2002 Trailblazer - Random Service 4x4 light

[fixnux]

I was driving in the city so I had A4WD. When I got out of heavy traffic I forgot to put it back in 2HI. Anyway randomly the "Service 4x4" light turns on. The light switches from A4WD to 4HI. I was going up a pretty steep hill that was unplowed and it was probably about 7-8in of snow. The 4x4 was working as I switched it to 2HI at the top of the hill. If the 4x4 wasn't working it would have been some work in the heavy snow.

Restarting the vehicle removed the light and any codes and everything works fine. But wondering if it's a "BTW asshole I'm going to cost you a lot of money soon, HAHAHA"

I checked for any fluid spray. Nothing. I didn't have my notebook with otherwise I would have gotten codes from HP. I'll keep that in my backseat if it happens again.

The previous owner was older and the truck is in great shape for going on 12 years old and 216,000mi. I checked fluids and they are flawless clean and right where they should be.

Anyone else ever seen that? Any advice would be welcomed.


EDIT

Probably doesn't matter but it does have the G80 diff. The tires are new (about 2k mi) and air pressure is EVEN in every tire.

 

[TollKeeper]

Sounds like either the front axle actuator is sticking, or the transfer case encoder motor.

By the sounds of it, you should have had that truck in 4x4 anyways.

Now I will warn you, you will probably get a bunch of replies telling you to never use the A4WD or something. I use it all the time, and have never had a problem with it. You just should not rely on it solely, and realize when road conditions justify putting it into 4HI. This will also save you wear and tear on the TC clutchs.

 

[fixnux]

Sounds like either the front axle actuator is sticking, or the transfer case encoder motor.

By the sounds of it, you should have had that truck in 4x4 anyways.

Now I will warn you, you will probably get a bunch of replies telling you to never use the A4WD or something. I use it all the time, and have never had a problem with it. You just should not rely on it solely, and realize when road conditions justify putting it into 4HI. This will also save you wear and tear on the TC clutchs.

I should have switched it. Long ass day shoveling snow.

I can hear a whine when I switch it now, so I suspect you are correct. It didn't throw a code but it's a obvious whine. I didn't try to re-engage but 2HI was fine
Already made tracks up my drive so.

I used the A4WD at times when it wasn't needed today but it was fun and the truck new. Figured use it a bit more than normal and see what works and what doesn't.
But even in 2HI it wasn't bad. I used 2HI more than anything today. Although a lot of people don't seem to like the G80 diff. It seems nice so far.

Thanks

 

[TollKeeper]

I love the G80. My issue is my 3.42 axle. Gearing is just to low! But it makes better use of torque for when the slippery stuff is out there. Just never seems to be in the right gear when I am pulling a trailer.

 

[Matt]

Besides the TCCM, it could also be a flaky 4WD switch. They are know to spaz out, which mine has done once, but hasn't done it again...one day I might actually change it as a preventative measure.

 

[Inert_Static]

Sorry to jump in, but is it better to leave it in 4hi than auto4wd ?

 

[fixnux]

Sorry to jump in, but is it better to leave it in 4hi than auto4wd ?

From the truck manual, it said to use a4wd when road conditions warrant. However DO NOT drive around town with 4HI or A4WD if it's not needed. That will burn your TC up in no time.

If you know you'll need 4x4 it's less stress to switch. At least that is what I got from the book and various reading here. a4wd if the road is shitty is less stress than full on 4HI.
But drive with caution and use 2HI as much as possible.

Others can probably give you more info but from my understanding of a4wd it engages the TC with 5% pwr vs 4HI which switches full power to front/rear. Although
I thought my manual said it engages in standby. But I could have read it wrong.

 

[fixnux]

Just to close this thread. It ended up being the encoder motor. Super easy fix, but the motor is pretty expensive.

Thanks all!

 

[Playsinsnow]

Inert: in auto4 there is a fork that slides over a collar gear when slip is felt in rear. Metal on metal at whatever speed. I had to replace my disco because I "beat on my 4wd"

Now I use 4hi and only shift in and out at low speeds, stopped if possible. Or get the AWD sleeve that keeps the front locked and bypasses the need of a fork. Check out offroadtb.com and search disconnect.

 

[tbyoda]

Inert: in auto4 there is a fork that slides over a collar gear when slip is felt in rear. Metal on metal at whatever speed. I had to replace my disco because I "beat on my 4wd"

Now I use 4hi and only shift in and out at low speeds, stopped if possible. Or get the AWD sleeve that keeps the front locked and bypasses the need of a fork. Check out offroadtb.com and search disconnect.

