GMC 99 pickup trans noise

marc · March 1999

I have a new 99 GMC ful size 4x4 ext cab. When I
cruise at about 40mph I can hear and feel a
vibration, it reminds me of a road noise like when
you run over those groves in the road to warn you
you are getting close to a stop sign. Kind a like
snow tire noise, when you speed up a few mph it
goes away. Someone told me it might be the locking
toqure converter? What could it be?

quadrunner500 · March 1999

I've heard it in mine too. My guess is that it is probably normal, since so many people seem to be reporting it. My truck is going into the shop later this week, and I'll ask them about it.

3436 · March 1999

I have the same noise in mine. I do not think it is normal it did not vibrate when I test drove my truck. It came later after a 1000 miles or so I took it back to the dealer who called gm. Gm said they know about the problem and the engineer's are working on the problem.

jxyoung · April 1999

I have noticed the same noise in the last few weeks! I now have 1800 miles and it does it at right around 40 miles per hour. When you are just cruising along at that speed. I notice if I am accelerating through I do not hear it? If any of you find any more info please let us know!

richram123 · April 1999

It is the lock up torque converter. Shift the trans into drive (not o/d) and see if the vibration at 40 mph goes away, I bet it does.

P.S Most all vehicles with overdrive experience some shudder at the torque convertter lock up speed. If you accelerate hard thru 40mph you probably wont feel it.

Rich

quadrunner500 · April 1999

richram123,
You might be right, but I never noticed a shudder in my Z28 that also has the same 4L60E electronically controlled automatic transmission used in the 1/2 ton Silverado. There could still be differences between the two. Mine's in the shop right now, and they are testing it, but I don't know what, if anything they have found yet. But I wish I had tried shifting into drive (3rd) to see if it would go away...I suspect it might.

bob85 · April 1999

I've posted on this noise/vibration previously. I have a 99 Sierra 1500 4x4 and my vibration/noise started around 500 miles. It usually was most noticable around 30 to 40 mph and while I was at low RPM's. Took it to the dealer several times and only with persistance did they finally find it was coming from the front transfer case. GM sent an entire replacement and they switched it out. I now have no vibration or noise at 40 mph. Dealer said that the front case was "out of harmonic balance". You all may want to bring this possiblity up with your dealers.

quadrunner500 · April 1999

bob85,
Thanks for the info, but I'm pretty sure what I heard was not coming from the transfer case, nor does it seem harmonic related. What I hear is perhaps different than what you were hearing. The noise was there from the first mile, never got worse, stayed exactly the same at 3,400 miles. I hear it at almost exactly 40 mph, but just barely, and it's gone by 41 mph, like a switch. I'm inclined to go along with the lock-up torque converter theory, since there aren't that many parts rotating inside the transfer case when 4x4 is not engaged. The service advisor had the technician spend quite a bit of time with it, and he feels it is normal, nothing to worry about. From my experience, G.M. will fix drivetrain problems even beyond the warranty if there is a service bulletin about a defect that comes out. I'm on record now for reporting it, should there be a problem in the future. I could demand more for the sport of it, but I'm persuaded this is a normal condition since other owners are saying about the same thing, nor am I bothered enough to want them tearing into everything. I would rather not cry wolf, in the hope that when I really do need help, they will be there for me.

quadrunner500 · April 1999

By the way, I had copied your previous posting (and several others) about the problem, and submitted it to the service advisor when I brought the truck in. It never hurts to let them know that you are communicating with an owner's group.

jxyoung · April 1999

I fell the same as Quadrunner. I will mention it if I need to bring it in but I do not want them to start swapping things unless they are sure of the
cause! I have heard similar noises from other OD trans vehicles. It sounds like when the vehicle goes into the very low rpm's at higher speeds(30 to 40) is when it happens. I almost thought it sounded like some resonating in the exhaust system??

mwinter1 · April 1999

I have this same noise. This started right from the beginning. It happens at a low acceleration when I reach the 40mph mark. I too thought it was some sort of exhaust low frequency vibration. I was surprised to see so many other people that have the same experience. My noise does not go away past 40mph, it just seems to be less noticeable due to engine/road noise. Has anybody had a concrete answer yet. I'm going to inform my dealer today. I only have 500 miles on my truck.

tungle · April 1999

I have same problem with my Sierra. Around 40
with OD on, felt a little vibration. Passed that
speed no more noise, or at least not so audible.
I will try to use the tow/haul mode around that
speed to see if the vibration is still there.

