1992 T4R a/t 4x4 problem

[1992 4runner]

Hi, I'm new and this is my first T4R and post. I have a 1992 T4R with 72,000 original miles. Everything works perfect on this baby except that the 4x4 tries to randomly engage as you drive down the road. Sometimes it stops trying by itself and sometime you can slow down to about 25-30 mph and it will 'bump' hard and quit. It makes a very noticeable noise when it starts trying to engage. The guy I bought it from said he thought it was the vacuum actuator and had a shade tree mechanic replace it with a used one but it still does the same thing. Anyone have any ideas? Thanks in advance.

 

[fourwd1]

The vacuum actuator is controlled by 2 VSVs which are controlled by a relay that is energized when you shift into 4HI or 4 LO. None of those is going to do something on its own to lock the front axle into 4WD.
However, if your shifter is actually in 4HI and you don't know it, an onteittent part could randomly start working and lock the front axle, which would then make the 4X4 light on the dash light.

 

[1992 4runner]

Thanks for responding so fast! It is in 2H for sure. When I shift to 4H or 4L the light come on in the dash. When I shift back to 2H it goes off. When it sounds like it's trying to engage while driving, no light comes on. If I slow down to about 25-30 mph you can hear and feel something 'catch' and the noise stops. The only time the light comes on is when you shift into 4 wheel drive.

 

[fourwd1]

OK, the next question is, are you sure the problem is actually what you think it is (the actuator locking the axle) and not something else?
Try this, make absolutely sure it is unlocked, then remove both vacuum lines from the actuator (mark them so you know which goes where). Then drive around a few days/weeks and see if the "bump" keeps happening.

 

[1992 4runner]

Will try that and let you know.