4HI to A4WD issue

[TheDave]

First post here so be gentle. Truck has 144,000 and is about due for fluid changes again. However, when I switch from 4HI to A4WD the switch light go crazy and flash like a Christmas tree. I am hearing the motors run but can not determine the cause. I leave the switch in 2WD 98% of the time but I occasionally rotate the switch to "keep things tested". Any thoughts. This is the first time I have heard that buzzing sound at the end before I switch it to 2WD.
Here is the video:
[video=youtube;2XsSEeJjY0g]http://www.youtube.com/watch?v=2XsSEeJjY0g[/video]

Regards,
TheDave.

 

[gmac310]

Sounds like either the TCCM or the spline disconnect is bad.

 

[The_Roadie]

Did you hear were harsh on noobs or something?!?!?! Heck, only if you put a downforce wing on the back of the truck or a fake carbon fiber hood wrap or something.

Welcome!

You have a most unique problem I've never seen before in 8 years on three forums, and that's saying a lot about the 4WD system, which is my specialty.

Your front axle actuator is fine. Your splined disconnect collar is probably OK. Have you ever taken it out into a parking lot, switched to 4HI mode, and then turned a tight corner to see if you get the traditional driveline binding or tire scrub that proves your front axle disconnect is indeed engaged?

In any event, your 2HI->A4WD transition is the only one that moves position of the front axle actuator, and your sounds EXACTLY like it should. The mode control switch would also flash the light over the A4WD mode if the front axle actuator failed to complete its commanded action. No problem there. When you move from A4WD->4HI, the only noise you should hear is the transfer case encoder motor, and indeed you do hear it, and it sounds OK on your video clip. The light also goes full on steady over the 4HI mode, so that means the TCCM was satisfied that the encoder motor completed ITS commanded action.

Your problem is inside the transfer case or the encoder motor when it tries to go into 4LO mode. Assuming you're in NEUTRAL when you do this (if you were not in NEUTRAL the TCCM interlock should prevent this activity at all), what's happening is (I predict) the transfer case has an internal problem preventing the encoder motor from completing its commanded action, and there's a mechanical jam that stalls the encoder motor and that makes the TCCM flash all the lights as a huge warning sign and causes the buzz noise.

How's the maintenance and ownership history? Has the transfer case fluid been changed religiously every 50K miles?

We have tech articles on ORTB about the internal theory of operation of the transfer case? Have you read them or do you need a link? There's a lot going on in the transfer case during the low range transition, and you may have broken bits inside the case preventing it.

 

[C-ya]

Why would the transfer case try to go into 4LO when he never turned the switch to 4LO? The issue was when he came back to A4WD from 4HI.

Maybe I missed something...

 

[The_Roadie]

I probably missed it because the video was way too dark and I was trying to be gentle on the noob by not pointing out the obvious.

 

[Robbabob]

Welcome to the forum!

We are humorous lot, filled with a ton of information. We look forward to you learning and giving a little back as you brouse and share your experiences, too.

Happy trails!
Robb

 

[DenaliHD66]

Sometimes when I am 4Hi, and I shift into neutral and brake, then switch it to 4LO, the light goes back to A4WD and it shifts into there. I have to hold it a couple seconds to engage 4LO.

I've been having issues with my A4WD. The 4HI/4LO settings work fine, but as soon as I start driving in A4WD, the service 4wd light comes on. I notice that shortly after I shift from 4HI to A4WD, there is still driveline bind. I replaced the front actuator motor, thinking that was bad, but I'm thinking another nightmare is true.... the 4wd disconnect is bad? Or would it just need serviced?

 

[linneje]

I've been having issues with my A4WD. The 4HI/4LO settings work fine, but as soon as I start driving in A4WD, the service 4wd light comes on. QUOTE]

The disconnect is in the same position when you switch fro 4HI/LO to A4WD. The actuator does not change position, hence if the service 4WD light comes on when you do this it is not the front disconnect nor the acuator motor causing this to light up. It is coming from your transfer case. The binding you are feeling is another indication of a problem with the transfer case.

