4L60-E Tie-up when shifting to 4th (Overdrive)

[clayndwoods]

What would cause a 4L60-E to entered a tie-up (starts to lock up) condition when shifting to 4th? Is this a common failure mode in this particular transmission.

It operates as advertised when the shift lever is in the "3" position. It operates normally in the "D" position as well until it shifts to 4th (overdrive), which occurs around 45mph under light throttle/load. The TCC is operating properly, as you can feel when it kicks in/out.

 

[shanba]

The forward clutch is not releasing properly, or the 2-4 band is coming on too aggressively.

 

[clayndwoods]

So the forward clutch is applied in 1-2-3, but released in 4 and reverse (reverse works fine, so this will give me a hint).

I do have the PCM programmed to be more firm, and I can reprogram it in about 5 minutes, but it worked fine for a year, just happened as I was driving down the road with very minimal load.

 

[clayndwoods]

On second read (making sure I understand), by "forward clutch" are you referring to the forward friction clutch pack, or the sprag clutch?

I'm hoping against hope (which doesn't look very hopeful at this point) that I am reading the manual wrong, but possibly the sprag clutch is locked up?

 

[shanba]

I was referring to the forward friction clutch. The forward sprag clutch is a one-way clutch device. If it is locked up (not free wheeling), than the clutch timing becomes much more significant to shift feel. This could be the issue. The forward friction clutch release timing is not significant if the sprag is operating properly (I believe). Also, if the coast (or overrun) clutch is dragging, this could cause poor shift quality for the 3-4 shift, but I think this symptom is pretty rare.

The band could come on too aggressively if calibration is too aggressive (line pressure too high) or servo spring is broken.

The best way to diagnose this is with pressure gauges on the suspect clutch ports, if they are available on this trans.

 

[clayndwoods]

Well, it's not really a "poor" shift quality, it's trying to lock up. In other words both the in and output sides are dragged down.

I did find that in 3 (as opposed to D, which is OD or 4) it does operate but not exactly. when it downshifts into 2nd the sprag is supposed to be overrun (or coast). It does not and I hear the same pained sound that I hear when it's attempting to lock up on the 3-4 shift, so I am almost certain it's the sprag clutch at least. It's locked up, and I understand this isn't an unusual failure mode on this transmission.

If so I can only hope it stays locked up for a trip to Houston (appx 3 hours) so I can get it on the lift and pull the transmission. I hate that.

Oh well, it will give me the opportunity to change the spark plugs and wires, which is a real chore unless the exhaust is dropped and you have it on a lift.

Thanks for the advice, and I'll continue looking, hoping I'm wrong.