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.

 

[Fless]

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.

Yes to your first question, if all is well the encoder motor shifts the transfer case into whatever mode you select.

I didn't read through the operation description, but in the earlier Auto T/Cs when 4x4 is selected, the T/C is shifted into 4WD and the front diff is locked into the drivetrain by the actuator (full time).

If AUTO is selected, the front diff is not actuated until the multiple speed sensors tell the TCCM that the rear wheels are slipping and to engage the actuator, and the diff comes in and out of the drivetrain based on the speed sensor inputs. Minor delay of a second or so for this switch to take place, unless you have a lazy actuator. All along the T/C has been keeping the front propshaft up to speed for a quick switch.

No idea on how easy or difficult it is to replace the encoder motor, but YouTube your gen of truck and look for an encoder motor replacement video from a reputable source. On the NBS NVG246 cases the front propshaft does need to be removed to access the encoder motor. As @swathdiver mentioned, AllDataDIY is a good source for troubleshooting.

You have to have a B- C- or P-code on that rig.

 

[Bill 1960]

The sequence is like this.

2H > Auto: first the transfer case shift motor engages the transfer case front drive clutch to spin up the front driveshaft. Next the front axle actuator engages the front axle disconnect collar. Finally the TC motor relaxes the front drive clutch and waits for slippage to demand more clutch pressure.

Auto > 4H: TC motor applies full clutch pressure.

4L: TC motor moves range shift fork from high to low range.

N: TC motor moves range shift fork to N.

You can hear the TC motor whirring and clanking if you listen closely, it’s easier to hear if you leave the engine off.

If you’re hearing nothing I’d say the chance of it being the front axle actuator is near zero, because the TC shifts before that actuator is commanded.

There’s a control module under the drivers side dash that connects to all the components and runs the show.

What you can do when shopping isn’t what you can do as an owner. I’d just factor it into the negotiations. IIRC the TC motor is about $250 just for the part. Which is likely the cheapest part that will solve the issue. If it’s mechanical, a GM reman case I think runs around $1600. Of course there are recycling yards and aftermarket vendors for other price points.

 

[Fless]

Bottom line, it could be something simple like a circuit, or something a lot more involved. If you can get the code that's triggering the msg, you'll be ahead of the game. Otherwise factor the possibilities into the price.

 

[UpAndDown]

Thanks guys. I got the code. It’s C0321 if that helps to battle it down. Again I appreciate it.

 

[George B]

Start here:
https://www.tahoeyukonforum.com/threads/service-4wd-message-with-c0321-code.121273/

 

[UpAndDown]

One other thing. I provided some bad info yesterday. The amber light on the 4WD selector switch is on but it stays lit in the 2WD position no matter what position I choose with the dial. Not sure if this changes anything. The owner did say that one day recently he got in and noticed that the 4WD selector knob was pushed inward and he pulled it back out. How do I remove this knob so that I can see if anything behind there got damaged or unplugged? Thanks once again

 

[UpAndDown]

Here’s a pic of the code readout

 

[UpAndDown]

One more thing too. Not sure if this is a related electrical issue or not but the tire pressure warning light is always illuminated. However the pressures through the dash display show all tires at 34-35 psi. I did the calibration reset procedure and nothing changed. Like I said I’m not sure if this is a clue to the original problem or not.

 

[wsteele]

One more thing too. Not sure if this is a related electrical issue or not but the tire pressure warning light is always illuminated. However the pressures through the dash display show all tires at 34-35 psi. I did the calibration reset procedure and nothing changed. Like I said I’m not sure if this is a clue to the original problem or not.

I can't see how it would be related. Did you see any C7xx DTC's in the scan?

Looking through my shop manual, parts wise, it looks like about $400 in parts is your maximum exposure for fixing the C0321 (likely quite a bit less, if you bought parts online and got the correct replacement part right the first time). Labor is anyone's guess if you don't plan to do it yourself. My shop manual calls for the use of the Tech 2 (factory style scan tool) through some of the decision matrix diagnosis to command certain functions if it is a part failure (likely) and not a wiring failure. You can get quite a ways through the diagnosis matrix with just a VOM.

Looking at my shop manual my guess is it would be a part (encoder motor, TCCM, etc.) versus a wiring problem. If you know anyone close to you with a Tech 2 (or clone) or an advanced scan tool that can mimic the functions of the Tech 2, it shouldn't take long at all to diagnose the problem and fix it.

 

[Fless]

As @wsteele mentions, the use of a Tech 2 is highly recommended.

My '04 had an issue with no fault codes, but when 4HI was selected -- and indicated on the switch -- the truck would act as if it was in AUTO. I monitored the "commanded" and "actual" states of the mode selection, and the Tech 2 showed me that although commanded to 4HI it switched to AUTO. Mine (2004) ended up being the encoder motor.