Anyone who has a definite answer to this, please
pass on. By the way, I've been having this vibr
since the first day. Besides that, everything is
great.
Tung

tungle · April 1999

tested last night. With either tow/haul on or shift in 3, no vibr/noise was observed at that speed. To me, it seemed to be similar to the "over gear" problem on a manual trans car.

Can someone please explain what over-drive really
is, and relate that to some analogy on the manual trans, if possible, so I can understand? This is my very first auto trans vehicle.

Currently, I plan to either drive with tow/haul on or 3rd gear if I have to drive at around 40 for
quite some time. Which is better, tow/haul or 3rd?
Please advice me. Thanks.

haulin · April 1999

My 89 3/4 ton does the same thing. It shifts into OD at 37-38 MPH. Some things never change I guess.

g8tr · April 1999

Checked out the 40 mph vibration/noise--nothing doing. I kept it in OD and it shifted at 40 mph and was turning 1000 rpm. I felt a slight vibration but it quickly went away as I increased speed. I guess that since its turning such low rpms in high gear is what makes the truck vibrate. In 3rd I never feel it. I even tried it with the AC on and while I was pulling my boat and did not feel it. My truck is 2WD, 4.8L, 3.42, reg. cab. It does, however, shift pretty hard into 3rd gear. It does not do it all of the time. It does not do it if I accelerate slowly or if I'm carrying a load. If I accelerate pretty hard, the shift into 3rd is pretty harsh. I'll keep an eye on it and see if I can narrow it down.

quadrunner500 · April 1999

tungle,
Overdrive just means that when you are in top gear, the transmission output shaft (drive shaft) is turning 30% faster than the engine crankshaft. This gets reduced by the ratio of the rear end gears, so that the drive axles are always spinning slower than the engine crankshaft. The meaning of overdrive is essentially non-important. Final drive ratio, which includes the rear end gears is the actual amount by which torque going to the rear wheels gets multiplied. I think 3rd is better than tow/haul if you will be doing most of your driving around 40 mph. But it will hurt economy. Is that vibration really than annoying? I barely notice it, although I know it is there. I am much more bothered by the occasional light engine ping I get driving at 50-55 mph.

tungle · April 1999

Quad,

Thanks for the info. Even though you said OD only
affects the top gear, I somehow saw that RMP was
higher for all (maybe except 1st gear) gears. Does
it means that OD gives us "extra gears" (like 1.5, 2.5, ...)? (I had a hard time express myself
here).

I did not concern about the annoyance caused by
the vibration but more about the health of the
trans itself. For now, I probably shift to 3rd when driving constantly around 40mph.
Thanks,
Tung

quadrunner500 · April 1999

tungle,
No extra gears with overdrive. Overdrive IS the 4th gear. The only reason it's called overdrive is because it's the only gear that turns faster than the crankshaft. The ratios in the transmission are something like this, give or take...
1st - 3.50:1
2nd - 2.70:1
3rd - 1.00:1
4th - 0.70:1
Numerically speaking all are greater than "1.0" except 4th which is the overdrive gear. It would be fair to call all the others "under-drive" gears! By the way, each ratio expresses engine revs to driveshaft revs. So in first gear, to get one turn on the driveshaft takes 3-1/2 turns of the engine. The term "overdrive" was frequently used to describe a tall highway gear in the days of 3 speed trannies, in the same understanding that a "granny" gear was used to describe a low-low 1st gear for creeping, towing etc. But no extra gears. Overdrive is pretty meaningless since all trucks today have a 4th gear which turns faster than the engine.

quadrunner500 · April 1999

Ooops! Third gear is neither an overdrive or underdrive, since it is exactly 1.0:1. When you are in third gear, engine rpm exactly equals driveshaft rpm.

quadrunner500 · April 1999

mwinter1,
Here is a link you can go to a G.M. site where you enter your VIN#, and they show all the calibration updates for the powertrain control module, PCM. When they have one for the transmission shift schedule, you can find out here.
http://calid.gm.com/vci/

mwinter1 · April 1999

Thanks Quadrunner:

I printed out a copy over the weekend. Now, we'll just have to wait for the GM recal.

GMC 99 pickup trans noise

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