I have a 99 GV- automatic with shift on the floor  4wd.  My 4wd light blinks when I put it in 4H or 4L.  I checked the air pump-unhooked the air line at the pump and pluged the pump into my pressure guage only-  6lbs of pressure no leaking at the pump.(4h or 4l---4wd indicator light was steady.)    Hooked up the pump back to the airline, Checked again where the air line plugs into the front diff- 6 lbs- no leaking. (front diff unhooked--this assures me no line leakage before the differential-- also steady 4wd light )   I hooked everything back up as it should be but also  I put a t in the line just before the front diff- to attach my pressure guage.    when it is put in 4h or 4l- pump cycles and pressure goes from 6lb to 5.5 and then back to 6-- (4wd light flashes accordingly)

My thought is that the actuator must have a bad diaphragm---??  or is something else  hokey in the differential

Here is the wierd part.   With the front wheels off of the ground, anytime it is shifted into 4wd the wheels rotate opposite each other--(even when the pump is continually cycling--)  This usually means the actuator is ok-- but I know for a fact there is no leakdown before the diff.

Lastly-- if it is inside the diif-- how hard is it to fix
Any help?
Thanks

Push Button 4wd you mean slec lever?

Perform a vacum check on the actual diff it self. Hook up a vacum tester to the port going to the diff and see if it holds air and check the vent tube near the left of the rad.

I am sorry- It has the shifter on the floor. 

Ok i was going to say when did they come with a push button 4x4 . Check the pumpkin for leaks. I had a problem like that once where the diff was leaking and the pump had to come one a few times off and on and the light flickers.

my 4x4 light began to flash at a rate of about once every 9-10seconds, seemed to be a non issue however over time it began to flash more often, to the point where it seemed to engage and disangage 4x4, this took about 1.5yrs from start to problem, no leaks in the system anywhere it was just the actuator itself that needed to be replaced, no more flashing light now.  just monitor the light and see if the flashing becomes more frequent, if so you might need to replace it..

The air-diff, or as I call, dog feces, is prone to problems.  This is one of them, and unfortunately, I think the frequency of problems like yours will increase over time.  First, I saw pumps just seizing up in the early years of this platform. Then lines began to age, crack then leak.  Then seals and other internals begin to leak, and the little air pump can't keep up with more than the tiniest leak.

If it's the acutator, you got to choices. 

Good: rebuild it with new or used parts (I think I got a box of these parts *somewhere* if you really need).  Prolly best bet for you if you like shift-on-the-fly without having to lock your hubs, of if your having a shop do the work.

Better: Do the air diff elimination.  While not easy or for the faint of heart, it puts the front axle back to they way it was under the Sidekick platform, and opens up gearing and locker applications if thats your thing.  No one has flashing 4x4 lights in a Sidekick unless the shift lever isn't fully engaged.

Hi-

I am going to do the vacuum test on the diff actuator tommorrow.  I have  afeeling this is the problem because the light flashes about once a second.   
I am mechanically inclined--- have access to a hoist-- what is involved in replacing an actuator?
I can not find specific manual pdf's(I have the pdf's of the repair manual however I do not have the free one's  of the UNIT REPAIR which inlcudes the actual diiffertial rebuild)
Anyone have this?

I am assuming that you must have done the full swap out and eliminated he actuator---do you still have one available and/or parts that I would need for this fix/
Thanks in advance

I am assuming that you must have done the full swap out and eliminated he actuator---do you still have one available and/or parts that I would need for this fix/
Thanks in advance

I'll look thru the buckets of parts this week.