TCC unlock for upshifts?

[buddy]

Anyone have any experience as to why you would want to unlock the TCC when upshifting? GM programmed it to do this on the 4L80E, as well as downshift which makes sense to me to get some RPMs up. I have been testing it without unlocking in upshift and seems to firm up the shifting from 3rd to 4th, but I was wondering if anyone saw some downside to keeping it locked? Others also defeat the programming and install a switch to keep the TCC locked all the time when towing in 3rd or 4th, so it would be there for downshift too.

Is there some downside to keeping TCC locked in upshift?

The older mechanical auto tranny TCCs that locked from RPM obviously wouldnt unlock to upshift. It also seems that unlocking and relocking could contribute to slipping in the shift. Its going to lock up again immediately after it shifts.

When I compared the 4L60E programming to 4L80E programming I also noticed the lighter duty tranny would not unlock when slipping, like near its stall speed, which for the 4L80E 6.5 is about 1600rpm. I notice that my TCC wont lock up around 1500rpm for this reason, and the program keeps saying because of slipping. And in the code I have found that it is set this way. I can unset that, but something tells me slipping and lugging like that would not be good anyway. This contributes to why the 6.5 with 4L80E will be more efficient around 1800rpm than at 1500rpm cruising.

In the same kind of area, I also found where the code decided to unlock on low or zero TPS as well, and stopping it from doing that works really well for going downhill, stabbing the brake a good amount will unlock it if you want to coast. This is from OBD1 6.5TD programming.

Thanks for any input,
Tom

 

[ak diesel driver]

I thought I read somewhere the input shaft was hollow and that it might be the weak link if doing the TCC switch

 

[NVW]

x2

 

[buddy]

Thanks, that makes sense. I will have to think about the different loads/forces and shock on upshift vs normal lockup during acceleration or unlocking and locking anyway in a matter of a second when it does shift from 3rd to 4th.

 

[handcannon]

I have the manual TC lock switch. I've been playing around with my rig trying to see how things work better for me. I've been manually locking the TC at about 35 MPH as I pick up speed. What I've noticed is that the upshift from 3rd to 4th will go easy if you are going easy on the go pedal, but if you are really pushing things the upshift will be hard. I haven't tried this yet when towing.

One thing I have noticed is that when locked at 35 and slowly picking up speed the tranny likes to shift into 4th real early. This has the motor at 1300 to 1400 RPM and locked which sure seems too low an RPM and tends to lug the motor. To counter the early shift to 4th I've been trying something different. If I start out with the tranny set in 3rd instead of 4th, paying attention to the RPMs, and manually shifting to 4th at about 48 MPH, the tranny stays locked, the RPMs stay a bit higher, and the motor seems to run better.

I did not know the stock program has the TC unlocking and re-locking between shifts. This is something I'll be trying to pay attention to along with some of the other TC shifting things I'm trying.

Don

 

[WarWagon]

The TCC locked during a shift does two bad things. First a harder shift you get indicates just that: harder on hard parts. Input shaft, sprags etc. The second thing it does is make it harder to make sliding friction static friction in the bands and clutches during a shift. It just takes a moment to get the static lockup that is difficult to break free. But if you don't get a static lockup the clutches quickly glaze and burn. They have almost zero heat sink and are not designed to slip.

So the TCC unlocked during the shifting allows a softer shift and makes it easier for clutches and bands to obtain static friction during a shift. Modern computers go further and de-power the engine for a shift.

All I will say about the TCC override switch is that it will add MPH under WOT at the expense of transmission life. At 50K miles since rebuild I am now getting a slipping component code and compression braking hasn't worked in 2nd for the longest time. I suspect the factory grade TCC clutch is NOT up to this abuse.

The shift from 3-4 and 4-3 is very harsh with the switch on and WOT.

 

[great white]

You want the unlock on upshift for the reasons ww mentioned.

Not designed for the abuse. Hense, the "torque management" or "abuse management" schemes.

Lockup on coast down with min throttle is basically what a popular PCM programmer (well, it's just a bit more than that) uses for a speed control scheme. It should unlock with brake application and re-engage with release as long as it's above the programmed min engagement speed.

Now, that's with obdII, where my "dabblings" take place. I would think obdi would be very similar if not virtually the same.

Get it right....works great! It was a godsend on my cross continent trip.

Get it wrong....... get out your wallet......

 

[THEFERMANATOR]

I thought I read somewhere the input shaft was hollow and that it might be the weak link if doing the TCC switch

What he said. GM unlocked the converter fro downshifts and anytime you let off the throttle due to the hollow input shaft. Mid year 99 GM came out with the solid input shaft for the 6.0L as it does not unlock the converter all the time. It seems that all of the 6.5's got the same basic trans tuning as even a 2000 6.5 will unlock teh converter all the time anytime you flinch basically.

 

[WarWagon]

Doesn't the transmission "freewheel" by a sprag clutch when you are in OD and the output shaft starts driving things? (I take this from the operation from a 4L70E found on Trailblazer Super Sports. 4L80E may be different.)

Thus applying throttle causes the clutch to grab, and with the TCC locked it could be harsh causing clutch failure from 'pounding' it to death. Selecting "D" causes overrun clutches to apply preventing the transmission from freewheeling if the output shaft tried to speed up the engine. This freewheeling gets a clunk when you apply throttle and the sprag clutch locks up. The over run clutch discs are fairly small in the 4L70E.

 

[handcannon]

The TCC locked during a shift does two bad things. First a harder shift you get indicates just that: harder on hard parts. Input shaft, sprags etc. The second thing it does is make it harder to make sliding friction static friction in the bands and clutches during a shift. It just takes a moment to get the static lockup that is difficult to break free. But if you don't get a static lockup the clutches quickly glaze and burn. They have almost zero heat sink and are not designed to slip.

So the TCC unlocked during the shifting allows a softer shift and makes it easier for clutches and bands to obtain static friction during a shift. Modern computers go further and de-power the engine for a shift.

All I will say about the TCC override switch is that it will add MPH under WOT at the expense of transmission life. At 50K miles since rebuild I am now getting a slipping component code and compression braking hasn't worked in 2nd for the longest time. I suspect the factory grade TCC clutch is NOT up to this abuse.

The shift from 3-4 and 4-3 is very harsh with the switch on and WOT.

Thanks for all the info. I appreciate the very knowledgeable people here, especially since I'm not very knowledgeable about these intricate details.

Don

 

[THEFERMANATOR]

Doesn't the transmission "freewheel" by a sprag clutch when you are in OD and the output shaft starts driving things? (I take this from the operation from a 4L70E found on Trailblazer Super Sports. 4L80E may be different.)

Thus applying throttle causes the clutch to grab, and with the TCC locked it could be harsh causing clutch failure from 'pounding' it to death. Selecting "D" causes overrun clutches to apply preventing the transmission from freewheeling if the output shaft tried to speed up the engine. This freewheeling gets a clunk when you apply throttle and the sprag clutch locks up. The over run clutch discs are fairly small in the 4L70E.

I don't b elieve the 4L80E uses the over run clutch set-up like the lighter duty series of trans does. I know if you take a TECH 1 and command lock-up manually it will hold lock-up right down until it stalls out, so I would say teh 4L80E can take it. And the newer 4L80E behind the 6.0L, 8.1L, and teh DMAX vans did not have this unlocking feature in them since they have the solid input shaft.