'98 Blazer 4x4 "BANG" on disengagement.

Been on this problem for a while now and thanks to the posts in this forum I think I'm making some progress. Here's the problem: 4 WD hi makes a loud noise and "thumps" the vehicle pretty hard when the front diff disengages. ('98 S10 Blazer, 4.3, standard, dual range x'fer case with buttons, about 145,000 mi.) Sometimes, if I do some reverse-forward-reverse-forward moves it disengages w/o noise.

The encoder motor apparently is working. Light sequence is correct, unit seems to work. Checked the vac actuator (under battery) and see it move. Powered it with a hand pump and 5 to 8 "Hg seems to do the trick. Changed the oil in the front diff (no visible chips).

I'm wondering about the sequence of the system and the time involved between pressing the 2 Hi button and the extension of the actuator. How fast should the actuator travel and how soon after pressing the button should the actuator disengage the diff? The encoder motor seems to act quickly but the vac actuator sometimes takes a minute or so to "de energize" when the vehicle is on the jack stands. Shorter time to disengage when on the road (with the bang) maybe 10 - 30 seconds. When the vehicle was up, the actuator takes about a second to go from retracted to extended once the vac signal is discontinued. It's to the point where I'm getting pretty confusticated. Could the vac switch be leaking or dragging causing a slow or weak actuator problem?

It's the wifes vehicle and the problem showed up last winter. Of course I must have screwed it up when I installed new calipers, disks, pads, park brake shoes, tie rod ends, and ider arm - you know how that goes. Any help will be GREATLY appreciated, I have to get over to my K2500 and dope out the low speed ABS problem.

Thanks in advance,
Tom

Disengage without having your foot on the throttle.

BlazerLT,
It seems like it won't disengage without a little power applied. I actually coasted to a stop from 35 mph a couple of times w/o disengagement. This is a new event. She (and I) have put over 100K on this beauty without anything of this nature happening.

T

For the ABS on your truck, you may want to read this:
http://www.automotiveforums.com/vbulletin/showthread.php?t=425516 (http://www.automotiveforums.com/vbulletin/showthread.php?t=425516)

As for your blazer... Are all your tires the same size? Are some of them more worn than others?

blazee,
I followed the posts on the ABS for a while now and already printed the info out. I should be OK there, the sensors are probably good. I just put new rotors and pads and one clliper on that one and I KNOW I didn't clean under the sensors well enough. Rushing as usual. I'll put a meter on them and determine which one has the lower output and tackle that side first. Those 8 lug units are a pain in the butt.

The tires on the Blazer are great. I have to keep that vehicle in real good shape, it has to run about 75 miles a day. Now I have to get the 4wd working right - before the cold weather gets here.

BlazerLT,
It seems like it won't disengage without a little power applied. I actually coasted to a stop from 35 mph a couple of times w/o disengagement. This is a new event. She (and I) have put over 100K on this beauty without anything of this nature happening.

T

Sounds like a vacuum leak going to the vacuum actuator.

Get under there and replace the vacuum line going to it.

Sounds like a vacuum leak going to the vacuum actuator.

Get under there and replace the vacuum line going to it.


I had the same thing just happen to mine, the bang that is.

Just had the Controll Moduale, what ever that is, replaced becuase it wasnt engaging and disengaging properly. Can you tell me were exactly the vacuum lines are, and is this something that a Tech looking into the reason is would or would not disengage at 35 MPH should have checked in addition to replacing the whole control module?

You mentioned the tires are great, but are they all the same tread depth? Had the same problem after a tire was replaced due to a blowout. 6mm on 3 of the remaining tires and 12mm on the new one was enough to prevent proper disengagement, and when it did disenage it came out with a bang.

Here's the latest (now that it's not as hot out):

Vac lines seem OK, VISUALLY. Put in a new vac switch - still the same.

With the vehicle on the jack stands, I had her start it up and put it in 4 wd hi. While under there I could here what I assume is the clutch gear engaging. When put back into 2wd I could hear the disengagement. The diff is not engaged when in 2wd as could be seen when rotating a wheel - the other does not move. When in 4wd I can see the diff working when I rotate a wheel. If I maintain a slight amount of force on a wheel in 4wd, and have her shift back to 2wd, I don't hear the disengagement until I release the wheel and maybe move it a bit in the opposite direction. The actuator does move to the end of its travel.

Do you still think there's a vac problem or could ther be a problem with the fork or spring?

Thanks again,
T

Your encoder motor in the transfer case is probably weak.

BlazerLT,
This thing is really startin' to bug me. The encoder motor seems to me to be alright. It shifts thru the ranges correctly with no strange indications on the lights. As the transfer case switches into 4hi or low the vac switch is signalling the actuator to engage the front diff. The front diff works, I see the cable pull to max. and the front diff engages and works.

