Need advice on "SERVICE 4WD" warning

[TJ]

I have a question for some of you experts out there. We went out off roading in some local mountains yesterday. When coming out of 4LO/rear axle lock, we heard a loud "clunk", all transfer case indicators on the dash went out and the message center popped up with the "SERVICE 4WD" warning. We restarted the engine, message disappeared, transfer case indicator lights began working and all seemed fine again. Any ideas on what might have caused this?

[TJ]

I have a question for some of you experts out there. We went out off roading in some local mountains yesterday. When coming out of 4LO/rear axle lock, we heard a loud "clunk", all transfer case indicators on the dash went out and the message center popped up with the "SERVICE 4WD" warning. We restarted the engine, message disappeared, transfer case indicator lights began working and all seemed fine again. Any ideas on what might have caused this?

[Doug]

Were you in neutral and stopped when switching out of lock? Check the manual, I beleive when switching in and out of lock this should be the case. ...Just a thought!

[TJ]

Yes, in neutral, going about 1 mph

[SJ]

TJ:

I've only used 4LO and Rear Axle Lock once. There is a "clunk" when the eLocker is engaged - the dealer stated that this is normal when they gave me my orientation back in July. The Owner's Manual also states that a "clunk" is normal as the splines engage.

What you don't want to hear is a grinding noise. I've blown 3rd members on previous vehicles and that's when a flat-bed is brought in to haul to the nearest shop.

Does your H2 drive OK in 4HI? Are the panel lights still off or is 4HI illuminated during normal driving? Have you tried re-engaging 4HI Locked or 4LO Locked?

My GMC Yukon had the same service light, but I was able to reset the message. The Transfer Case's electronic sensor detected high temperatures and thus gave a warning.
I don't know if the H2 is equipped with the same sensor, but excessive heat may have caused a temporary problem.

Last question: were you running fast (e.g. above 25 mph) in 4LO w/ Rear Axle Locked?

[Doug]

????? Time for a service check, just to be safe. Good luck!

[TJ]

SJ - We were creeping in 4LO/rear axle locked, maybe 5 mph tops, very temporary situation before shifting back up. It wasn't exactly the clunk that concerned me, we've heard it before and understand it's normal, but this was somehow a much louder clunk, then all indicator lights went out and we got that SERVICE 4WD message. After restarting the engine, all the indicator/panel lights started working again. It seems to be running fine in 4HI and also in 4HI lock, but didn't try 4LO again. Truth is we were probably afraid to--we were on some very remote roads in the middle of the snow, not another soul in sight, although we could have used our OnStar [img]/infopop/emoticons/icon_eek.gif[/img]

[txsurveyor]

My h2 has 14,700 miles and the service 4wd light comes on during real cold weather (not freezing, but plenty cold to those of us here in the south) in 4hi. It goes off after punching the reset button and does not come on again after warm up. I suspect it is the sensor. Anyone else has this situation especially those of you in snow conditon?

[SJ]

The only thing that freezing temperatures impact on my H2 are the folding mirrors - no big deal.

There are so many sensors and computer controlled devices on today's vehicles that it becomes difficult to determine some problems, unless you have the proper diagnostic equipment.

I used to get a loud clunk on my old Chevy Blazer, when the Transfer Case had not yet engaged the Front Driveline until The vehicle had moved forward for a couple feet. Since our 4WD operation is entirely different and torque is split front/back to begin with, I'm at a loss for what the problem may be.

I'd call the Hummer dealer and get their take on what it might be. Further problems warrant a trip to the service department.

[TJ]

Thanks...I'll call the service manager tomorrow morning and see what he says. We were going to need to take it in to get our side mirrors fixed again anyway, along with a couple of other things, so I'll let you all know what happens.

[Steve R]

Hey there....don't be alarmed.

I too have been in tough situations and had a complete system freak-out. I've seen it happen on two other H2's...it's no biggy: just stop the vehicle and park like you normally would....then just shut off the engine. Wait a moment or two...and then turn her right on again. The whole system will do it's run-up and your good to go!

It's literally just like our home computers...just gets wiggy now and then. Nothing to worry about! You probably had a lot of load on the gears when you made the transition in gearing/locking...that's why it was extra-loud. It probably just freaked your computer.

Play with it some more...try to make it fail: I'd bet you got nothing to worry about at all! Been there, seen that....no biggy! [img]/infopop/emoticons/icon_biggrin.gif[/img]

[Hummie2]

Steve R is right. Its no biggie.

What happens is when you are in 4lock (either high or low) the front axle and rear axle don't travel the same distance when you turn corners and go over uneven terrain and this results in a tortional load on the gears in the drivetrain if you are on a hard surface. Thats why there is a differential in the transfercase that operates in "4 high open". This differential in the transfercase is locked solid in "4 lock". I have seen manual shifted transfercases that you could not shift out of 4WD no matter how hard you pulled on the shift lever. The solution was to backup a few feet while keeping pressure on the shifter and when things unwound alittle it would shift out usually with a loud clunk.

The H2's electric shifter on transfercase didn't have enough power to overcome the torque load on the gears. When you requested the shift into "4 high open" the computer starts a timer and if the shift doesn't happen within the alloted time the shift aborts and triggers the warning message. This is a sign that you have been in 4lock on too hard of a surface and there was no slippage in the tires to relieve the tortional twist in the geartrain. If it won't shift out try backing up a ways to unwind the twist and try the shift again.

Tortional load or twist places alot of unnecessary wear on tires and gears. Only use 4lock when on a slippery or poor traction surface otherwise stay in "4 High Open".

Hope this explanation was clear.

Don

[Steve R]

Don....I hail you my superior!! awesome/perfect explanation!! (I learned something myself...I thought it was just a class-D fully-suspended gaseous gremlin causing the problem)

[TJ]

Torsional load certainly makes sense, but we shifted into 4LO lock when we were going up an incline over snow and ice. The terrain was a little uneven in spots and not really very steep, but it did seem plenty slippery.

I did call the service department yesterday and he said that it was probably because we were rolling forward and weren't at a complete stop when shifting out. But, I read the manual and it says the "preferred method" is to have your vehicle moving at 1 to 2 mph. So which is better, a complete stop or 1-2 mph? Then I'll do like Steve said and go out and play with it some more [img]/infopop/emoticons/icon_biggrin.gif[/img]

[Hummie2]

Its much easier for gears to come into mesh when they are moving at nearly the same speed than it is when they are completely at a stop. 1-2 mph will not harm anything, but anything faster might cause a pretty good bit of gear clash.

The dealer might be right, but one thing for certain the geartrain had a load on it ( either tortional twist or load from the rig not being on level ground) or it would have shifted. Play with it a while and see how it does.

Don

P.S. Make sure transmission is in Neutral when shifting transfercase. This will help prevent load on the gears from the vehicle not being on level ground.

[This message was edited by Hummie2 on December 27, 2002 at 10:50 AM.]

[TonkaH2]

. . .a few times in a row as I was on and off-road one day. But then it locked in 4-LO. The electronic transfer case [they call it a selector] died at about 6K. Left me stuck in 4-LO 300 miles from home on a weekend. It's the same one as on the H2. Call me gun-shy, but I'll be careful about using the push-button options exactly as directed. And I think you may be able to get it to malfunction if you choose one setting and then change your mind -the computer may not have a proper 'ingore the next command' sequence.

Anyway, I wish for a good, old-fashioned transfer case mounted lever.

-Jack
Yellow H2 adv. on order
"I think I can make it. D'OH!"
www.sunspotnatural.com