4 wheel drive whines and bangs

On my 2000 Jimmy, when I press the 4WD button, I hear the clicking and I feel it shift. REALLY feel it shift! Feels likes I've got the brakes on. Vehicle starts to slow down slightly and there is a low whine. When I press the 2WD button. There is a loud Bang or thump :eek: and it goes back to 2WD. My daughtere recently had a ball joint replaced by a shade tree mechanic. I switched vehicles with my daughter so not sure how it has been cared for. She did take it to the dealer last year due to a loud...REALLY loud siren like whine from the 4WD. That cost $800+. Winter is coming so would like to get the 4WD working correctly. Thanks all for your help.

I had the same problem in my 98 blazer, the mechanic said the front diffrential was a miss. He said it needed a to be switched out but as an alternative he said they could try replacing the megnents in there. So they did and it works better now but still the same noises are going on. But it hasnt left me stranded yet but the loud clunk and stuff kinda makes you nervous if its really popping out of gear. Like mine wouldnt come out of 4wd until like a took a corner or something. it was like being fully disengaged

My 97 Jimmy is doing something similiar. It seems to be vacuum related. At about 70mph, if I excelerate to get around someone, ok, but when I let off, the noise starts. Continues until I press 4H. Then when I go back to 2H its fine. Also, If I turn the vehicle off,,, then back on again a minute later and pull away, it makes the same noise... same solution stops it. I figure there is a vacuum control on the front axle some where thats not holding due to a vacuum leak some where,,,, not sure yet.... still looking for a solution.

My 97 Jimmy is doing something similiar. It seems to be vacuum related. At about 70mph, if I excelerate to get around someone, ok, but when I let off, the noise starts. Continues until I press 4H. Then when I go back to 2H its fine. Also, If I turn the vehicle off,,, then back on again a minute later and pull away, it makes the same noise... same solution stops it. I figure there is a vacuum control on the front axle some where thats not holding due to a vacuum leak some where,,,, not sure yet.... still looking for a solution.

I have a 2000 and experienced similar problems. Have the vacuum switch taken out on top of the transfer case and see if it is stuck. It is relatively inexpensive, but can cause major problems if it sticks.

My 97 Jimmy is doing something similiar. It seems to be vacuum related. At about 70mph, if I excelerate to get around someone, ok, but when I let off, the noise starts. Continues until I press 4H. Then when I go back to 2H its fine. Also, If I turn the vehicle off,,, then back on again a minute later and pull away, it makes the same noise... same solution stops it. I figure there is a vacuum control on the front axle some where thats not holding due to a vacuum leak some where,,,, not sure yet.... still looking for a solution.
Check all the vacuum lines you have one running to under your battery box where an actuator is that locks and unlocks your front axle then they run back to the transfer case. You need to check these for dry rot or holes and check the diaphragm in the actuator to see if it is busted.

Bad vacuum switch, top left side of transfer case

thanks..... that was it J

thanks..... that was it J

Hi all! My 1996 Jimmy exbits the same behavior. Wines under load (not in park or neutral) 4wheel engages fine but when go back into 2 wheel drive, 4x will not disengage right away but after a munute or so it does with a very loud thump. Hey flipper, all you did was replace the check your vacumm hoses and replace the vacumm switch and both issues were solved? I would be soooo happy if the same solves my issue. Thanks in advance for the feedback.

Yeah, this, like the person above, is usually caused by poor vacuum control by the actual vacuum lines, the switch on the transfer case, or the actuator under the battery in the front. The front diff is not totally disengaging, and thats where you get the whinning and slow disengagement, until you get the bang, and its out.

You need to check and make sure your getting good vacuum to the switch, if that checks out, make sure the switch is working properly, you have to take the switch out to take a good look at it. If that checks out, you have to take a look at the vacuum lines to the actuator up front, and lastly the actuator itself. If you notice tranny fluid in the vacuum lines, its because of a bad vacuum switch and the fluid is causing poor control of the actuator, and not fully disengaging the diff up front. If this is the problem, you have to get a new switch, and clean or replace the vacuum lines to get the fluid out of there, to make the system work properly.

Hope this helps!

Also everyone, you have to run 4 of the exact same tires on these trucks, never replace two, always replace in sets of fours.

Well, I replaced the item below the battery tray, the dealer calles it a servo (actuator) but it diddn't make a diffrence. I have the part number for anyone who needs it. The whinning noise happens in both 2 and 4 wd. I am getting vaccumm to the actuator. A line runs from the firewall, then tees off, one side to a electronic plug positioned above the battery, the other runs to the actuator. Its a small diameter hose which is getting vacumm, how much I have no idea but when it reaches the actuator, there is a fitting which takes the smaller diameter hose to a larger one which fits into the actuator. Is there any way to bench test the switch on the TC. I assume it is the item on the other end of the cable running from the actuator. Thanks for the speedy response, blazes. Im still hoping I can (you) get this issue figured out. I am 110.00 in the hole but nowhere near what a transmission shop would charge!

Yeah, this, like the person above, is usually caused by poor vacuum control by the actual vacuum lines, the switch on the transfer case, or the actuator under the battery in the front. The front diff is not totally disengaging, and thats where you get the whinning and slow disengagement, until you get the bang, and its out.

You need to check and make sure your getting good vacuum to the switch, if that checks out, make sure the switch is working properly, you have to take the switch out to take a good look at it. If that checks out, you have to take a look at the vacuum lines to the actuator up front, and lastly the actuator itself. If you notice tranny fluid in the vacuum lines, its because of a bad vacuum switch and the fluid is causing poor control of the actuator, and not fully disengaging the diff up front. If this is the problem, you have to get a new switch, and clean or replace the vacuum lines to get the fluid out of there, to make the system work properly.

Hope this helps!

Bad vacuum switch, top left side of transfer case




yeah ok genius, you mind telling the rest of us how you replaced that nightmare other than taking it to a mechanic??

Bad vacuum switch, top left side of transfer case




yeah ok genius, you mind telling the rest of us how you replaced that nightmare other than taking it to a mechanic??

Please refrain from posting in a thread 3 years old, If you have a question about the vacuum switch start a new thread related to your problem.

Bad vacuum switch, top left side of transfer case




yeah ok genius, you mind telling the rest of us how you replaced that nightmare other than taking it to a mechanic??

This thread is old, closer to 2 1/2 years, but still. If you need help you should start your own thread.
That being said, Its opened up pretty nice if you take out the front drive-shaft. Its also a good time to change out the front shaft U-joints if you pull it for something like this.

Bad vacuum switch, top left side of transfer case




yeah ok genius, you mind telling the rest of us how you replaced that nightmare other than taking it to a mechanic??

You do it by reaching above the transfer case and feeling for it Genius.

i've been cruising this forum for the last THREE MONTHS looking for what might be going on with my S10. i found a thread, although it might be an OLD THREAD, that has the same systematic issues as i have been having.


i changed out the vacuum actuator under the battery tray(great time that was) and now thanks to duke350, i will attempt to replace the vacuum switch on the transfer case in the morning. i wasnt sure if i had to remove the front driveshaft or not. the more i looked under there the more i thought thats what i had to do but wasnt sure. sorry for dredging up a canker sore.