'98 Blazer 4x4 "BANG" on disengagement.

[firepaw]

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

[BlazerLT]

Disengage without having your foot on the throttle.

[firepaw]

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

[blazee]

For the ABS on your truck, you may want to read this:
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?

[firepaw]

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]

Originally Posted by firepaw

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.

[chigger23]

Originally Posted by BlazerLT

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?

[swalt]

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.

[firepaw]

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

[BlazerLT]

Your encoder motor in the transfer case is probably weak.

[firepaw]

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

[BlazerLT]

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.

[blazee]

Originally Posted by BlazerLT

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.

[BlazerLT]

Originally Posted by blazee

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.

[blazee]

Originally Posted by BlazerLT

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.