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2004 gmc
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Discussion Starter #1
The front axle actuator, that engages a splined disconnect when you go into A4WD, 4HI, or 4LO - is a reportedly high failure rate item. I don't carry a spare on the trails, but I've been considering it. My wheeling buddy Teebes has had intermittent problems with his 4WD system for many months, and rather than shotgun it at a dealer, I got him to be my guinea pig at finding the root cause and totally understanding the part.

He had previously swapped the actuator with a supposedly good new one, and also bought a user transfer case control module. One day, we swapped parts back and forth with my vehicle, but never found the root cause. The symptom was that the actuator looked like it worked fine outside the vehicle, but got flaky when installed. Looked like an electrical problem, so we concentrated on tracing wiring with the factory service manual for reference for a day. That eliminated the wiring, but didn't find the real cause.

The actuator is basically a severely geared down permanent magnet motor that drives a probe into the front diff disconnect from the passenger side. Total in-out stroke is 0.8", and you can push in on the internal lever with 2-3 pounds force. Here's the actuator in place.



There's a 4-wire harness on the right side (not connected in the pic). It needs +12 and ground. The control signal from the TCCM goes high to command the actuator to extend its probe for 4WD, and goes low to command retraction for 2WD. There is a feedback signal that confirms to the TCCM that the actuator is working, and it sends a low for extended and a high for retracted. Upon first use, the TCCM senses the feedback signal, and if the actuator doesn't succeed in changing state when it's commanded to, the TCCM gives up, starts blinking one of the control switch LEDs, and (importantly) gives up. It won't try again until you turn the car's power off and reset the system. So if the actuator ever fails once to go to the position it's told to, the TCCM stops trying. It's not a serial data bus kind of module like the door switches or HVAC control box.

So what ended up happening for Teebes was that the actuator when it was installed, would attempt to go to the 4WD position, the motor would whine once, but it would stall out and never make it to the fully pushed-in position. By clipping taps onto the wiring harness, I could see the command line go high, the motor would move, but stop short of the fully-out position. The feedback wire would never go low, so the TCCM assumed the actuator had failed, and then it just gave up trying.

Removed from the vehicle, the actuator would whirr and go both ways, the feedback wire was correctly reporting its success, and the system worked fine. So it was obvious that the permanent magnet motor was stalling or the gears needed lubrication or something was binding.

The actuator's cover is stiff, and it's not easy to disengage the locking tabs as it is on a HVAC system mode/temp actuator, but if you break a couple or use toothpicks, you can get it off.



In this, you can see the motor, reduction gears, and a lead screw on the left that brings signals from the 4-pin connector.



Empty housing.



Sorry for the poor pic, but I'm really not going back to take this all apart to get better ones. :p This is the lead screw with four spring fingers that slide up and down on the bottom of the PC board.



This is the PC board with the 4 stripes. The second from the top is the feedback signal, showing the break in the stripe where it goes from reporting high to low when extended. Note the actuator has to be near the end of the travel for this signal to change state, so if the motor stalls due to high load, like broken internals in the differential, it will fail to extend all the way, and the TCCM will see that and give up trying.



Taking it apart, I didn't feel any unusual binding in the gear train or the actuator probe, but there was obvious grease on the copper fingers and the PC board stripes. The mounting angle in the vehicle would tend to keep grease away from the fingers, but this unit was one Teebes bought as a replacement, so it had been on the shelf a few years perhaps. I'm guessing the box it was in had the PC stripes at the bottom and grease migrated over the years.

Grease caused a higher resistance than normal on the finger to PC stripe contact. The PM motor has to draw 2-3 amps to do its job, and it doesn't overheat because it's only for a few seconds. When the actuator was out of the vehicle, the motor would spin and work OK, but it couldn't handle the increased mechanical load of the differential internal lever. The motor current went up, half the power was wasted in the resistance of the greasy copper finger contact, and the actuator stalled out before it got to the end of the travel.

Took it apart - degreased the fingers and the PC stripes, and viola! The motor whined even faster when out of the vehicle, it never fails to actuate now when installed, and Teebes is back in business! Saved $90 on a new actuator and got the satisfaction of knowing how the sucker really works inside. The factory manual again saved almost its price by avoiding a dealer shotgun troubleshooting episode. And by repairing and not discarding the flaky part we now have a trail spare and it stays out of the landfill.
 

