The open collector input is defined as a single wire input that alternates between a high voltage of at least 4 V DC and a low voltage of 1.0 V DC or less. Typically, the input is connected to a transistor collector output whether open or through a pull up resistor. A pull up resistor is preferred as this eliminates the need to configure the signal type as open collector. See Figure 3-45 for open collector VSS information.
Allison Transmission Electronic Controls have an open collector output. DDEC IV circuit #556 is connected to Allison circuit #205 (Allison Transmission Electronic Controls) or Allison circuit #157 (World Transmission). This device is an electrically operated switch that grounds or opens the input signal. The VSS frequency (pulses/mile) may range between 7,000 and 145,000 pulses/mile.
The open collector requirements are listed in Table 3-15. Only circuit #556 is used. The #557 cavity must be empty.
SAE J1939 Data Link
A VSS wired to the ECM is not required if the transmission output shaft speed message is being transmitted over the SAE J1939 Data Link.
To obtain accurate vehicle mileage, the parameters, listed in Table 3-16, must be programmed with the DDR, DDDL, VEPS, or DRS. The VSS type will automatically be set to SAE J1939 when the appropriate transmission type is selected (trans type = 16).
Two faults (SID 216 FMI 14 and PID 84 FMI 12) will be logged simultaneously if DDEC is calibrated to receive output shaft speed over the SAE J1939 Data Link and the data is not being received or the data is bad. This indicates there is a problem with the sensor on the transmission or the transmission controller. The fault is available with Release 27.0 or later software. If these faults are received in addition to an SAE J1939 Data Link failure (SID 231 FMI 12), then the problem is with the SAE J1939 Data Link itself.