99 Tahoe 4wd not engaging.....however.

[res02p35]

Since everything has checked out except the tccm, I will replace that as I see nothing else that could be stopping this from engaging.
The removal of the tccm is straight forward if you get a pair of pliers to pull down on the tccm after removing the two 10mm bolts that hold it in place.
After the bolts are removed grab the bottom and pull hard, there is a bent tab at the top and black tar adhesive stopping the tccm from dropping out. Once it is loose you will be able to remove the 2 connector plugs.
There was no need to pull the dash, lower dash or gauge cluster as that reveals nothing.

Here are a few pics of the tccm and connectors.
I will order another tccm and update when replaced. Figured this would be easier and cheaper than pulling the diff.

 

[res02p35]

BTW, those two wires appear to go to a missing abs unit that should mount on that bracket.

 

[Eman85]

Yes the front driveshaft turns when 4wd is engaged.

I took it off the stands, put it in 4wd, drove it then jacked up the front.
The wheels and shaft turn by hand, but they are not turning together. Something in the diff must not be engaging.
Not sure how the stub shafts get engaged.

The right side axle has the actuator. The actuator moves a fork that has a slider that engages the right axle. If you say the actuator is good then it's in that assembly, but that is all hard parts and not prone to failure. The factory actuators were prone to failure as they would get weak and not engage the axle. No pulling of the diff required for any of it. I would remove the actuator and see if you can engage the axle manually through the actuator hole.

 

[89Suburban]

The right side axle has the actuator. The actuator moves a fork that has a slider that engages the right axle. If you say the actuator is good then it's in that assembly, but that is all hard parts and not prone to failure. The factory actuators were prone to failure as they would get weak and not engage the axle. No pulling of the diff required for any of it. I would remove the actuator and see if you can engage the axle manually through the actuator hole.

Exactly, he can stick a screw driver in there to manually engage it. That plug for the actuator looks like hell too. The waterseal is missing and the plug is full of dirt. That's not good either.

Here is a great video of the breakdown and operation of that front axle / diff, a little lengthy but very educational.

 

[res02p35]

I watched that video yesterday.
Ok thanks guys, I will attempt that today. Maybe the actuator is not coming out far enough for engagement.

 

[89Suburban]

I watched that video yesterday.
Ok thanks guys, I will attempt that today. Maybe the actuator is not coming out far enough for engagement.

Is your gen the thermal one or the motorized? Did you try to manually engage the fork?

 

[res02p35]

Yes I just tried, I was able to push the fork, it locked both wheels together. However the actuator does not appear to travel as far as I had to push in the screw driver.
The actuator travel is only an inch. It's an electrical motorized actuator. The travel is set but a stop on the ram.
I guess the spring in the diff could be weak?

 

[Eman85]

The original actuators were thermal actuators, why I don't know. The actuator heated up internally and the heat caused expansion that pushed the fork to engage the axle. They would get weak and not engage the axle. My money is on a bad actuator.

 

[res02p35]

Maybe on older models but this has always had an electrical one. This is a 99. I will get another and try it.

 

[Fless]

Maybe on older models but this has always had an electrical one. This is a 99. I will get another and try it.

They're both "electrical" but the older ones' internals are thermal; the newer ones are some kind of solenoid. The newer ones shouldn't take more than about a second to fully extend.