Will a 4L80 shift at high rpm while the tires are spinning?

[rhackstall]

Still chasing a shift issue. Im trying to see if I have a transmission issue or a traction issue. Some past logs show the tires are spinning, should the trans still make the shift? 1-2 is the problem. Thansk

[RDF1]

Sure it will still shift but the rpms will be accelerating much faster with loss of traction.
Does it show to be commanding the 1-2 shift and its taking awhile to actually make the shift?

[rhackstall]

Sure it will still shift but the rpms will be accelerating much faster with loss of traction.
Does it show to be commanding the 1-2 shift and its taking awhile to actually make the shift?

yes, I tried lowering the shift rpm and mph with no luck

[Chad D.]

Hoping to hear something on this as well! I'll be trying to help a friend with a similar issue this evening. He has to lift to get it to shift when the tires are spinning. This is in a rock crawler, so

[Chad D.]

Still struggling with my friend?s transmission tune. But, I just learned that his speedo sensor is being read from the trans output, rather than t-case output because his Atlas transfer case doesn?t have a spot on the output. All this really should mean is that the trans won?t know if the T-case is in high or low range.


It shifts fine in high range, but won?t upshift in low with any significant throttle input. Shifts at idle speed, but not anything more.

If I can get it figured out before he buys and swaps in a full manual valve body, I bet he?d appreciate it.

[kingtal0n]

4l80e is very simple. The shift solenoids are either on, or off, there is no PWM.

First you check the speed is measured correctly by the ECU.
Then you check to make sure the trans shift occurs when the solenoids are kicked on or off. There are only 2solenoids for shifting. When both are off, that is 2nd gear. This allows the trans to default to 2nd if the solenoids are not being controlled at all I guess.

Once you see the solenoids are controlled properly and the MPH/RPM is reading right, thats it. The trans will turn on or off solenoids as it shifts and you can see that in the log.

The rest is diagnosis. If you see the solenoids aren't being commanded early enough then adjust the tune. If you see a solenoid is commanded but the shift doesn't happen you need to diagnose the pressure of the transmission or internal parts config. Low pressure could be a cause of slipping or lack of shifting.

Remember the trans is mechanical, the parts move more slowly than the computer can react. The time it takes to shift depends on the fluid flow of the transmission, e.g. larger feed holes fill drums faster.
The rate of change of RPM also influence the shift points, the more powerful the engine the higher the RPM and MPH will rise after a shift is commanded due to delay of mechanical and fluids.
For example if you want a shift at 6000rpm you might need to command it by 5,500 RPM for 400rwhp, or 5,200 for 500rwhp. Or 4,900 for 600rwhp. The more power the engine makes the higher the RPM will rise after the shift is commanded.
If the tire spins that is a whole other consideration. The trans doesn't know if the tire slips or spins or lost traction all it does it respond to the ECU. The ECU commands a shift or not depends on the MPH and RPM as a trigger. Setting up the trans to shift comfortably during a spin involves a bit of trial and error, adjusting the pressure and triggers to deliver an ideal shift.

[Chad D.]

4l80e is very simple. The shift solenoids are either on, or off, there is no PWM.

First you check the speed is measured correctly by the ECU.
Then you check to make sure the trans shift occurs when the solenoids are kicked on or off. There are only 2solenoids for shifting. When both are off, that is 2nd gear. This allows the trans to default to 2nd if the solenoids are not being controlled at all I guess.

Once you see the solenoids are controlled properly and the MPH/RPM is reading right, thats it. The trans will turn on or off solenoids as it shifts and you can see that in the log.

The rest is diagnosis. If you see the solenoids aren't being commanded early enough then adjust the tune. If you see a solenoid is commanded but the shift doesn't happen you need to diagnose the pressure of the transmission or internal parts config. Low pressure could be a cause of slipping or lack of shifting.

Remember the trans is mechanical, the parts move more slowly than the computer can react. The time it takes to shift depends on the fluid flow of the transmission, e.g. larger feed holes fill drums faster.
The rate of change of RPM also influence the shift points, the more powerful the engine the higher the RPM and MPH will rise after a shift is commanded due to delay of mechanical and fluids.
For example if you want a shift at 6000rpm you might need to command it by 5,500 RPM for 400rwhp, or 5,200 for 500rwhp. Or 4,900 for 600rwhp. The more power the engine makes the higher the RPM will rise after the shift is commanded.
If the tire spins that is a whole other consideration. The trans doesn't know if the tire slips or spins or lost traction all it does it respond to the ECU. The ECU commands a shift or not depends on the MPH and RPM as a trigger. Setting up the trans to shift comfortably during a spin involves a bit of trial and error, adjusting the pressure and triggers to deliver an ideal shift.