 

[DenaliHD66]

I've been having issues with my A4WD. The 4HI/4LO settings work fine, but as soon as I start driving in A4WD, the service 4wd light comes on. QUOTE]

The disconnect is in the same position when you switch fro 4HI/LO to A4WD. The actuator does not change position, hence if the service 4WD light comes on when you do this it is not the front disconnect nor the acuator motor causing this to light up. It is coming from your transfer case. The binding you are feeling is another indication of a problem with the transfer case.

Transfer case motor then?

 

[The_Roadie]

Or grabby clutches, broken bearing mount, or other internal transfer case damage. Binding in A4WD mode often is the first symptom of a leaking or poorly maintained transfer case after a new owner gets a vehicle.

 

[DenaliHD66]

Okay, I was under the impression when in A4WD, the transfer case locks the front and rear driveshafts together, but the front driveshaft spins freely at the front axle until it slips, and then something in the front axle engages the front to lock up? Which would mean that there should not be any driveline binding at all, because the front is not actually engaged, unless it senses slip. However, when it is in 4HI, and I shift to A4WD, and the front still feels engaged, how is that the transfer case? Unless the engagement all happens within the transfer case, when in that case I was under the aforementioned impression.

If this is the case, how does it actually work, and what is the 4WD disconnect for?

 

[The_Roadie]

Sorry....I've been living with and explaining this system for so long I forget to repost the details sometime.

Required reading:
Offroadtb.com Transfer Case Operation: NVG 226 (NP8)

The front axle splined disconnect ONLY changes state during the 2HI->A4WD and A4WD->2HI transitions. Its only purpose is to save a tiny amount of fuel in 2HI mode to help GM sell vehicles by making people think a heavy SUV gets good fuel economy.

The transfer case contains the clutches that engage and disengage to send torque forward in the various 4WD modes. In A4WD mode (a mode I highly discourage using for a few reasons that have been previously discussed), the minimum torque sent forward is 5%, to keep the clutches ready. When wheel slip is detected, the encoder motor engages the clutches more until wheel slip is reduced. The TCCM is in charge of this process. You should not be able to detect any driveline binding in A4WD mode because the tire traction easily overcomes 5% torque bias. Grabby clutches from old/bad/low fluid or disintegrating clutches or an encoder motor that fails to go to the right position to limit the torque to 5%, can all cause the driveline binding behavior.

In 4HI and 4LO, the transfer case clutches essentially lock up and all torque is equally shared fore/aft. In this case, ONE slipping wheel won't cause all the torque to leak out of the driveline - you need one front and one rear to slip at the same time. If you have a G80, then you need one front and BOTH rears to slip before you quit moving forward.

Without all this working, I could never have climbed this steep lumpy trail, and it's a fantastic demo of the transfer case AND the G80 in operation. Watch the front tire in the air at one point while I feather the throttle until the G80 engages, then I can continue the climb. Engaging the G80 at full throttle after a wheel slips is very risky.

[video=youtube;24SwmSN0-bI]http://www.youtube.com/watch?v=24SwmSN0-bI[/video]

 

[DenaliHD66]

Sorry....I've been living with and explaining this system for so long I forget to repost the details sometime.

Required reading:
Offroadtb.com Transfer Case Operation: NVG 226 (NP8)

The front axle splined disconnect ONLY changes state during the 2HI->A4WD and A4WD->2HI transitions. Its only purpose is to save a tiny amount of fuel in 2HI mode to help GM sell vehicles by making people think a heavy SUV gets good fuel economy.

The transfer case contains the clutches that engage and disengage to send torque forward in the various 4WD modes. In A4WD mode (a mode I highly discourage using for a few reasons that have been previously discussed), the minimum torque sent forward is 5%, to keep the clutches ready. When wheel slip is detected, the encoder motor engages the clutches more until wheel slip is reduced. The TCCM is in charge of this process. You should not be able to detect any driveline binding in A4WD mode because the tire traction easily overcomes 5% torque bias. Grabby clutches from old/bad/low fluid or disintegrating clutches or an encoder motor that fails to go to the right position to limit the torque to 5%, can all cause the driveline binding behavior.

In 4HI and 4LO, the transfer case clutches essentially lock up and all torque is equally shared fore/aft. In this case, ONE slipping wheel won't cause all the torque to leak out of the driveline - you need one front and one rear to slip at the same time. If you have a G80, then you need one front and BOTH rears to slip before you quit moving forward.

