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4x4 head scratcher

bison

bison

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Location
Near Peace River-Alberta
Got a problem with the Wife's truck.
Put it in 4 x 4 and the truck starts bucking at 30-35 km,under and over that its ok.drives smooth as a baby butt

When in bucking range the engine is sluggish to make speed
No bucking experienced in 2 whl drive.

4 x 4 is positively engaged in the T-case and front axle.It don't appear to jump out of gear either.

I have the feeling the AT is hunting between gears if that would be possible at all.But why only at between 30 and 35?
Speed sensors wacky???

When i start out in second till i reach 40 km i can upshift manually to 3 and then 4 and there's hardly no bucking.
No problems in reverse in either 2 wh or 4

I didn't check for codes,no SES light though.

your collective thoughts please
 
Different tire diameters will definately make a 4x4 buck. Even with same sizes but different psi. It's always at certain speeds too. With my truck it's right around 45 mph
 
Bucking like an engine miss or bucking because the truck has a disagreement somewhere? I'm guessing a mechanical binding somewhere rather than electrical. Hows the front end alignment? Could affect things when the front wheels are driving rather than just along for the ride. Front driveshaft ujoints worn out? Mechanical problems seem to always have a "perfect storm" for speed when the cosmos align just right for the problem to be the worst. 30-35 could just be it.
 
Nope on the tires and drive shaft .
the problem happened a couple times last winter but started to do it more this winter,now it is every time when driving in 4x4,slow down at a corner to below 30 and trottle up on the straight it has a definete violent buck to to it,like i said it feels like rapid up and down shifts in the tranny.
same when just taking of,smooth going till i hit 30.
 
bison said:
But why only in 4x4?
Click to expand...

I wish I knew.....But then again PCM won't give you lock up till operating temp and when you have a unplugged CTS you have no L/O either.......So is that PCM or tranny control?....Or both?

Got to be something like speed sensors or some kind of tron tranny malfunction......But you already figured that out.




Change the Batts and clean your grounds, they seem to fix almost everything tron related on these Dinosaurs......jk:rolleyes5::);)
 
Does it seem to correlate to a certain RPM and speed ? Or is it just speed?

An accumulation of slop in driveline that now has a harmonic at that speed due to tire and differential differences and leaf spring wrap / unwrap like BT said. I suspect its at a sweet spot in rpm that just a little fuel changes torque output of crank appreciably and you get a little "feed back loop" of add fuel takes away fuel. Its the pcm is trying to maintain smooth rpm control and might have some feedback from your foot too. If its violent your leg will move ever so slightly and change fuel just a bit.

Similarly, I can get into a little hop, skip, skid and tire bark at idle control or very light fuel in 2nd gear driving slow (rolling through a parking lot when the truck wants to roll and coast) if PCM control gets out of sinc with some leaf spring wrap/unwrap. Its fairly random and I have grown accustom to when it might happen and am better at anticipating it and getting out of it smoothly. The first couple of several times I thought what the heck something has to be wrong but it seems normal just a funky combination of torque feed back.
 
I'm pretty sure it has to do with the drive line.It is solely speed related.
One other thing that bugs me is that when i step on it to get out of the that speed window the engine is sluggish to respond like its running out of power and the bucking becomes more like a bad vibration.

Pull it in 2H and all symtoms disappear,wait till te speed is over 40, shift on te go to 4H and all is fine.

I'll run GMTDscan on it one of these days ,see it it tells me something.
 
Based on what you've described so far, I would lean on mechanical driveline issue.

Under load is smooths out.
Speed sensitive.
You stated it's getting worse, as in it wasn't as bad last year.
Sounds like a joint going bad.

What happens in a turn? In 4x4 and 2x4?
What condition are the drive shaft universal joints?
Condition of the CV joints / knuckle joints, do they have play?


I had a outer CV that made noise at various times. The CV seemed tight in the truck. But once the shaft was removed, you could feel the rough spot in the tulip. Changed the axle and the noise went away.

Don't over look the obvious. I've spent a lot of money that way.
 
I wonder if you have some kind of binding in the transfercase? Simon, I wonder if one of the two solenoids in the transmission are acting up. I had one act up last winter but mine just wouldn't shift into the higher gears.

It sorta sounds like it could also be electrical. But it is strange that it just happens at that speed. Wish you luck on this one. If anyone can figure it out you can.
 
doober said:
I wonder if you have some kind of binding in the transfercase? Simon, I wonder if one of the two solenoids in the transmission are acting up. I had one act up last winter but mine just wouldn't shift into the higher gears.

It sorta sounds like it could also be electrical. But it is strange that it just happens at that speed. Wish you luck on this one. If anyone can figure it out you can.
Click to expand...
Oh , i'll figger it out somehow but i was hoping some one would have the holy grail on it.
Might save me a lot of frustration:agreed:
 
I think i got it figured out.
Been picking my Wife's brain(they are and work deffinetly diff than a male brain) about the when that hickup started(she drives the damn thing).Turns out she used 4x4 only one time last winter and it bucked then so she quit using it.she did't tell me that till last night (I had understood it bucked only once in a while).This winter because of the lack of snowplowing she leaves it in 4x4 all the time lately and had trouble all the time

I replaced the T-case 2 yrs ago(jumped out of 2H) with the one out of my 95 parts truck(same yr but standard tranny)Hers is AT.
The shift and speed sensors where diff between the 2 T-cases and won't fit each other,the connectors where diff too.
I can't remember which one but i think the shift sensor of the donor T-case had 2 pins,the AT truck wire harnass had 3,It lacked a ground pin.i robbed power from the hot wire of the harnass to power a solenoid to get that ground when 4x4 was activated.I think that part works OK.
On the speed sensor the wire colors where the same,so i just spliced the matching connector to the harnass .

I think that speed sensor is not working in cue with the Tranny speed sensor Cause a Standard tranny has only one speed sensor and a AT uses 2.
When i said it feels like the tranny is hunting between gears at low speeds,it might actually be doing just that.

Does that makes sense to ye?
 
You last post is in line with rapid transmission shifting due to possible speed sensor issues. There is a switch that shifts the speed sensor for 4 Low, switch being bumped by ice etc? I have the speed sensor wires with the insulation falling off of them and shorting them out. I replaced them and they are expensive connectors!

Sluggish acceleration is because the TCC has locked up. Mine will lock the TCC and stay locked after rapid gear hunting. It will change gears fast enough to make me worried about hard part damage.

Still hunting this down on my 1993. I do not see any fault in the TPS, computer swapped with spare...
 
Before going trough the process of rebuilding the orig T-Case,
I figured on just swapping the tailpiece and reluctor wheel from the failed AT mounted T-Case with the wrong one that is in the truck right now.

The VSS reluctor wheel in the Orig T-case is 2 3/4" in dia,the shaft its on is 1 5/8" dia.
Has any of you perhaps a New Process T-case from a standard tranny kicking around to get the dia of the reluctor wheel and shaft size.
They prob fit each other but i ain't sure.
 
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