4x4 Engages by Itself

Any help??

While driving at approx 45 to 50 miles per hour you begin to hear the 4x4 beginning to engage. If you slow the sound goes away or if you engage the the 4x4 hi by engaging the switch the noise goes away but comes back shortly. Transfer case switch has been replaced and the noise was greatly dimminished. (Was a low grinding sound in the left front wheel) any direction would be appreciated.

VMI07

What transfer case and what year?
What are the shift indicator lights doing?
MT

I have been reading your reply to others with similar problems. Your input has help me significantly Thanks,

My transfer case is 233 either C or G.

As for my dash lights in 2 wheel mode no lights are on. In 4 wheel HI the 4HI light is on steady and the axle is engaged. I have not tested 4 LO.

My problem is slightly difference than others. While driving in 2 wheel mode approx 45 - 50 MPH the front axle begins to whine. Sound seems to be coming from the front right. If I push the 4 wheel HI button the noise goes away and the 4Wheel HI light comes on steady. When switched off the noise slowly returns.
Seems to me the 4 wheel is trying to engage but never gets there. I have replaced the vacuum switch on top of the transfer case. Help the noise diminish greatly both in sound and frequency. I used to have to slow to 5 MPH before the noise would stop.

Do you think I have a vacuum leak somewhere else? Without a gauge is there a test I can perform.

Question. If there is no vacuum to the cable actuator under the battery tray is the cable in the engage mode 4wheel or 2 wheel, meaning the cable engages the 4 wheel axle only in the presence of vacuum. If so and my axle does engage does this mean vacuum or the actuator is not the problem..

In 2 wheel drive the dash light should come on indicating the transfer case is in 2 W position.
And front axel should be unlocked.
Actuator not pulling the shift cable .
Ground pin 13 on diagnostic connector and see if any codes flash a code on shift buttons.


It realy sounds like transfer case is not shifting all of the way into 2W drive.

Does it have a good 4W High to 4W low shift?
Can you hear a good shift from 4W high to 2 W drive?
MT

As long as I owned the car The 2-wheel drive button light has not worked. Didn't think it was designed to light. When you first turn on the ignition the 4 HI and 4LO buttons illuminate but not the 2 wheel drive. Bulb? I will check tonight.

As for the solid shift sound, 2-wheel to 4-wheel and back is smooth with a slight clunk. 2-wheel to 4LO and back is more pronounced clunk. 4 HI to 4LO and back is a also a solid clunk.

I have moved the car in all drive modes. The transfer case is disengaging the front drive shaft when shifting out of 4-wheel drive mode. Obviously can't tell at high way speeds.

If I find pin 13 where can I get information on diagnostics. I assume the lights, 2-wheel 4HI and 4LO will flash and also to get the proper diag all light will have to work.

Ever used the reapir bearings designed for slightly worn rear axles? Good idea don't know how durable they are..

This Blazer has 165,000 miles. Body in good shape. If I can get these gremlens fixed I'll have reliable transporation.

Thanks for the help.

VMI07

2W button light should light up in two 2W mode.
Also all 3 buttons should come on for bulb check when key is first turned on.
Ground pin 13 and see if any lights flash and post back the number of flashes.

If front axel grinds going down the road it is trying to shift in or lock up.

http://www.automotiveforums.com/vbulletin/showthread.php?t=572914

ok fixed all bulbs. During bulb check all bulbs illuminate.

Grounded pin 13, no flashes. ( Key on grounded 13 no flashes)

While driving car 2-Wheel drive bulb stays constant and lit at all speeds.

If the natural position of the vacuum actuator and cable is disengage and I disconnect the vacuum system and plug the lines the 4 wheel drive system will not work but the sound should go away.. Your thoughts. Not a fix but I have localized the problem.....I am leaning towards the axle or the vacuum acuated cable.

VMI07

A quick way to narrow this down is to unhook the actuator cable at the actuator under the battery tray. Push the cable in and go for a drive. If your noise is now gone, your problem lies in the vaccuum /actuator side of the system. if the noise is still there, you have a problem on the differential side of the system.

Check it this way and we'll go from there.

Instead of disconnecting the cable I disconnected and plugged the vacuum line. (Cable connection was rusted.) Noise gone. Not sure what this means but I suspect that vacuum is getting to the accuator at highway speeds. I would suspect the transfer case valve but I have just replaced it. Not a dealer part. Could this be the issue?

For a tempory fix I installed an in-line small motor fuel valve in the actuator vacuum line. This will allow me to use the 4 wheel drive but no more shift on the fly. More like the old days when you had to get out and lock the hubs. Anyway I will need a more permanent fix for the Pennsylvania winters are swiftly approaching. Any suggestions?

Anybody with the same issue, I have read many posts which this seems to be common the same the in-line valve works but its only temporary. Maybe it will stop damage from occuring until a better fix comes along and its only $7 dollars.

If I find it I will post it...

one more thing.. the actuator is working fine, no leaks, Holds a strong vacuum ( easy to test with the valve installed.) problem must be on the vacuum line transfer case side.

VMI07

VMI07

Yup. This is my problem too. I'll be outside tomorrow hunting it down. My problem is that is doesn't happen very often. It happens when driving around 40+mph and feels like it disengages(with a heavy clunk) either when I slow down or hit the 4hi button. The noise stops immediatly when I hit the 4hi and stays gone(until the next occourance) when I put it back into 2wd. I have someone interested in buying this beast but not with the noise. I wouldn't want to see it to someone while it has a problem either. So, I need to fix this fast and cheap. Anyone have any other advise?

FYI update. My issues were solved with the replacement of the transfer case vaccum switch. Can't beat a $32 fix. To check the switch pull the vaccum hose off the top and look for moisture. The moisture is not water and travels into the HVAC system and can screw with those systems as well. When I pulled the switch(which takes a 7/8" deep socket or wrench) the check ball was stuck almost all the way in. The ball on the new switch was out about 1/4". And for the record, after the replacement the switch from 2 to 4 and back to 2wd was better than I ever remember.