1. 9
Electronic Transmission Controls

2. FIGURE 9–1 The throttle valve (TV) cable on a 4T-60 transaxle.

3. FIGURE 9–2 A vacuum modulator moves the modulator valve depending on the vacuum of the engine. A heavy load on the engine causes the vacuum to be lower than when the engine is operating under a light load. The spool valve applies mainline pressure to the boost sleeve of the pressure regulator valve, which causes the mainline pressure to increase.

4. FIGURE 9–3 A governor assembly is used on older hydraulically controlled automatic transmissions/transaxles to control shift points based on vehicle speed.

5. FIGURE 9–4a This control solenoid assembly contains four transmission fluid pressure (TFP) switches, a line pressure control (PC) solenoid, four pressure control (PC) solenoids, two shift solenoids (SS), a torque converter clutch (TCC) solenoid, a transmission fluid temperature (TFT) sensor, and the transmission control module (TCM). It also has a vehicle harness connector and connectors to the shift position switch and the input and output speed sensors.

6. FIGURE 9–4b A simplified view is also shown.

7. FIGURE 9–5 The transmission range switch is usually located on the case where the shifter cable attaches to the manual valve lever. The switch also includes the switch for the backup lights and the park/neutral switch which is used to prevent the start being engaged unless the shifter is in park or neutral.

8. FIGURE 9–6a Moving the shift lever to the M (manual) position (a) activates the up/down, +/− switches that will cause an upshift or downshift.

9. FIGURE 9–6b Moving the shift lever to the M (manual) position (a) activates the up/down, +/− switches that will cause an upshift or downshift.

10. FIGURE 9–7 Speed sensors are used by the powertrain control module (PCM) or the transmission control module (TCM) to control shifts and detect faults such as slippage when the two speeds do not match the predetermined ratio for each gear commanded.

11. FIGURE 9–8a The speed sensor switch will close as the magnet moves past it.

12. FIGURE 9–8b It will generate a sine wave signal, which is converted inside the PCM/TCM to a digital signal. The frequency of the signal is used to measure the speed.

13. FIGURE 9–9 The pressure switch manifold (PSM) used in a GM 4L60-E consists of diaphragm switches with seals around each one that are bolted to the valve body over holes for each clutch circuit.

14. Frequently Asked Question What Is Pressure Logic
Frequently Asked Question What Is Pressure Logic? Pressure switches are used to monitor which clutch has pressure but the PCM/TCM can use the information from the switches to verify which gear the transmission/transaxle is operating. Some pressure switches are normally open (N.O.) and others are normally closed (N.C.) and the gear that the unit is operating in can be determined by the switch positions. An open circuit is represented by a binary code “1” and measures 12 volts while a grounded circuit binary code is “0” and measures 0 volts. Depending on the position of the manual valve, fluid is routed to the pressure switch manifold (PSM). The PCM/TCM uses information from the on/off positioning of the switches to adjust line pressure, torque converter clutch (TCM) apply, and to control shift solenoid operation. SEE FIGURE 9–10.

15. FIGURE 9–10 Some switches are electrically normally open (N. O
FIGURE 9–10 Some switches are electrically normally open (N.O.) and others are normally closed (N.C.) and are used to provide gear selection information to the PCM/TCM.

16. FIGURE 9–11a A transmission fluid temperature sensor can be checked by connecting an ohmmeter to the harness connector terminals.

17. FIGURE 9–11b The resistance should change as the temperature changes.

18. FIGURE 9–12 The brake (stop light) switch is mounted at the brake pedal. It provides a brake-apply signal to the TCM.

19. FIGURE 9–13a The normally closed solenoid blocks fluid flow when it is off while opening the exhaust; and when it is on, it opens the valve.

20. FIGURE 9–13b The normally open solenoid allows fluid flow when it is off; and when it is on, it closes the valve while opening the exhaust.

21. FIGURE 9–14 The signal from the TCM can cause the EPC solenoid to change the pressure regulator valve to adjust line pressure.

22. FIGURE 9–15 Line pressure increases as the duty cycle of the EPC solenoid decreases.

23. FIGURE 9–16 Solenoid control occurs when the PCM/TCM completes the circuit to ground (top) or switches on B+ (bottom). The ground connection is also B−.

24. Chart 9–1 Typical electronic pressure control (EPC) current and line pressure comparison.

25. Frequently Asked Question What Is Torque Control
Frequently Asked Question What Is Torque Control? Accurate control of shift timing and quality provides a smoother driving experience. In addition to improving shift quality, altering the ignition timing during the shift decreases the load on the transmission and increases transmission life. This is called torque management or torque reduction and is controlled by the PCM/TCM. SEE FIGURE 9–17.

26. FIGURE 9–17 When the transmission control module (TCM) is ready to begin an upshift, it signals the powertrain control module (PCM) to reduce engine torque. This produces a smoother shift with less wear in the transmission.

27. FIGURE 9–18 Using data from the various sensors, the TCM can apply or release the clutches. During an upshift, solenoid 1 can control how fast clutch 1 releases as solenoid 2 controls how fast clutch 2 applies to keep the shift time at the proper speed.

28. FIGURE 9–19 A diagram showing the relationship between the electronic and hydraulic controls.

29. FIGURE 9–20 A scan tool display showing the adaptive (TAP) pressure changes at various throttle positions.

30. Frequently Asked Question What Is Fuzzy Logic
Frequently Asked Question What Is Fuzzy Logic? A method used to improve shift timing is through a process called fuzzy logic. In most situations, shifts simply match vehicle speed and throttle position. Fuzzy logic adapts shifts to driving conditions such as mountains, upgrades and downgrades, and while turning corners. The shifts will be delayed and firmer because of increased load and multiple changes in throttle position. Fuzzy logic and advanced electronics allow improved shifts for many different situations. SEE FIGURE 9–21.

31. FIGURE 9–21 The fuzzy logic part of the TMC receives input signals, compares what the driver is doing with the throttle and what the vehicle is doing with normal operation, and adapts shift timing.

32. CHART 9–2 In this example, the vehicle would start out and remain in third gear if there was a fault with the computer or wiring.

33. CHART 9–3 In this example, the vehicle would start out and remain in second gear if there was a fault with the computer or wiring.