Truck won't shift at WOT

[thunder550]

I already posted this on LS1tech so I apologize for those who are seeing this as a duplicate....

My WOT shift settings are set as follows:

Shift....RPM.....Speed
1-2......5700....55.0
2-3......5700....95.0
3-4......5700....145.0

I have logged two runs and I reach both RPM and MPH shift points and the PCM still is not commanding a shift. I have a log showing 63 mph, 6300 RPM in 1st, scan tool shows 1st gear commanded still, and I have a log showing 104 mph, 6150 RPM in 2nd and scan tool shows 2nd gear commanded. What's the deal?

[thunder550]

And just to show where the LS1 tech thread has gone so far...

Tires are 285/75-16's (32.83")

Keep in mind I have a 4L80E (1st gear is 2.48, 2nd is 1.48, 3rd is 1.00, 4th is 0.70) and 3.73 rear end.

6300 RPM in 1st gear
2540 driveshaft RPM
681 axle RPM
70,237.25 inches per minute :jest:
1.109 miles per minute
66 mph (TCC is unlocked, so I can see a 3mph loss there....)


Both the VSS and RPM need to be met before it will shift

That's what I was thinking too, but I was over 5700 RPM and over 55 mph, so I don't understand why it won't shift.

[joecar]

Try an experiment: set the WOT MPH to 40 and WOT RPM to 5500 and see if it shifts.

[redhardsupra]

fill up your trans fluid ;)

[driver456]

If your truck still does not shift after trying different shift points.Try putting up your shift pressure's.

[thunder550]

Tranny is vacuum modulated, so that won't help unfortunately. I will try changing the shift points...and yes, the fluid is full :tongue:

[thunder550]

OK this is weird...I changed my 1-2 shift points to 40 mph and 5500 RPM as advised, now the truck shifts, but it shifted at 50 mph/5100 RPM the first time and 52 mph/5200 RPM the second time. I guess I'll just have to mess around with the settings till I get them where I want them.

[joecar]

OK this is weird...I changed my 1-2 shift points to 40 mph and 5500 RPM as advised, now the truck shifts, but it shifted at 50 mph/5100 RPM the first time and 52 mph/5200 RPM the second time. I guess I'll just have to mess around with the settings till I get them where I want them.Ok, now leave the WOT MPH at 40, and just fine tune the WOT RPM.

[thunder550]

Will do, thanks!

[thunder550]

Well, just an update. I now have my 1-2 shift set at 40mph/5900 RPM and my 2-3 shift set at 75mph/5000 RPM and they both shift right around 5900. Another member told me that vacuum modulation messes with some of the settings (confuses the PCM??), so I guess it's kind of a crapshoot to figure out the correct settings to use. Oh well, at least it's working now. Thanks for the help everyone.

[Redline Motorsports]

My experience with this issue has lead me to believe that this is a mechanical vs. electronic issue. You can command things all day long but the delay can be because you are commanding things that also rely on a mechanical movement to take place. I have seen a few "broken" parts in the tranny cause the 1-2 shifts not to respond to any shift changes............

This may not apply in your case but some food for thought regarding shift points.........

[joecar]

Well, just an update. I now have my 1-2 shift set at 40mph/5900 RPM and my 2-3 shift set at 75mph/5000 RPM and they both shift right around 5900. Another member told me that vacuum modulation messes with some of the settings (confuses the PCM??), so I guess it's kind of a crapshoot to figure out the correct settings to use. Oh well, at least it's working now. Thanks for the help everyone. Instead of 75MPH, try setting this down to 65MPH and see if the shift moves closer to 5000RPM; if it does then just fine tune the RPM.

[thunder550]

I like the 5900 RPM shifts, that's what I was trying to get to anyway. Now my problem is this: the lower I go with the upshift mph, the lower I have to go with the downshift mph as well. If my downshift mph is higher than my upshift, the PCM never commands a shift. I want my downshift mph's to be as follows: 2-1 @ 50 mph, 3-2 @ 95 mph.

Since my upshift speeds are set 1-2 @ 40 mph and 2-3 @ 75 mph, I have to have the downshift speeds set lower. I'm not sure that there's going to be a way around this one...

[joecar]

you can try this:
set your WOT RPM low (say 4500 RPM) and increment the WOT upshift speed until it shifts at the MPH you want,
and then set the WOT downshift speed to 5 MPH less than this.

I don't know why it's so fuzzy like this, it seems to be way off.