Without all this working, I could never have climbed this steep lumpy trail, and it's a fantastic demo of the transfer case AND the G80 in operation. Watch the front tire in the air at one point while I feather the throttle until the G80 engages, then I can continue the climb. Engaging the G80 at full throttle after a wheel slips is very risky.

[video=youtube;24SwmSN0-bI]http://www.youtube.com/watch?v=24SwmSN0-bI[/video]

Okay thanks for clarifying. So I KNOW that in 2HI, the front driveshaft is NOT spinning... because you can turn it freely both ways under the truck when it is parked. When you engage A4WD at 70 mph, what brings the front driveshaft up to speed seemlessly? Again, I know HOW to use the 4WD system, I am just trying to figure out how exactly A4WD functions. Like you said, I've heard enough against using it, so I never use it on hard surfaces... only in mild off roading scenarios.

 

[The_Roadie]

When you engage A4WD at 70 mph, what brings the front driveshaft up to speed seemlessly?

Outstanding question! I had to have it explained to me by a mechanical engineer, my partner over at ORTB, James Downing.

When you're rolling along in 2HI, assuming the front driveshaft has enough friction in its seals to not rotate on its own, the driver's side tire and CV shaft are rotating. The passenger side tire, is also rotating, but because the splined disconnect collar is not engaged, it's not connected to the intermediate shaft going through the oil pan. If the front driveshaft (from the transfer case to the front diff) is not rotating, then neither is its pinion gear or the differential's ring gear, or the differential's carrier. But the driver's side CV shaft is rotating its end gear, which rotates the two little spider gears in the carrier, and that rotates the passenger's side end gear that's connected to the intermediate shaft. Backwards! I didn't visualize this for a long time. The intermediate shaft is rotating in reverse from the passenger side CV shaft, which means the splined disconnect collar has some rotating mass (the intermediate shaft, its end gear, and ultimately the differential carrier, ring gear, pinion, and front driveshaft) to get up to speed instantly. This can wear the splines or stress the collar in the disconnect.

Soon as I understood this, I quit engaging A4WD at any speed. People do it all the time, and it hasn't been proven to break things, but it could. And many offroaders rebuild their disconnects to put in the permanently-connected collar from AWD vehicles like the Bravada, and accept the lower gas mileage.

The fact that the differential and transfer case still have spinning internal parts, even in 2HI mode, has disappointed a few owners who don't want to service or repair their units, and thought they could just drive in 2HI mode and not worry about things.

Added: In the 2HI->A4WD transition, the transfer case encoder motor is ALSO in the process of engaging the clutches to that 5% torque bias position, and if they engage before the front axle disconnect, then the clutches have a gentler time of getting the front driveshaft to spin, but there's no guarantee the transfer case wins that race.

 

[DenaliHD66]

Outstanding question! I had to have it explained to me by a mechanical engineer, my partner over at ORTB, James Downing.

When you're rolling along in 2HI, assuming the front driveshaft has enough friction in its seals to not rotate on its own, the driver's side tire and CV shaft are rotating. The passenger side tire, is also rotating, but because the splined disconnect collar is not engaged, it's not connected to the intermediate shaft going through the oil pan. If the front driveshaft (from the transfer case to the front diff) is not rotating, then neither is its pinion gear or the differential's ring gear, or the differential's carrier. But the driver's side CV shaft is rotating its end gear, which rotates the two little spider gears in the carrier, and that rotates the passenger's side end gear that's connected to the intermediate shaft. Backwards! I didn't visualize this for a long time. The intermediate shaft is rotating in reverse from the passenger side CV shaft, which means the splined disconnect collar has some rotating mass (the intermediate shaft, its end gear, and ultimately the differential carrier, ring gear, pinion, and front driveshaft) to get up to speed instantly. This can wear the splines or stress the collar in the disconnect.

Soon as I understood this, I quit engaging A4WD at any speed. People do it all the time, and it hasn't been proven to break things, but it could. And many offroaders rebuild their disconnects to put in the permanently-connected collar from AWD vehicles like the Bravada, and accept the lower gas mileage.

The fact that the differential and transfer case still have spinning internal parts, even in 2HI mode, has disappointed a few owners who don't want to service or repair their units, and thought they could just drive in 2HI mode and not worry about things.

