4WD Questions

[UpAndDown]

Hi I am new here and new to this platform, although I’ve owned a TrailBlazer for the past 16 years so I am familiar with GM products. I am looking at buying a used 08 Tahoe LT 4WD with the 5.3 flex fuel engine and about 125k on the clock. The “Service 4WD System” light comes on at startup and the selector switch, although the light on it lights, doesn’t shift the truck into any of the 4 wheel modes. It will blink as the knob is turned to each setting but always reverts back to 2WD. Is the most common failure point for this symptom the actuator that’s threaded into the front axle? Is there a definitive way to prove that it’s the actuator, the switch or another part without getting too far into it as it’s not yet my vehicle? Thanks for the help. All replies are appreciated.

 

[OR VietVet]

Welcome to the forum from Oregon. The actuator motors are a known weak spot and so are the switch assemblies. Others will chime in too.

I would remove the actuator and press switches/buttons and see if it moves. Check for the signal being sent to the actuator. Because it does the same thing no matter which button you push I would lean toward the actuator.

 

[UpAndDown]

Thank you for the reply. Not sure the owner would be too happy with me removing the actuator to test it. If the actuator is bad would it act the way I described? That is, no shifting into any 4WD modes and the light blinking when the knob is turned, but always reverting back to 2WD? Thanks again

 

[OR VietVet]

Those symptoms you just listed point to signals being sent for the actuator to work but it is not working. The fact that it does it in all modes means to me the signal is sent but not used. Can I be 100% for sure, NO! But is a guess on the forum. Others will chime in too. Give it a chance and others will be here but remember it is Memorial Day Weekend.

 

[Fless]

I doubt that the front diff actuator would cause this. More likely is the encoder motor that attaches to the transfer case.

Do you have a pic of the RPO codes from the glovebox, or know which transfer case it has?

EDIT: would the owner allow an advanced scanner be used to read the transfer case code, for the one that is causing the error msg?

EDIT again: looks like the T/C would be the MP3023.

 

[UpAndDown]

Floss thanks for the reply. I actually have a SnapOn Solus Pro and I scanned it but there were no codes found. I went into the Transfer Case option and got nothing. Can I ask why you think it would be the encoder motor as opposed to the actuator? Not second guessing you just trying to educate myself. Thanks again for the help.

 

[Fless]

I've never seen a diff actuator generate a code, although I've worked on only the later NBS models (2003-2006). But the systems are very similar in design and operation even though the hardware is different.

There has to be a code somewhere for that to generate the "service" message. If you know someone with a Tech 2 you might want to try that tool.

Here are a couple of articles on operation and troubleshooting. Probably premature in your case but good info nonetheless.


MP3023 Operation:

https://www.transmissiondigest.com/the-magna-powertrain-mp3023-3024-nqh/


MP3023 Troubleshooting:

https://www.transmissiondigest.com/magna-powertrain-mp3023-3024-transfer-cases/

EDIT: theoretically the code could be set by a malfunction in the diff actuator circuit, but there's not a good way to know without knowing the codes.

 

[OR VietVet]

I've never seen a diff actuator generate a code, although I've worked on only the later NBS models (2003-2006). But the systems are very similar in design and operation even though the hardware is different.

There has to be a code somewhere for that to generate the "service" message. If you know someone with a Tech 2 you might want to try that tool.

Here are a couple of articles on operation and troubleshooting. Probably premature in your case but good info nonetheless.


MP3023 Operation:

https://www.transmissiondigest.com/the-magna-powertrain-mp3023-3024-nqh/


MP3023 Troubleshooting:

https://www.transmissiondigest.com/magna-powertrain-mp3023-3024-transfer-cases/

Sorry Fless, I reacted to his using the word actuator and I meant to say encoder motor. Thanks for catching that for me.

In my mind I was thinking the encoder electrics problem and said actuator.

 

[UpAndDown]

Thanks again guys. So if I am to understand correctly the actuator is just to lock the front axle and the encoder motor is what actually shifts the transfer case between the 2 and 4WD modes? I guess then the question would be what sequence the operation takes place in, between the actuator abs encoder, as I don’t hear anything at all happen when I turn the knob on the dash. The actuator replacement looks very straight forward. As far as the encoder replacement on this truck goes, does the front drive shaft have to be unbolted right get at it to swap it out? Thank go for all the good info.

 

[swathdiver]

Floss thanks for the reply. I actually have a SnapOn Solus Pro and I scanned it but there were no codes found. I went into the Transfer Case option and got nothing. Can I ask why you think it would be the encoder motor as opposed to the actuator? Not second guessing you just trying to educate myself. Thanks again for the help.

You could get an online subscription to AllDataDIY for the shop manual and run the troubleshooting guide with your Solus to find the problem. Alternatively, knock off $1500 from the price to cover the worst case scenario to replace the whole thing.