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2005 chevy trailblazer_lt
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Very interesting Roadie. Thanks for the teardown. :thumbsup:

I find it interesting how they attached the feedback sensor to one of the drive gears. Why not make it part of the output mechanism itself?

I guess they were attempting to keep it away from the grease?, which was the cause of the problem in your case anyways. (so maybe a wise choice on their part)

Was this the same problem that Teebes had originally? How did grease get onto the contact feelers when in the mounted orientation? Something we should be doing preventive maintenance on?
 

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2004 gmc
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Discussion Starter #3
I forgot to get a timeline of the entire story, but I think the original flakiness was cured by a replacement switch, but he and/or the dealer also fiddled with the TCCM or performed a reflash, and he also changed the actuator along the way from his original to a replacement. It was the replacement that had been on a shelf in storage that I found was flaky from the grease.

After figuring out the grease issue on the actuator, he took apart his original switch, and found grease there! So months of flakiness and troubleshooting expense could have been some misplaced grease in the control switch as the ultimate root cause.

There's no other good reason for so many reports of flakiness, and I'm happy to see it's grease because you can fix that without buying replacement parts.

I don't think there's anything to PM, just be ready to take it out and degrease it if it happens to you.

I'm extremely glad he's fixed up because we just discovered we can get the same week off in October, so this year's Sierra expedition will be TWO trailvoys, not just one!
 

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2002 chevy trailblazer_lt
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371 Posts
Grease :duh::hissy:

Thanks again for all of the assistance roadie - so glad to be driving with 4wheels again :hail::thumbsup:

As I still have an 4wd switch handy, I'll snap some pics of that one for the archives later this eve.
 

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2002 chevy trailblazer_ltz
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237 Posts
Hey Roadie,
all i can say is wow!
Sometimes i am really impressed how things work, but it is more impressive that you have found the failure.
Good to know that it can be repaired easily..:thumbsup:
 

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2005 chevy trailblazer_lt
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This wasn't overly obvious to me after thinking about it. I was assuming this was the transfer case output to the front diff, but some of your wording made it sound like this may be on the front diff itself. It became further confusing because that looks like a CV boot in your picture, which I thought were only on the front half-shafts.

To verify, this is the control that actually activates the front drive shaft, right? This is the motor we hear moving when we change the selector knob, right?



Sorry, I may have missed something, or maybe this was painfully obvious, but thanks for the clarification. :eek:
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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3,700 Posts
Nope... This is on the front diff, in place of what used to be known as "locking the hubs in", except it's automatic now, and on this vehicle, it's not on the hubs, but on the hog head, so the half shafts spin all the time... It disconnects the half shafts from the ring and pinion, so the front drive shaft and front ring and pinion just sit there instead of being back-driven by the front wheels.

Mike
 

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2004 gmc
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Discussion Starter #8
This item is the "actuator" they call it, on the passenger side of the oil pan, just in front of the CV shaft. It actuates the splined disconnect that makes the front differential go into freewheel mode when you're in 2WD. The motor is a very high pitched whine, and ONLY makes noise going from 2WD to A4WD and back again.

The transfer case "encoder motor" is on the driver's side of the transfer case, and is a lower pitched motor sound that makes some noise between EACH of the four modes. You can turn the 4WD mode switch with the engine turned off and listen to the noises. I recommend this HIGHLY so you'll know what it sounds like on a good day, and you can diagnose some things just by listening.

The front drive shaft is always turning, driven backwards from the front diff in 2WD mode.

The transfer case theory of operation is too much to type in, but I may scan in part of the manual someday. Sorry.
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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The front drive shaft is always turning, driven backwards from the front diff in 2WD mode.

.
um... shouldn't be... when in park, I can turn my front drive shaft by hand, and it is free-wheeling...

ETA: I guess if only one side is disconnected, the spider gears will spin, and the unattached shaft will spin the opposite direction, but since the one isn't tied to anything, the carrier, ring, pinion, and drive shaft will still be at rest, or free-wheeling as it were...

Mike
 

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2002 chevy trailblazer_ls
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377 Posts
Great information!

Thank you for posting!

Now I'm thinking that I should get some wrenching done and take mine apart to see if that's my problem... :undecided
 

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2005 chevy trailblazer_lt
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um... shouldn't be... when in park, I can turn my front drive shaft by hand, and it is free-wheeling...

