disable 4wd for testing purposes?

 

Are you driving in 4WD 65mph+?
If it's in park and engine running, will it fully shift into 4wd? And not blink?

 

i am not purposely driving in 4wd. whatever it's doing, it's doing it by itself upon deceleration. in fact, the 2high light stays lit when it happens, but it sounds and drags like 4wd.

in park with the engine running it will shift into 4high and 4low and not blink (and sometimes will not disengage until the engine is turned off).

 

Parking brake not sticking right?
Will the transmission shift normal in acceleration?
So you do not feel nor hear anything upon accerating? Just trying to narrow down whats going on here.
I wonder if its downshifting too early. Does the tach jump up RPM's when you are off the throttle?

 

the parking brake light isn't on, but i haven't checked the brake to see if it's on.

haven't noticed the transmission shifting abnormally. it seems to shift great.

i do not feel or hear anything upon acceleration, but the moment i let off the accelerator at roughly 65+ mph, it starts to wail like it's in 4wd and will continue to make the noise unless the truck slows down to a few miles per hour at which point the noise and drag vanish (sometimes with a thud/thump).

i'll have to keep an eye on the tach next time it happens, but it doesn't seem to be downshifting early and iirc, it's only at about 2k rpm when it happens.


edit:
i'm probably not describing the problem very well, but i don't know how else to describe it. maybe it's not going into 4low. the noise it makes sounds like the same noise that 4high makes, only much louder. it sounds quite a bit like an air raid siren going from high to low frequencies. it also sounds kind of like road noise from deep tread tires, but if it's road noise, why does it only happen when decelerating from 65+ mph, and why would it then continue to make the noise down to roughly 3mph?

 

here's some new information: if i hit the 4high button while it's making the noise, the 4wd engages and the noise stops. so apparently it's not going into 4wd while decelerating, but the noise has something to do with the 4wd system.

 

and a little more information: it seems to only happen under very specific conditions. based on my testing, it seems i have to be going uphill in third (at roughly 65mph), then let off on the accelerator for it to make the noise. i can't make it happen every time i try, but there's one point on my way to work where it happens almost every drive if i'm going fast enough.

 

is there anything else i can add to this that would help with diagnosis or troubleshooting?


edit:
someone in another forum mentioned that some of the blazers had 'soft' spider gears that would cause a howl over 35mph unless 4wd was engaged. can anyone get a hold of a tsb for this?