Ok, so I have been searching the forum like crazy over the past few months trying to diagnose my 4wd problem on my 1997 Blazer.

Here's the info:
-transfer case engages fine
-replaced vacuum switch on TC
-3 button 4wd
-vacuum actuator under battery works fine, used a vacuum pump to check for leakdown and there was none
-cable was removed, lubed up and moves very easily

I still have a whirr then a clunk when in four wheel from the front axle. 4HI and 4LO. It only happens under at least 1/4 throttle.

I have even tried applying vacuum from my hand pump to the actuator to ensure that it has nothing to do with engine vacuum!

The only things that I can see as a possibility now are:

1) The cable has stretched and is not being pulled far enough. How far is it supposed to move anyways?

2)There is something internal going on in the front differential or where the cable pulls the axles together.

I am getting so frustrated!

Please help...

Kevin.

 

I doubt the cable has stretched. It sounds like the cable is gummed up a bit. There is a spring inside the front axle that pulls on the collar to disengage the passenger side prop shaft, but if the cable gets gummed up, it can cause more resistance than the actuator should see and may cause slow engagement. I would look over your cable for kinks, disconnect it from the front axle, then with some penetrating oil, work the cable back and forth until it moves nice and free as far as it can go in either direction.

 

I already took the cable off and did exactly what you are suggesting. The problem persisted...

I should have mentioned that as I drive down in 4WD it constantly does the "whirr and clunk" like it is engaging and kicking out again. Speed doesn't seem to affect it, it will happen going really slow or really fast. It won't do it under light throttle application, but starts at about 1/4 to 1/2 throttle, and it isn't vacuum related.

 

Could the prop shaft collar be damaged? Preventing full engagement? Is there a diagram of exactly how the collar locks in?

 

It is quite possible that it is damaged. You have to remove the passenger side axle tube to get to it.

I would check the vacuum at the actuator with a vacuum gauge first to make sure that it is very close to full manifold vacuum when the 4wd is engaged. If it is not, you may have a leaking line that is just not pulling the actuator properly. You can also take the vacuum gauge, push the actuator all the way in, then connect the vacuum gauge. If the gauge shows a drop in vacuum, the actuator has a leak. With the design of the vacuum system, your vacuum pressure should not be very dependent on the operating conditions of the motor. The vacuum reservoir will even out the changes in vacuum pressure.