Shift Pointers
- Author: Pete Luban, ATSG Technical Supervisor
One of the more-common complaints that I hear when it comes to Isuzu NPRs or Chevy Forwards and GMC Tiltmasters (all the same vehicle) is late and harsh shifts.
There are a number of reasons for this complaint, such as TPS problems, speed-sensor problems or even problems with the exhaust-brake system.
Another problem area also can be the throttle-pedal cable. This cable is so long that it eventually stretches (see Figure 1). When you step on the throttle, the TPS responds by lowering the voltage (yes, lower voltage; the TPS ranges from 4.5 to 0.5 volts), at which time the injector-pump lever has not been moved.
Of course, when it finally does, the TCM also has responded to the TPS’s lower voltage by delaying shift times and raising line pressure, so by the time the first shift occurs, it is late and harsh.
If you can pull a loop in the cable with your finger without moving the injector-pump lever, the cable needs replacement.
The meat of this article, and the cause of late and harsh shifts in this scenario, is the hand throttle with which these trucks are equipped (see Figure 2).
The hand throttle can be turned easily to open the throttle and can be left that way, causing this complaint. The hand-throttle mechanism also can freeze, causing the same problem.
The adjustment mechanism is inside the plastic tube at the end of the cable where the knob is and cannot be seen or serviced (see Figure 3).
If this cable has a mechanical defect, the entire cable assembly must be replaced.
So, if the mystery of the late and harsh shifts on one of these trucks is plaguing you, and you’ve addressed all the other areas that can cause this, perform a little “sleight of hand” and don’t overlook the hand-throttle cable.