Added: In the 2HI->A4WD transition, the transfer case encoder motor is ALSO in the process of engaging the clutches to that 5% torque bias position, and if they engage before the front axle disconnect, then the clutches have a gentler time of getting the front driveshaft to spin, but there's no guarantee the transfer case wins that race.

Oh okay then so it really does stress it to do that. Obviously there is never a reason to, but if you go from 2hi to 4hi at 70mph, bypassing the A4WD option, does it BYPASS the previous process or does it still have to do it because the driveshaft is still being forced to be brought up to speed?

 

[Sparky]

Oh okay then so it really does stress it to do that. Obviously there is never a reason to, but if you go from 2hi to 4hi at 70mph, bypassing the A4WD option, does it BYPASS the previous process or does it still have to do it because the driveshaft is still being forced to be brought up to speed?

The process from 2HI to A4WD or to full 4WD is the same. The only difference is the transfer case switches to full power transfer vs the initial 5% on the A4WD.

 

[BRomanJr]

Oh okay then so it really does stress it to do that. Obviously there is never a reason to, but if you go from 2hi to 4hi at 70mph, bypassing the A4WD option, does it BYPASS the previous process or does it still have to do it because the driveshaft is still being forced to be brought up to speed?

IMHO:
Actually, because of the 5% front drive connection, the front driveshaft and all front differential components are always spinning when moving, hence going into A4WD does not bring the parts "up to speed" they already are spinning. (unless there is abnormal drag in any of the front drive components)

Because it is only ~5% you can turn the front driveshaft after breaking loose the slight engagement of the clutches. This ~5% engagement approaches 100% if the TC Fluid is old or low and turning produces the hopping and binding.

If you don't feel ANY drag inside the transfer case when turning the driveshaft, something is probably wrong.

Logically if the front end parts were not spinning while driving in 2WD, the engagement would cause noises and eventual destruction of the front axle disconnect.

 

[Sparky]

IIRC the 5% is only in A4WD mode. In 2HI it is at 0%

 

[DenaliHD66]

YAY FIXED.

Got a real time diagnostic scan, so basically one of the sensors was not reading the speed when the front driveshaft was spinning. The sensor was pulled, cleaned off, and reinstalled with some dielectric grease. Completely solved the issue. Works like brand new. There is now no hesitation at all when shifting between 4wd modes. I don't know if it is just in my head or not but the transmission was shifting smooth like butter afterwards too. Either way, drove around switching between modes and it was fine.

The thing was, when it was initially on the lift, switching between modes is perfectly fine. But as soon as you begin driving in any mode, that's when the light would come on, and it wouldn't shift out of that mode, until engine was turned off and on again. Major repair dodged though.


Was kinda neat to watch the 4WD operate on the lift. When in A4wd mode, the rears would rotate just at idle under no throttle, and the fronts would try to catch up, and if left on idle, the rears would be spinning at say 10 mph, while the fronts were rotating only half that speed.

 

[linneje]

Where was the location of this sensor? Are you talking about a wheel speed sensor?

 

[DenaliHD66]

Where was the location of this sensor? Are you talking about a wheel speed sensor?

It was in the transfer case, not sure which one. I was dressed up, and the melting ice/snow was pouring out from the under side so I didn't join the mechanic in pulling it out.



BUT. I F#@$@$ SPOKE TOO SOON. 4WD LIGHT BACK ON AGAIN. I'M GOING TO STOP POSTING ON HERE because it seriously seems like the opposite happens every time.

Okay, so driving home tonight, everything is fine. Then, Engine light comes on. Okay... keep calm, its just being difficult. Mind you I was in TWO WHEEL DRIVE, the entire 25 mile trip, and then 5 minutes after the SES light comes on, SERVICE 4WD light comes on. WTF.... I was in 2wheel the entire friggin time. The 4wd selector light shut off, as it always does when the service light comes on. So even after shutting the truck off, and even just trying to turn the ignition on, BOTH lights come on. Before, when you shut it off, the 4WD light would go away and it would work again.

Beyond frustrated, considering the day of work I had too..