IIRC That "fork" you talk about is in the disconnect and when in A4wd or 4whi the fork moves the gear shift collar to connect the inner and outer gear to keep the front locked the thought the cv shaft and intermediate Shaft. As stated in a4wd the transfer case is now at the 5% load and when tire slip is in the rear the transfer case can engage the front 5-100% and shock load the clutches. Hence the wear issue and/or damage. I am sure Roadie will chime in. But this should help.

http://www.offroadtb.com/articles/tech-docs/transfer-case-tech-the-nvg-226-np8-theory-of-operation/

http://www.offroadtb.com/articles/tech-docs/front-axle-4wd-disconnect/

 

[Playsinsnow]

Yep, those are a couple. I read a few before replacing my disconnect two years ago.

 

[fixnux]

That is pretty neat. Guess I'll be doing that soon.

My only concern is void a warranty.

 

[The_Roadie]

Glad you're working better now. Just make sure you religiously keep up with the 50K mile transfer case fluid changes.

Also, nothing the encoder motor can do will cause a spontaneous mode change from A4WD to 4HI or any other mode. Only a switch with flaky contacts or a much more rare connector oxidation can do that. Recommend running the switch full CW to CCW 50-100 times to clean up the contacts, or consider replacing it with the 2005+ redesigned switch (the one with arrow international 4WD mode symbols instead of the words HI and LO), which is shockingly cheap even at full retail.

 

[fixnux]

Glad you're working better now. Just make sure you religiously keep up with the 50K mile transfer case fluid changes.

Also, nothing the encoder motor can do will cause a spontaneous mode change from A4WD to 4HI or any other mode. Only a switch with flaky contacts or a much more rare connector oxidation can do that. Recommend running the switch full CW to CCW 50-100 times to clean up the contacts, or consider replacing it with the 2005+ redesigned switch (the one with arrow international 4WD mode symbols instead of the words HI and LO), which is shockingly cheap even at full retail.

I haven't changed the motor yet. I was going to take it in but the weather turned bad and I wasn't able to get out of my drive. Don't know yet what it is. Hopefully my drive gets cleared this weekend. However is there a vacuum line to the TC for shifting?

 

[The_Roadie]

There are no vacuum-controlled anythings on the vehicle. People sometimes think the HVAC actuators are, but that's an old design concept that our platform doesn't use. Same with the TC. All electric, with a position feedback sensor that tells the TCCM if a command succeeds or not.

 

[fixnux]

There are no vacuum-controlled anythings on the vehicle. People sometimes think the HVAC actuators are, but that's an old design concept that our platform doesn't use. Same with the TC. All electric, with a position feedback sensor that tells the TCCM if a command succeeds or not.

That's what I was seeing from reading threads, too. Thanks for the conformation. I have very little knowledge of 4x4.

From the reading I have done, I hear a motor trying to engage on the front driver side. If I understood correctly that would
imply the encoder motor?

As far the truck switching to 4HI from A4WD, I have no clue why it did that. It won't switch to any mode now. I have moved the switch CW/CCW about 100 times to no avail.

Is there any way to pull the encoder motor and test it? Or is it a crap shoot? It's pretty easy to remove anyway.




Off topic:
From some reading, replacing a 02' TCCM for a 03' is a good ideal? Even if it's not a problem now, it seems like it could be down the road. Could be mistaken.

 

[The_Roadie]

The front noise if it's on the passenger side, a higher-pitched whine for around a second, that ONLY happens in the 2HI->A4WD (and vice versa) transitions, is the front axle splined disconnect actuator. The transfer case encoder motor is a lower pitched noise, on the transfer case, that only takes 1/4 to 1/2 second to move, and it makes a noise in every possible mode transition. Get a second person to listen for noises - you can do all this with the ignition on but the engine off, to make it easier to troubleshoot.

If you have a problem changing modes, what is the behavior of your switch lights? If they're all dark, and the dash "Service 4WD" light is on, then your TCCM has lost communication with the PCM and it's dead or flaky. Swap it with any 2003+ unit. Junkyard or a used one is better, because you don't have to waste money at the dealer getting it programmed. Virgin ones from a supplier like a GM source need to be configured, and dealers often charge $100 for a 10 minute job.

You can pull the encoder motor and see if you can make it move with your switch, but it all depends on if your TCCM is seeing the switch and that story is told in the behavior of the lights on the switch. Describe in a lot of detail what you see if anything.

 

[fixnux]

The front noise if it's on the passenger side, a higher-pitched whine for around a second, that ONLY happens in the 2HI->A4WD (and vice versa) transitions, is the front axle splined disconnect actuator. The transfer case encoder motor is a lower pitched noise, on the transfer case, that only takes 1/4 to 1/2 second to move, and it makes a noise in every possible mode transition. Get a second person to listen for noises - you can do all this with the ignition on but the engine off, to make it easier to troubleshoot.

