400th 'front axle not engaging' problem

[Pressureangle]

'04 Yukon XL 2500.

Transfer case shifts range and engages driveshaft.
Front axle actuator not extending, new actuator not extending.
'Service 4x4' illuminated on dash info. No DTCs present.
Dash control buttons self-test good, all illuminate; 4x4 hi engages driveshaft, 4x4 low shifts range and engages driveshaft.
Dash button 4x4 lamps flash, 2wd lamp stays lit solid.
DeOxit dash button connectors
DeOxit all connectors, serviced grounds at DF body mount, serviced driveshaft speed sensor connectors.
DeOxit all fuses underhood and dash panel.

This is a Michigan truck, so some rust and corrosion but well cared for and generally in very good condition. It did sit for a few months with wet driver interior due to sunroof drains plugged.

I'm an old SAE truck tech, so I can bulldoze this but I'm time crunched for a very long trip towing (no 4x4 action, so merely annoying) hoping somebody's experience will point to something simple I don't know. I've read every thread I can find, read the schematics and bulletins.

Who's holding?

 

[Scottydoggs]

if the drive shafts spinning but not the front tires it could be actuator in the front diff.

 

[rockola1971]

Lock it in 4x4 and check for power and ground at the plug for the front diff actuator or just pull the actuator and see if it extends and retracts normally. You got caught with the parts cannon out of its case.

 

[Pressureangle]

Lock it in 4x4 and check for power and ground at the plug for the front diff actuator or just pull the actuator and see if it extends and retracts normally. You got caught with the parts cannon out of its case.

Neither the original nor replacement actuators extend. I checked, have B+ and ground at actuator plug, but I'm alone with no helper or DMM available so can't test signals.

Possible control module on the buttons? Will 4x4 DTCs show up on my cheap OBD scanner?

 

[rockola1971]

Neither the original nor replacement actuators extend. I checked, have B+ and ground at actuator plug, but I'm alone with no helper or DMM available so can't test signals.

Possible control module on the buttons? Will 4x4 DTCs show up on my cheap OBD scanner?

Not sure how you have B+ and Ground at actuator but actuator isnt extending? Unless you got a bad "New actuator. If you got ground and 12v then that actuator is getting everything it needs to extend from the TCCM to engage front diff. So everything from your plug at the actuator back to the TCCM is good.

 

[Fless]

SWAG: there should be B+ to the actuator. Might test for the B+ and ground with 4x4 engaged, and loaded in-circuit to make sure it can carry the amperage that the actuator takes. You can also bench test the actuator to verify it's good. See this video for test:

or

If the ground is weak, then tests on an unloaded circuit might show "good" when it's not. The ground for the actuator is on the driver's side body mount under the driver's door hinge. There will be two ground bolts there with (usually) 3 wires -- two on one bolt and one on the other. Clean all of those grounds and re-seal with paint or something that will keep corrosion away for at least a while.

Or maybe the center pin control ground is not getting sent?

 

[Pressureangle]

SWAG: there should be B+ to the actuator. Might test for the B+ and ground with 4x4 engaged, and loaded in-circuit to make sure it can carry the amperage that the actuator takes. You can also bench test the actuator to verify it's good. See this video for test:

or

If the ground is weak, then tests on an unloaded circuit might show "good" when it's not. The ground for the actuator is on the driver's side body mount under the driver's door hinge. There will be two ground bolts there with (usually) 3 wires -- two on one bolt and one on the other. Clean all of those grounds and re-seal with paint or something that will keep corrosion away for at least a while.

Or maybe the center pin control ground is not getting sent?

I think you're halfway there. I already cleaned and remade the body grounds.
I have ground signal from the buttons.
BUT testing the actuator ground with a test light instead of my power probe shows no ground. (PP has a broken feed switch. )

So, this will get put down until I have the time to trace it out unless somebody knows where the hidden point of failure is likely to be. I took off the DS kickpanel, couldn't find the ground splice indicated in the schematic.

Thanks for the input, I'll close this up in a few weeks when it's resolved.

EDIT; It's always what you thought it was, it's always what the internet said it would be. It's always something simple.

Although I removed, cleaned, DeOxit-ed, wire brushed and thoroughly cleaned the frame grounds, I took myself to school and got dirty. Short story is, the connection between the ground wire and the terminal loop had failed, though it looked ok. New ring terminal, new cleanup, new punch mash on the remaining terminal loops, covered with Cosmoline and FIN.
Dangit, I'm all sweaty now.

 

[Fless]

My memory is a bit foggy but I thought there was a splice in the circuit very close to the brake master cylinder, in the engine compartment. I could be wrong, but it seems like it's working for you now?

I haven't looked at a schematic but I would bet that the ground activation signal comes from the ATCM behind the driver's knee panel, meaning the dash switch talks momentarily to the ATCM to switch modes, and the ATMC holds the activation (ground) signal to the actuator while in that mode.

 

[Pressureangle]

My memory is a bit foggy but I thought there was a splice in the circuit very close to the brake master cylinder, in the engine compartment. I could be wrong, but it seems like it's working for you now?

I haven't looked at a schematic but I would bet that the ground activation signal comes from the ATCM behind the driver's knee panel, meaning the dash switch talks momentarily to the ATCM to switch modes, and the ATMC holds the activation (ground) signal to the actuator while in that mode.

The axle harness has a connector on the inner fenderwell just in front of the brake lines. I peeled the harness back where that connector goes into the larger harness to the frame ground, no evidence of a splice. <shrug> lots of gymnastics for something so simple, but that's one of the downsides of being retired. You forget that it's always the simple stuff, particularly in systems you're not familiar with and in this case didn't have my head in the game until last quarter.

 

[89Suburban]

Holy rust Batman!