Valve Body Assistant (VBA) The VBA is a voltage limiter that limits the voltage returning from the transmission governor pressure sensor. The computer sends a 5 volt reference signal to the governor pressure sensor inside the trans and monitors the returning signal to determine what governor pressure is, the computer also monitors the output shaft speed sensor. After looking at the speed sensor and determining what gear the trans should be in, the computer will vary the ground circuit on the governor pressure solenoid and the governor pressure to the shift valves will go up or down as needed. The computer checks the returning voltage from the governor pressure sensor to see that the governor pressure is correct. The original valve body will only produce about 110 psi max, and the governor pressure sensor will return about 4 out of 5 volts back to the computer and the computer will read this 4 volts as prox 110 psi.
Your new valve body will produce about 175 psi so now the governor pressure sensor will return almost 5 volts back to the computer. Because the computer is programmed to only see about 110 psi it now thinks the sensor is shorted out when it sees over 4 volts returning to it.
The VBA is on the return side of the circuit and dumps any excess voltage over 4 volts to ground so the computer will not see it. This keeps the computer from going to limp mode.
Limp mode is when the computer takes all voltage away from the trans and now the trans will start out in 3rd gear only, during limp mode the trans will start in 3rd gear. If you pull the shifter to 2nd it will go to 2nd but it will not go to 1st.
If you want to get to 1st gear wile in limp mode you must stop and shift to neutral for about 5 seconds and then quickly all the way to 1st, and then upshift manually.