1989 Z71 auto 4wd not doing 4wd

[Adam McLaughlin]

Hey Guys,

I am a bronco guy myself, but I am trying to fix a friend's 4wd 1989 Z71 pickup. The truck won't go into four wheel drive when I throw the lever. It is a 5.7liter TB injected truck with an automatic transmission (4l80e?) and shift on the fly 4wd.

The 25 amp fuse in the fuse box continually blows when I shift that lever on the floor. If I pull out the thermal activator (hot dog looking item) out of the front end and connect it straight to the battery, it extends out of it's "bun" and gets VERY hot. So hot that it melts the wire going to the battery. Could this mean that the thermal activator is toast?

No, I have not tried disconnecting the thermal activator and seeing if the fuse blows yet. I'll try that Tuesday night.

I bought a haynes book on the truck but the haynes book doesn't even begin to cover the schematic. Can anyone give me a schematic or a place to start so I can get this truck back in 4wd?

Thanks,

Adam

[aztec777]

Adam,
It's the thermal 4x4 actuator that's gone bad. You can either get the newer Suburban part to keep it OEM or the better choice is to use the Warn aftermarket part. Really easy to install and works. I've replaced 4 now with the same awesome results that Warn is famous for. Costs around $125 or so. Look online and in the 4x4 mags. Good luck.

Steve

[Adam McLaughlin]

Ah, okay.

So it IS the thermal activator! I had a sneaking suspicioun about that.

Thanks for the tip. I'll start flipping through some 4x4 mags today for that part.

Adam

[aztec777]

Adam,
Find it in the mags or online and then go to your local parts store and they will probably match the price or come close enough to make it worth it to you. Good luck.

Steve

[mudrunner78]

Insted of buying a new thermal accuator just buy a 4x4 posi lock which will eliminate the problem for good, and allow low range 2wd. Price is $199 from 4wheel parts wholesale but have seen them for 169.