When shifted to 2hi the transfer case shifts and the actuator returns to normal. It's as though the mechanism that does the disengaging of the diff is dragging when released. Do you know of any web sites that have a GOOD diagram of my front diff?

T

It is not the front diff that is actuated, it is the encoder motor that removes the drive front the front drive through the transfer case.

The fron differential is just like the rear and operates in the same manner.

It is not the front diff that is actuated, it is the encoder motor that removes the drive front the front drive through the transfer case.

The fron differential is just like the rear and operates in the same manner.

The transfer case only changes between low and high range. 2wd and 4wd are controlled by the front diff.

The transfer case only changes between low and high range. 2wd and 4wd are controlled by the front diff.

No, the transfer case controls the drive to the front wheels.

No, the transfer case controls the drive to the front wheels.
No, to switch between 2wd and 4wd, the front axle is engaged at the front diff. The transfer case switches between low and high range.

No, to switch between 2wd and 4wd, the front axle is engaged at the front diff. The transfer case switches between low and high range.

The transfer case is what supplies the power to the front wheels.

Actuating the front diff doesn't supply the front wheels with power, the transfer case does.

The transfer case is what supplies the power to the front wheels.

Actuating the front diff doesn't supply the front wheels with power, the transfer case does.
The transfer case supplies power to the front diff. It always supplies power to the front diff, it doesn't matter if the vehicle is in 2wd or 4wd.

The front diff engages to supply power to the front wheels.

Here's my diagram of the front diff. Anyone have any ideas? Part nomenclature is at http://home.comcast.net/~firepaw/blazer.html/frdiff.doc

http://home.comcast.net/~firepaw/blazer.html/frdiff2.JPG

I hope you don't mind....I blew up the picture and added the nomenclature to it.

http://img4.imgspot.com/u/05/226/18/FrontDiff.jpg

No, to switch between 2wd and 4wd, the front axle is engaged at the front diff. The transfer case switches between low and high range.

OK GREAT GUYS> SO WHAT IS THE F'ING BANG ALL ABOUT???????
Had TCC Motor Changed, solved some problems but their is still a F ing Bang. Spider Gear, worn out Shaft? What is the Bang? To be more specific, when it disengages it is a bang that feels like you just hit a Pot Hole With the Right Front Wheel.

WHAT IS THE BANG. Please do not responde with how something goes into or out of gear if its the front or rear dif, or if its just 2wd 4wd.

JUST TELL US WHAT THE FING BANG IS. Can you do that, Thank you.

Did you check the tread depths on all your tires? Any variation from even one tire will cause the symptom you are having.

Did you check the tread depths on all your tires? Any variation from even one tire will cause the symptom you are having.


Have 2 new ones in Front and 2 older ones in back. ASked a tech about that and he said it si only an issue if they are differnt size, and that thread depth will not afftect it that much. Areyou sure you dont work for Goodyear? Just kidding.

Honestly, this could be the issue.

With the backs being different than the fronts, this could cause the clunking and the lack of immediate disengagement.

On a 4x4 you replace all tires, not just two.

I ditto that sentiment - all tires must match in size, wear level and tread patterns for there to be trouble free 4x4 operation. Oh, and rotate them every 6k Miles, when you change your oil and filter so that they continue to wear relatively evenly. When it's time to get new skins - get *all 4* replaced at the same time.

OK GREAT GUYS> SO WHAT IS THE F'ING BANG ALL ABOUT???????
Had TCC Motor Changed, solved some problems but their is still a F ing Bang. Spider Gear, worn out Shaft? What is the Bang? To be more specific, when it disengages it is a bang that feels like you just hit a Pot Hole With the Right Front Wheel.

WHAT IS THE BANG. Please do not responde with how something goes into or out of gear if its the front or rear dif, or if its just 2wd 4wd.

JUST TELL US WHAT THE FING BANG IS. Can you do that, Thank you.
If you would pull your f ing head out of your f ing ass and read the f ing thread, you will see that I posted the most common f ing cause in f ing post # 4 and Swalt followed it up with more f ing details in f ing post # 8. From what we f ing posted you should be able to f ing realize that if you have two new f ing tires on the f ing front and two raggedy ass f ing tires on the f ing back... that is probably causing your f ing bang. Try not to be such a tool. Can you do that, Thank you. f head!

If you would pull your f ing head out of your f ing ass and read the f ing thread, you will see that I posted the most common f ing cause in f ing post # 4 and Swalt followed it up with more f ing details in f ing post # 8. From what we f ing posted you should be able to f ing realize that if you have two new f ing tires on the f ing front and two raggedy ass f ing tires on the f ing back... that is probably causing your f ing bang. Try not to be such a tool. Can you do that, Thank you. f head!

