I'm In Panic Mode 4x4 Not Working

[Bruce Wayne]

Hello gang, its been a while since I've posted. Any help or advice will be greatly appreciated.
 
Ok I have a 1991 GMC classic body style K1500 4x4 5.7. The truck runs great has 356k miles, the original engine got 286k. Transmission is also new about 50k miles.The truck turns 30 this year the last 11 years of her life she's been a garage queen mostly. Yesterday my daughter and I went scouting for turkey season. going through some soft mud we got stuck i went to shift the light comes on. Nothing, the tires dont do anything. I carry her to one of the best mechanics in town. He told me the solenoid actuator was disconnected, so he reconnected it. Nothing, hubs still won't lock. He looked to replace the solenoid actuator and according to him the solenoid actuator for the front diff for this model truck is obsolete GM quit making them and he couldn't find them at Napa or anywhere.

So now what?

[newdude]

Does it still have the old thermal style actuator?

 

Dorman still makes the old thermal style actuator, part # 600-100.  4WD Axle Actuator | 600-100 | Four Wheel Drive Actuator | Dorman Products  

 

These are still available and just about any local parts store can get them.  Including Napa.  Napa part # FWD12SB or FWD12.

 

If its been converted to the electronic actuator, those are still available from GM and the aftermarket.  

Edited January 26, 2021 by newdude

[newdude]

You will have to see what style plug you've got.  If its a 5 wire plug directly into the actuator, you've got the electric actuator.  Plug looks like this:

 

 

If its the thermal style, its a two wire plug that connects to a lead off of the actuator.  Plug looks like this:

 

Edited January 26, 2021 by newdude

[LDM]

Up thru 95 they came with the thermal style actuator.  Had the same problem in my 95 Tahoe where the 4x4 would not engage due to the actuator not getting hot enough anymore.  Since you already have to replace it, go with the newer electronic style actuator from the 96-99 trucks, they work much better than the old thermal style.  I did this in my 95 Tahoe year ago and it was one of the better upgrades for that old truck.  There used to be a good writeup online with all the part #s and walk thru but it seems like the link is broken these days.  If I remember correctly you'll need to add a 12V ignition switched power to the new actuator.

[Bigfutz]

How does the thermal one work? Just runs the little motor until it hits something, stalls, overheats and turns itself off until it’s disengaged? Or is it some other electrical sorcery?

[Bigfutz]

I’ve also seen full-time 4WD actuators, which seems to be basically a bolt of a certain length.