Ok. Thanks for this detailed post! Very appreciated.

If I?m wrapping my head around it correctly, he may still be in a bit of a pickle since he has two drive ranges within the transfer case. Since he can build RPM much faster in low range (I believe his Atlas is 3.8:1 low), he may need to live with a little rev limiter bounce if he?s really gettin? it in low range. Also, it may shift up a little early in high range since it?s revving up slower. I don?t know that we will be able to make it work really well in both ranges since they?re so different. We?ll try though!

He just emailed his tune to me, so I?ll play with it tomorrow?

[kingtal0n]

Yes it may shift a bit sooner in the high range where the engine accelerates more slowly. I wouldn't let it smack a limiter if possible. Just tune for the fastest rate of rpm so it never does.

If an engine absolutely must hit a limiter, make sure its a fuel only limiter, and that it happens at a nice low rpm, like in the 5000's range. The higher the engine rpm when a limiter comes on the more risk.

And to re-iterate. The solenoids are on or off. The ECU just throws a switch- the solenoid. Turns it on or off. At that point, the fluid can flow somewhere to shift the gear, but the fluid has to fill a drum. So the speed of the fluid, filling the drum, the pressure and volume flow rate, you can mod the trans fill holes and separator plate holes to fill faster for example. Its all hydraulic and mechanical once the ECU throws the 'switch' and the amount of time between the switching and the actual shifting is dependent on those fluid filling devices of engagement.

[Chad D.]

Thanks again; this is making sense.

Not knowing where he?s currently shifting, I multiplied all shift point initiation rpm by 75% to see if that gets us going through the gears.

I also looked at his tire size and saw that I need to input the trans input shaft revs per mile. To figure that, I took his tire size (40?) and determined that it rolls 517 revs per mile. Multiplied by his 5.38:1 axle ratio, I believe the trans input shaft spins 2,780 revolutions per mile. I?m assuming this needs to be in 3rd (1:1 ratio).

We?ll write the new tune into the ECU and see if we gain some ground on controlling this thing?. If we start to see shifts, we know we?re making progress.

Thanks again for the help!!!

[STAGEUP]

What does your input speed sensor say? Input shaft speed is needed to make the WOT shifts happen

[Chad D.]

What does your input speed sensor say? Input shaft speed is needed to make the WOT shifts happen

Not 100% sure how to find that?. The only thing I?ve done related to that was determine the revs per mile based on tire circumference and axle gear ratio. Tires go round about 517 times per mile and axle gears are 5.38:1. I entered 2,780 for the input shaft revs per mile, which is a little different than where it was set previously.

If we?re supposed to log to get that number, what rpm should it be checked at? Assuming where he wants it to shift under WOT? Then we?d have to work backwards (reduce input speed) to get the mechanical timing of the shift to jive?

No wrong answers, as this is all pretty much Greek to me still!!! I appreciate everything you folks have shared already.

[STAGEUP]

Log transmission Input shaft speed with the VCM scanner on Hp Tuners. Verify that input shaft RPM doesn't flake out during WOT. Input speed should obviously rise with engine RPM. However, it sounds to me as if you're not getting any input shaft speed RPM signal at all.

[Chad D.]

Log transmission Input shaft speed with the VCM scanner on Hp Tuners. Verify that input shaft RPM doesn't flake out during WOT. Input speed should obviously rise with engine RPM. However, it sounds to me as if you're not getting any input shaft speed RPM signal at all.

Thank you for that explanation. I'll see if he can do some logging soon and confirm that is (or is not) receiving input shaft RPM. Scanning and logging is new to both of us, so hopefully we can spot that area...

[rhackstall]

Thank you for that explanation. I'll see if he can do some logging soon and confirm that is (or is not) receiving input shaft RPM. Scanning and logging is new to both of us, so hopefully we can spot that area...

Did you resolve this issue?