2002 Bluebird Bus Shifting Hard (Trip tomorrow 3/22) Help please!!!

[mikeicnd]

Hey there! I have a 2002 Bluebird bus with a Kodiak 7500 8.1 liter engine. I have added some speed tunes to the bus to get it above 45mph however I am having some shifting issues:

1) Shift from 1st gear to 2nd gear doesn't happen until close near 3,300 rpm. Very high.
2) when shifting from 2nd gear to 1st gear it jerks very hard. believe also because it is still a high RPM.

I have lowered the RPM values for the transmission but still getting the same results. I have attached the tune file along with the drive recording. Any help would be very much appreciated before I head out on a trip tomorrow.3-21-24 shift points.hpt3-21-24 test ride.hplBus.jpg

[Fast4.7]

If it has an Allison AT545 in it, I recall those somewhat relying on the engines rpm governor to help upshifts. When the rpm governor activates, the increased vacuum draws on the modulator helping it complete an upshift.

There is nothing shift point wise in the tune on an non-electronic transmission.

Look at the transmission general tab, I feel like it will show non-computerized automatic. On those the transmission governor and modulator combined with the engine RPM governor work to control shifting. Maximum governed input speed is like 3,600-4,000 rpm.

[Fast4.7]

I also want to say those Kodiak 8.1Ls were detuned for 250-280 hp and ~4,000 rpm maximum engine speed. It is the same engine, GM just limited the throttle body opening and engine speed electronically. The throttle body does not open more than 50-60% throttle on them even at WOT, the vacuum generated by the partially closed blade contributes to the vacuum signal on the modulator as well. Good new is the modulators I have seen are externally adjustable either with a small flat blade or allen key screw. Turn the screw 1/2 turn CCW, see if it improves the part-throttle shifting. If it does, go another 1/2 turn if still not to your liking. On a transmission like a TH350 and TH400 using lower than stock vacuum, I have even backed the screw out entirely to correct late and hard shifts from lack of vacuum on the modulator. The modulator has the largest effect on part-throttle shifting, more throttle opening, less vacuum, more line pressure, firmer shifting and later upshifts.

[edcmat-l1]

Yeah that's a non electronic trans so you're not going to find an answer here. It's a mechanical problem.

[Fast4.7]

Yeah that's a non electronic trans so you're not going to find an answer here. It's a mechanical problem.

I have not downloaded or looked at the HPT file, since I am responding from my phone, but figured that was the case. He needs to look at the vacuum modulator and adjust it for the new vacuum levels the engine is running at with whatever throttle remapping he has done.

[mikeicnd]

Thank you!!! We are going to try this today before we leave! This awesome and thanks again!

[04silverado6.0]

Do you have the stock tune for this?