Why won’t my front diff engage?

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Schurkey

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I see a few on rock auto listed as only fitting vehicles that have already been upgraded from thermal to electric. Is it just saying that because of the adapter harness? Or is there something else?
Adapter harness AND the metal spacer supplied with the 600-600 adapter harness. I guess you've gotta cram the spacer in place before you attach the "upgraded" actuator to a truck that used to be thermal-actuated.

But I've never done the conversion, so I don't know for sure.
 

Supercharged111

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Adapter harness AND the metal spacer supplied with the 600-600 adapter harness. I guess you've gotta cram the spacer in place before you attach the "upgraded" actuator to a truck that used to be thermal-actuated.

But I've never done the conversion, so I don't know for sure.

One could also swap in a newer front diff, or do what I did in my 1500: I swapped in an older style 4:10 diff but changed the actuator housing out so the newer style actuator fits without any spacers or trickery. I wouldn't bother with this in the OP's situation.
 

Chillin Dylan

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One could also swap in a newer front diff, or do what I did in my 1500: I swapped in an older style 4:10 diff but changed the actuator housing out so the newer style actuator fits without any spacers or trickery. I wouldn't bother with this in the OP's situation.
I’ve considered swapping a newer front diff in because I swapped in a 4.10 11.5 AAM from an 03 8.1 truck. But I already have the 9.25 front diff so I’m not sure what, if any, upgrades would be had
 

Supercharged111

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I’ve considered swapping a newer front diff in because I swapped in a 4.10 11.5 AAM from an 03 8.1 truck. But I already have the 9.25 front diff so I’m not sure what, if any, upgrades would be had

Just the newer style housing and seals is all it really gets you. I was thinking more along the lines of not having to use that spacer is all.
 

Macs Garage

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That's what I think of when I hear "Posi-lock". Wonderful pullers...but heavy.
I had to replace the Actuator on my 98. I went to O'Reilly's and they couldn't figure out what I was talking about. I finally went behind the counter and found it my self. If it wasn't Sunday I could have went to Auto Value and the experienced parts men would have just walked to the shelf and grabbed it. Turned out the 4x4 fuse was blown.
 

Night Bomber

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I have a 1995 K1500 Z71 (that I dearly love) and finally got tired of the stock thermal actuator in the front differential.
Went to the wrecking yard and pulled a servo motor actuator out of a 2004 or 2005 K1500 and did some research. I decided to make my own wiring harness and am very pleased with the results.

Attached below are the schematics I got my information from.

I also found a helpful GM Service Bulletin (No: 76-43-01A) That clarified some.

I only used three conductors and three pins on my wiring harness.

Pin C (+): Brown Conductor - +12vdc Fused Actuator Servo Motor Power
Connect to Brown Conductor found on switch on Transfer Case above manual shift linkage)

Pin E (-): Black Conductor - Chassis GROUND

Pin D (+): Light Blue Conductor - Actuator Command Conductor
Connect to Light Blue conductor found on switch on Transfer Case above manual shift linkage)

When ignition key is on and transfer case is pulled INTO 4WD, the transfer case position switch CLOSES and SUPPLIES +12vdc onto the Light Blue Command Conductor and the actuator is told to EXTEND.

When ignition key is on and transfer case is pulled OUT OF 4WD, the transfer case position switch OPENS and REMOVES +12vdc from the Light Blue Command Conductor and the actuator is told to RETRACT.

I connected the wiring and did my tests to make sure the new GMT800 actuator would behave properly before I threaded it into the front diff.

I DID NOT USE PINS A & B
Pin A goes to the GMT800 4WD ENGAGED indicator light
Pin B goes to the GMT800 EXTEND/RETRACT GROUND CONNECTION.

A couple of details:

Actuator length differences:
I did a test with an older GMT400 thermal actuator and compared it's length while extended and retracted with the length of the GMT 800 servo actuator while extended and retracted. The GMT800 actuator I got from the wreck was shorter so I decided to make a small block of white UHMW around 1/2" long and inserted it into the axle so that the newer actuator would be sure to fully engage the splines. It sits in the hole loosely and I can reach in with a small screwdriver and pull it out if I want.

Wiring Harness
I did not want to ****** the stock wiring so I could go back to a thermal if I wasn't pleased with the results so I used a burned out old thermal actuator wiring harness plug and used it as my adaptor to connect to the stock wiring harness. I also bought some black plastic "conduit armour" to protect the new harness. Looks good and no carnage!

Results
It used to take a few seconds or even longer to lock in the front axle before my mods.
Today it is -34 degrees Celsius (which is -29 in Amrikaan Farenheit degrees) and it works beauty, eh! (Yes I'm from Candada). Instant lock!

Not sure who supplied some of the drawings I used but most grateful for the info on this site.

Hope this helps!
 

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  • Four Wheel Drive Axle Actuator Wiring Diagram GMT400 210828.PNG
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  • 1995 GM 4x4 Actuator Wiring Diagram BENZ 210618 02.jpg
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  • GMT800 Front Axle Actuator 210827 01.jpg
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  • GMT800 Front Axle Actuator PINOUT 210827 02.PNG
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HotrodZ06

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There are also two different connectors for the thermal actuators one with round pins and one with flat pins.
 
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