ETA: I guess if only one side is disconnected, the spider gears will spin, and the unattached shaft will spin the opposite direction, but since the one isn't tied to anything, the carrier, ring, pinion, and drive shaft will still be at rest, or free-wheeling as it were...

Mike
To verify:

It disconnects the passenger side of the differential only and disconnects the front drive shaft at the transfer case?

I guess they disconnect the half shaft so there is no torque pulling through the differential which would lead to unnecessary wear?

I would imagine the front drive shaft would rotate a bit just due to parasitic drag through the diff from the driver's half shaft?

Its interesting how different designers do the same tasks with different methods.
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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To verify:

It disconnects the passenger side of the differential only and disconnects the front drive shaft at the transfer case?

I guess they disconnect the half shaft so there is no torque pulling through the differential which would lead to unnecessary wear?

I would imagine the front drive shaft would rotate a bit just due to parasitic drag through the diff from the driver's half shaft?

Its interesting how different designers do the same tasks with different methods.
Your first sentence is correct, but it's 2 different things that disconnect... What we're talking about here is the one that disconnects the half shaft from the diff...

Yes, unnecessary wear, but mostly unnecessary fuel usage, messing with the CAFE standards...

I don't think it's the different designs from the different designers, but it was changed to make it cheaper to mfg... No more auto-locking hubs... hurts a little bit on fuel economy, but it's the way they played it...

Mike
 

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Front Actuator

Can anyone tell me what the front actuator physically engages (drivefork ?), and what should I see in the opening when you unbolt the actuator from the side of the front axle housing. I removed mine and there's a half-moon opening about the size of a dime but I didn't see anything that the actuator could engage with.

Thanks,
 

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2004 gmc
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Discussion Starter #15
Welcome! Glad you found the forum, even though it was when you have a problem. The switch lights can blink for a lot of reasons. Both the actuator and the transfer case encoder motor report back to the control module the success or failure of any motion they're told to do. Are you having intermittent or permanent problems, and have you listened to the two kinds of motor noises? You can do this with the engine off, ignition on and the shifter in park. Then move the 4WD control switch among the modes.
 

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2007 buick rainier
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Roadie,
Just a comment , in one of your photos showing the motor, it looks like a slot car motor from the 1960's. We used to wind a 3v armature and run them on 12v. I still have a few laying around somewhere.:hijacked
 

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Welcome! Glad you found the forum, even though it was when you have a problem. The switch lights can blink for a lot of reasons. Both the actuator and the transfer case encoder motor report back to the control module the success or failure of any motion they're told to do. Are you having intermittent or permanent problems, and have you listened to the two kinds of motor noises? You can do this with the engine off, ignition on and the shifter in park. Then move the 4WD control switch among the modes.
Thnaks, the problem I'm having is intermittent, we have snow in Lexington and I got it to work but ever so often it fails (turns off and go the 2WD)
 

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2002 olds bravada
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Roadie, thanks for the information. This is a good thread because it seems like this is the likely suspect for those with the Stabilitrak AWD issues as well.
 

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2004 gmc
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Discussion Starter #19
Full-time AWD vehicles don't have this actuator. The front diff is always engaged. The transfer case is also different - no LO range, and no 2WD mode.
 

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2004 chevy trailblazer_lt
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It was -37 last night. (Don't ask - it's freaking cold!) At that temp - and I don't think it matters much if I say celsius or fahrenheit because at that low a temp, it is pretty much the same thing - I flipped from 4Hi to 4Aawd. The lights blinked for about 8 seconds, then illuminated the service light.

This morning, I tried from 2Hi to 4Aawd and again the service light illuminated but it went to Aawd this time.

I just checked the TSBs for my year and sure enough they state that a problem with extended shifting in very cold weather could illuminate the service light. GM says to replace the front axle actuator in these circumstances.

Obviously, it is rare to experience these temps. I could see how that build-up of grease would exacerbate the problem in very cold temps, and I am thinking the actuator may not have failed but is maybe just slower than normal at these temps. As far as I have read, if the actuator does not engage the axle within 8 to 10 seconds, it illuminates the service light, even if sometimes it eventually engages the axle and sometimes it fails to engage completely.

Would you suggest waiting for warmer weather and seeing if the problem tends to go away, or would you think this is just the start and it is going to get worse eventually? (Regardless, I don't relish replacing that actuator in a cold garage.)

Once it warms up, I don't have a problem taking the actuator apart to see what it looks like on the inside, especially if I have a new one ready to go if I screw something up.
 
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