Electronic 4WD won't disengage

Old 12-24-06, 12:27 PM
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Electronic 4WD won't disengage

Hi, thanks in advance for any help you can provide.

I have a 1998 Chevy extended cab shortbed, 4WD, 5.7 L, automatic trans Z71 and towing package.

The problem I am having is that when I engage the 4WD, it takes a little time to engage, then it won't disengage the way it should. I made the mistake of not using the 4WD for the entire spring through fall, and now it won't work.

Does anybody have any idea what the problem might be? and how to fix it? I have decent automotive skills, by no means an expert but I do pretty well for an amateur. Is this something I should even attempt, or just take it to a professional?

Thanks again for your help!

Old 12-24-06, 12:35 PM
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Happy Holidays.

I would look for a vacuum or air leak. I believe the 4 wheel drive is controlled either by vacuum pressure or air pressure.
Old 12-24-06, 12:48 PM
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Not sure which you have, but

<Edit - caveat - I have a diesel, yours could be different >

at least thru '96, GM used a "heated gas charged" CAD ( Cental Axle disconnect. ) When the switch was thrown for 4wd, the unit would heat a the 'gas" and it would extend the plunger, engaging the 4wd. This would obviously take a little time to warm the gas, and the colder it was, the longer it took.

Then they came out with the electronic CAD, which immediately extends the plunger. This unit has 3 wires, versus the gas charged units of 2 wires. The trird wire needs to be connected to a Hot in Run wire, usually back by the transmission to the brown wire in a 4 wire connector above the transmission.

Now, assumming you have the 'upgrade', you're not getting the hot in run power that would pull the plunger back and disengage the front axle.
This unit is on the front axle housing, on the passenger's side. It's about a 5 inch long cylinder mounted in front of the right axle housing.

Since your's is taking a little longer to engage now, and not retaracting, the unit might have gone bad. It will unscrew from the housing, with no fluid loss, and you could bench test it to see if it does indeed extend and retract when 12v is applied to the various wires.

It's not that hard to get in and out. Wiring connectors are on the back of the right axle housing, and there is a plug in to the CAD unit that you can undo to remove the unit.

Note that there is a metal slug ( at least on the upgrade kit - which I did on my '96 ) that goes into the hole before the electronic version get's installed.

If the unit test good, then you mave have a wiring 'abnormality'. Check for blown fuses, should be fuse 24 in the under the hood fuse box. Duh - do this first!
Good Luck

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