A: PWM stands for “pulse width modulation.” In transmission control applications, PWM is a way of controlling a solenoid so that it applies varying amounts of hydraulic pressure according to computer control. PWM torque converter clutch systems use apply and release the clutch gradually to improve shift feel. Unfortunately, many of the PWM controlled systems cause the clutch to slip, and over time, eventually fail. Over time, manufacturers have introduced and then later removed this gradual apply/gradual release feature. Some types of transmissions have been built with both PWM and non-PWM versions of the converter clutch. For those transmissions, Compushift II has a setup menu selection that lets you choose PWM or non-PWM. Whether PWM or not, the Compushift II always does a quick apply and release of the torque converter clutch. We feel that it is more important, especially in high-horsepower applications, to preserve the life of the clutch rather than improve shift feel.

Line Pressure: The amount of hydraulic pressure applied to the clutches determines how the transmission shifts. Under low pressure, the clutches engage slowly and softly, making a gentler shift. Under high pressure, the clutches engage rapidly and the shift is firm.


Q: Do I have to purchase the display programmer in order to correctly install Compushift II?

A: No. Every Compushift II comes with the digital display. Throttle calibration and shift point/shift pressure adjustment can be done without the display. The display does offer the greatest flexibility in setup, though. It can also serve as a “digital dashboard,” providing you with running information about vehicle and transmission performance.

Q: Do I need a laptop computer to program the system?

A: No. All adjustments and programming to Compushift II can be done with the handheld display/programmer.

Q: Does the torque converter clutch (TCC) unlock during gear changes?

A: The torque converter clutch always unlocks during downshifts, and sometimes during upshifts, depending on the vehicle speed and throttle position. It is safe to upshift under light load while the TCC is locked, and most OEM systems do so.

Q: How do I run an electronic speedometer with Compushift II?

A: Compushift II provides an output signal for driving speedometers. This signal is called reproduced TOSS. This signal replicates the signal coming from the Transmission Output Shaft Speed (TOSS) sensor. A device called the Cable-X can drive mechanical speedometers if necessary. Built by Abbott Enterprises, the small Cable-X box motor rotates a speedometer cable at a rate proportional to an electronic speed input, and is calibrated to match your speedometer using internal switches. HGM is an authorized Cable-X reseller.

Q: How do I use an existing throttle position sensor?

A: A throttle position sensor usually has three wires: +5V power, ground, and signal. The power and ground wires supply a reference voltage to either side of a variable resistor. As the shaft rotates, the signal wire has a voltage that varies between +5 V and ground. In order to connect Compushift II to an existing TPS, you only need to connect two wires, the ground wire and the signal wire. These can be piggybacked on top of the existing wiring using an in line wiretap, but they should be done as close as possible to the sensor. The +5V wire coming from the Compushift II should be left disconnected, but insulated to prevent a short circuit.

Q: I don’t understand the function of the “always upshift at” setting. What exactly does it do?

A: The “always upshift at” places an absolute engine rpm limit on each gear. The controller will always upshift when this engine rpm is reached. The controller measures engine speed from the tachometer input, if connected, or calculates engine speed based on driveshaft speed and gear ratio. The tachometer is the more accurate measurement because all torque converters slip. As the “shift speed” adjustment is increased, the transmission shifts at greater speeds for a given throttle position. The “always upshift at” provides a cap for this adjustment. This allows you to have the transmission shift at high rpm with light throttle, but still not exceed the engine’s rpm limit at full throttle.

Q: What is the difference between PWM and non-PWM torque converter clutches?

Gearstar Performance Transmissions
132 N. Howard St.
Akron  44308
24009 Hawthorne Blvd
CA  90505