So, are you saying that I should look into replacing the 2 back tires as well? Just kidding, I saw your advice and thought that you guys were full of shit, still do to a point, well maybe not full of shit as much as just a little purest. Anyway I will try them, Sam I am. And if it doens't work which I F ing dont think it will. I will be forced to abuse you till you cry like Nancy Fucking Kerrigan, BIATCH>

Stay Tuned. This is just too funny;l

What's really funny is watching the center differential on a Subaru bind up and burn out when someone with the same line of thought as yours comes along. You can't just swap one tire for new on any axle on 4x4/AWD cars and trucks. I blew a tire on my Scooby a few years back and got the one tire replaced. Same type and tread as the others. However, since it was of a larger diameter (because it was newer) she was binding and making the car lurch in low-speed, parking lot like turns, or sharp right hand turns to launch down a side-street. The solution was to take the car to a specialty tire shop that had a machine that would *lathe* the new tire down to match the diameter of the other tires. The binding/chatter going around a turn disappeared. We're all seriously not making this sh*t up my friend - funny or otherwise. :2cents:

What Dies Everyone Else Think

Now I need a little more help. If I bend over, can a couple of you guys give me a hand pulling my head outta my butt. I went down and put 4 new tires on the beast today - problem gone. The old ones looked plenty good to me but I DID NOT measure tread depth. Hmmmm.

There is however, still a couple of questions bothering me that someone may be able to clear up. When you switch out of 4hi what exactly is the sequence of operation of the x'fer case and front diff? Does the X'fer case vac switch get actuated after disengagement of the x'fer case or does everything happen simultaneously. I can't seem to understand how there can be a load on the front diff gears when the input is decoupled.

Anyhow, thanks a ton for the help and the laughs, I appreciate both greatly.

Tom

Now I need a little more help. If I bend over, can a couple of you guys give me a hand pulling my head outta my butt. I went down and put 4 new tires on the beast today - problem gone. The old ones looked plenty good to me but I DID NOT measure tread depth. Hmmmm.

There is however, still a couple of questions bothering me that someone may be able to clear up. When you switch out of 4hi what exactly is the sequence of operation of the x'fer case and front diff? Does the X'fer case vac switch get actuated after disengagement of the x'fer case or does everything happen simultaneously. I can't seem to understand how there can be a load on the front diff gears when the input is decoupled.

Anyhow, thanks a ton for the help and the laughs, I appreciate both greatly.

Tom

Glad to have helped you.

The front end loads up because you have a differential up there that does not allow for front wheel slippage. You will note this as wheel hop when you say, turn into a parking space and the 4x4 is on when snowing out. Even if the transfer case disengaged, if the front axle shift fork hasn't decoupled the passengerside wheel yet, you'll still get a draggity-hop from the inside tire whenin a tight turn, there's really nothing in there to allow for inside/outside wheel speed differential that occurs in a turn, unlike your rear diffy that's got a full clutch and spider gear set. The frnt axle locks right up like a rigid Dana Jeep Axle on a CJ-5. ;)

Oh, and the best apologies always come when you're able to stand up tall and straight and breathe for the first time. It gets crowded in in there! :D Glad you have it fixed dude...oh, and for being such a hardnose about it? We told you so! *laugh*

What's really funny is watching the center differential on a Subaru bind up and burn out when someone with the same line of thought as yours comes along. You can't just swap one tire for new on any axle on 4x4/AWD cars and trucks. I blew a tire on my Scooby a few years back and got the one tire replaced. Same type and tread as the others. However, since it was of a larger diameter (because it was newer) she was binding and making the car lurch in low-speed, parking lot like turns, or sharp right hand turns to launch down a side-street. The solution was to take the car to a specialty tire shop that had a machine that would *lathe* the new tire down to match the diameter of the other tires. The binding/chatter going around a turn disappeared. We're all seriously not making this sh*t up my friend - funny or otherwise. :2cents:

OK So smack my ass and call me Sally, it fucking worked. I can not believe it. It goes in and out of 4WD Smoothly, no bang, and it is much quiter in 4WD as well. I apologize for my skeptisicm (and my spelling), Somebody should tell these F ing mechanics to take a refreseher course, becuase none of them knew it.

OK So smack my ass and call me Sally, it fucking worked. I can not believe it. It goes in and out of 4WD Smoothly, no bang, and it is much quiter in 4WD as well. I apologize for my skeptisicm (and my spelling), Somebody should tell these F ing mechanics to take a refreseher course, becuase none of them knew it.

*SMACK*

Take that Sally. :lol: