Apparatus and method of neutral start control of a power transmission

A control system for a vehicle transmission, the vehicle including an engine, a starter coupled to the engine, and a transmission including an input shaft coupled to the engine, an output shaft for driving the vehicle, a plurality of intermeshing gears, and a plurality of fluid clutches associated with the gears, the clutches being selectively engageable in predetermined combinations via fluid valving to establish desired gear ratios between the input shaft and the output shaft, the control system comprising:

• a clutch engagement circuit including a plurality of solenoid coils associated with the clutches and selectively energizable to engage the clutches, and a driver circuit coupled to the solenoid coils for selectively energizing the coils in response to control signals;
• an analog electrical neutral start circuit for placing the transmission in a neutral condition upon startup of the engine;
• and
• a digital control circuit coupled to the clutch engagement circuit, the control circuit redundantly placing the transmission in a neutral condition upon startup of the engine and until the engine is in a running condition and a neutral condition is commanded by an operator.

A method for controlling a transmission in a vehicle of the type including an engine, a transmission coupled between the engine and a drive train, and an operator-movable command device for placing the transmission in preselected forward, neutral and reverse drive conditions, the method comprising the steps of:

• (a) sensing a parameter representative of whether the vehicle engine is running and generating an engine running signal representative thereof;
• (b) sensing a position of the command device and generating a command signal representative thereof;
• (c) disabling electronic driver circuits for shifting transmission control solenoid valves to place the transmission in a neutral condition;
• and
• (d) enabling the electronic driver circuits to permit the transmission to be shifted to forward and reverse gear ratios and thereby to transmit power from the engine to the drive train when the engine running signal and the command signal indicate a neutral condition is commanded and the vehicle engine is running.