Malfunctioning governor valve.

Clean or replace the governor screen and/or the governor. Refer to the applicable Allison Transmission Service Manual.

The shift signal valve adjustment is too high.

Adjust the shift points. Refer to the applicable Allison Transmission Service Manual.

The vacuum modulator vacuum hose-to-engine restricted or leaking.

Replace or repair hose.

The modulation is improper.

Inspect the modulator valve.

Repair or replace as necessary.