 

[Playsinsnow]

That sucks man. Not to hijack.... but... does anyone know if the Torque Pro app would be able to read what tripped a "service 4wd" light? Saves a guy some time after a day like above.

 

[linneje]

This really sounds like a ground issue is acting up on you. Someone here can chime in with more knowledge on where to look to troubleshoot this. I seem to remember a ground on the underside somewhere underneath the driver's feet, the bolt holding it and washer getting corroded. Not sure if that is the ground for your problems though ...I think an advanced scan tool might really help here.

 

[linneje]

OK, I checked on the site whose name shall not be mentioned ... as I had recalled some posts about the grounds ... but several posters noted that changing the ignition switch actually solved similar problems.

If you have not done that yet, it is a cheap part that these vehicles are noted to have problems with. Yes, it can actually mess up the 4WD and other electrical systems according to many posters.

 

[The_Roadie]

We say trailvoy 's name all the time around here. We're not threatened like little babies by THEIR existence.

 

[DenaliHD66]

OK, I checked on the site whose name shall not be mentioned ... as I had recalled some posts about the grounds ... but several posters noted that changing the ignition switch actually solved similar problems.

If you have not done that yet, it is a cheap part that these vehicles are noted to have problems with. Yes, it can actually mess up the 4WD and other electrical systems according to many posters.

Aaaaand today it was working fine again at least stationary. I had a lot of traveling to do so I didn't mess with it. Neither light was on today.

 

[linneje]

We say trailvoy 's name all the time around here. We're not threatened like little babies by THEIR existence.

No, I know that, it was meant in humor. I didn't know anything about the background stuff before, and I posted in the old site suggesting they come over here - and when I reviewed it I found that it did not say GMTNation.com ... so I tried it again. They have some sort of automatic substitution so that you can't even enter it in a post.

I also didn't understand the weird posts ... like tons of posts saying things that made no sense or posts like "An informative discussion ...." Obviously they are trying to artificially create traffic.

I get it now, and so that is why my post was meant as a "reverse" type of dig at them.

 

[TheDave]

I am really sorry to you all for not responding back to this post for all the questions that were asked. After reading and thinking about it I decided to replace the TCCM since mine is an 02 and we all know the stories about them. I was able to get to a pick and pull today and get a TCCM from an 2004 TB. I checked the RPO codes and got the GT4 like mine as what I had read is that the TCCM is programed with gearing in it. Many said that it would not really make much of a difference unless you are using the A4WD. I do not really use that position or at least I would not admit it round here. Anyways I had easy access to it as the dash was already torn apart. I just reached up pulled the holder and TCCM out and cut the wires. To install I removed the 2 plates under the steering wheel and the side cover, reached up and pulled the whole bracket out and disconnected the wires. Did the reverse to install. I found I really easy to just replace the whole bracket. Connected the battery and what do you know SUCCESS. It worked. Such an easy job that I could have had my 7 or 10 year old daughter do it. I bet they could have done it faster too with their little hands. I just can not believe that I have heard that people are paying up to $600 for this job. It is so simple and it only cost me $40 and a little amount of time. As to some of the questions I have been very religiously changing my Transfer case and Differentials fluids as well as the other fluids in this Trailblazer. I use it to tow and want to keep this around as I really like my Trailblazer. 155,000 and running great. Thanks again for all your comments and suggestions.

Regards,
David

 

[The_Roadie]

No, I know that, it was meant in humor. I didn't know anything about the background stuff before, and I posted in the old site suggesting they come over here - and when I reviewed it I found that it did not say GMTNation.com ... so I tried it again. They have some sort of automatic substitution so that you can't even enter it in a post.

Yep. It's their way of making sure nobody can spread the truth about where the community went. Type in "gmtnation.com" and they automatically edit it to say "trailvoy.com". Juvenile.

I also didn't understand the weird posts ... like tons of posts saying things that made no sense or posts like "An informative discussion ...." Obviously they are trying to artificially create traffic.

That's usually spammers at work. The payload of their spam is in their signature. Or they search for a thread about HIDs and then post their lame Chinese HID vendor link. All spammers all the time. Since there's no longer any mods on patrol, the spammers have a field day. And Vertical Scope doesn't care, because as you figured, even spammers inflate the post count so they can charge more for advertising on such a HUGELY POPULAR site. Liars.