If you have a problem changing modes, what is the behavior of your switch lights? If they're all dark, and the dash "Service 4WD" light is on, then your TCCM has lost communication with the PCM and it's dead or flaky. Swap it with any 2003+ unit. Junkyard or a used one is better, because you don't have to waste money at the dealer getting it programmed. Virgin ones from a supplier like a GM source need to be configured, and dealers often charge $100 for a 10 minute job.

You can pull the encoder motor and see if you can make it move with your switch, but it all depends on if your TCCM is seeing the switch and that story is told in the behavior of the lights on the switch. Describe in a lot of detail what you see if anything.

Regardless of where I move it, A4WD, 4HI (didn't try 4LO) it blinks about 10 times then "Service 4x4" comes on and the 2HI light is lit up.

I'm also trying to understand removing the disconnect fork.

Offroadtb.com Front Axle 4WD Disconnect

If you don't mind, some quick questions.

I assume from reading 2HI doesn't hop as someone on here or another forum said it didn't.

If in A4WD, what is the difference or benefit?

And does 4HI act the same?

Sorry to ask questions off topic.

 

[The_Roadie]

Your encoder motor or transfer case is jammed. Definitely recommend removing the encoder motor from the case and see if the behavior changes.

Removing the splined disconnect assembly only needs to be done if you're trying to renew the grease or fix a bad one. What do you need to do?

2HI will never crow-hop, unless the transfer case is seriously borked.

A4WD is almost never useful except to lazy drivers who like to leave it on one mode and never work the switch. It gets 4WD mode "ready to go" but unless and until it detects wheel slippage, it doesn't send much torque forward so the front wheels don't help much. When it detects wheel slippage, the sudden appearance of torque on the front wheels, combined with spinning in the rear, can cause the vehicle to break loose and go into an unexpected slide. Drivers used to full-time AWD can crash after this surprise behavior.

If it's slippery enough to temp you into using A4WD, just go into 4HI. Front and back are locked together and you get the full benefit of the system. If the pavement is merely wet, that's not worthy of 4HI mode, and you'll crow hop. Stay in 2HI mode.

 

[fixnux]

Your encoder motor or transfer case is jammed. Definitely recommend removing the encoder motor from the case and see if the behavior changes.

Removing the splined disconnect assembly only needs to be done if you're trying to renew the grease or fix a bad one. What do you need to do?

2HI will never crow-hop, unless the transfer case is seriously borked.

A4WD is almost never useful except to lazy drivers who like to leave it on one mode and never work the switch. It gets 4WD mode "ready to go" but unless and until it detects wheel slippage, it doesn't send much torque forward so the front wheels don't help much. When it detects wheel slippage, the sudden appearance of torque on the front wheels, combined with spinning in the rear, can cause the vehicle to break loose and go into an unexpected slide. Drivers used to full-time AWD can crash after this surprise behavior.

If it's slippery enough to temp you into using A4WD, just go into 4HI. Front and back are locked together and you get the full benefit of the system. If the pavement is merely wet, that's not worthy of 4HI mode, and you'll crow hop. Stay in 2HI mode.

I was asking about the fork disconnect for the next TC in the vehicle. The truck has 216k miles on it so looking at various upgrades if and when parts go bad.

This is my first vehicle with 4x4 other than a F150 which I rarely use. Maybe 2k miles a year. Most those miles are during the summer, not winter.

A4WD was something I tested during the 1st snow. There was a lot of snow and it was ice on slush with 4-6in of snow. It was nice. When it locked, it was like dry pavement. Admittedly, I drove a little more aggressive than I would on my previous 2WD vehicles. I only recently bought it and it has 216k mi. I wanted to push it more than I normally would. Glad I did, a problem cropped up.

Just wondering how removing that piece on a rebuild TC would effect the different functions. More so, would it remove any warranty? I learned a huge lesson when my car was totaled last year, if you replace a part, get a warranty. Could be wrong on that assumption, too. Just trying to see if it's practical. Regardless, 20k out the window on a car that was nearly fully rebuilt. A lowly $1800 after some shit bag ran a stop sign... I'll quote him, "My GPS didn't say there was a stop sign."

 

[The_Roadie]

The full-time A4WD platforms like the Bravada (using the NP126 transfer case) don't disconnect the intermediate shaft from the right side CV shaft ever. The sliding collar doesn't slide, so it's stronger. The only reason anybody changes the disconnect to a full-time connected unit is to increase the strength for very, very aggressive rock crawling. Anybody into that sort of fun needs to join us over at offroadtb.com. Lots of other necessary mods (like armor and lockers and lifts and tires) before that specific one becomes useful. The downside is that you're always spinning the front differential guts, and that can wear out parts or the front driveshaft seals.

The fork and sliding collar are part of the front axle disconnect, not the transfer case.

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