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How Holden’s electronic control unit works; Pt.2 In last month’s issue we showed how the software con­trolling the Holden engine management system works. This time we examine how the Commodore’s automatic transmission is electroni­cally controlled. By JULIAN EDGAR I N BOTH THE VR and VS model  Commodores, an electronic con­ trol unit dubbed a Powertrain Control Module (PCM) is used to control both the engine management and the transmission. The PCM is a physically larger package than the Electronic Control Module used in previous Holdens but it is closely related, with similar software and hardware used in both packages. As with last issue’s story, this article draws heavily on computer programmer Ken Young’s DynoCal (now known as Kalmaker) software program. The program allows the man­ ipulation of all the variables within the Holden PCM program. It also allows the down-loading of the PCM’s EPROM program, meaning that actual data maps can be seen. The VR V6 Commodore of Awesome Automotive in Ade­ laide was used in conjunction with the Kalmaker Late-model Holdens use a Powertrain Control Module which inte­grates automatic transmission control with engine management. software package to gain much of the information shown here. The automatic transmission used in both the VR and VS Com­modores is the GM Hydra-Matic 4L60-E trans- mission. It is basical­ly a hydraulically-controlled transmission with some added elec­tronic control, as signified by the ‘E’ suffix. The items electronically controlled August 1997  3 Transmission Controls OPERATING CONDITIONS SENSED SYSTEMS CONTROLLED VOLTAGE •   BATTERY COOLANT TEMPERATURE •  ENGINE SPEED (RPM) •  ENGINE POSITION (TP SENSOR) •  THROTTLE FLUID TEMPERATURE (TFT) •  TRANSMISSION GEAR POSITION •  TRANSMISSION • VEHICLE SPEED SENSOR (VSS) POWERTRAIN CONTROL MODULE (PCM) SOLENOIDS •   SHIFT CONTROL SOLENOID •  PRESSURE SOLENOID – “ON” – “OFF” •  TCC SOLENOID – “PWM” •  TCC • 3-2 CONTROL SOLENOID Fig.1: this diagram shows the parameters sensed (left) & the systems controlled (right). PRESSURE CONTROL SOLENOID AUTOMATIC TRANSMISSION OUTPUT SPEED SENSOR (OR VEHICLE SPEED SENSOR) TORQUE CONVERTER CLUTCH “ON-OFF” SOLENOID TORQUE CONVERTER CLUTCH PWM SOLENOID 1-2 SHIFT SOLENOID “A” 2-3 SHIFT SOLENOID “B” 3-2 CONTROL SOLENOID TRANSMISSION FLUID PRESSURE SWITCH ASSEMBLY Fig.2: all the automatic transmission components under the electronic control system are shown here in the transmission are: •  line pressure control solenoid; •  1-2 and 2-3 shift solenoids •  3-2 control solenoid; •  torque converter clutch on/off solenoid; •  torque converter clutch pulse width modulated solenoid. The operating conditions sensed by 4  Silicon Chip the Powertrain Control Module are: •  battery voltage; •  engine coolant temperature; •  engine speed (rpm); •  throttle position; •  transmission fluid temperature; •  transmission gear position; •  vehicle speed. A further input comes from the driver-controlled Power/Economy switch mounted on the centre console. Fig.1 shows the operating conditions sensed, the PCM and the items controlled. The physical shape and location of the transmission components relating to the electronic control system are shown in Fig.2. Functions of controlled systems The line pressure control solenoid (PCS) takes the place of the throttle valve used in the hydraulically-controlled version of this transmission. The PCM varies line pressure via this sole­noid, which is controlled by the current flow through it. Line pressure is increased during times of high engine load which is sensed from various input sensors, including throttle position, speed and engine intake air temperature. Controlling line pres­sure with the PCM means that the pressure can be better correlat­ed with the engine’s torque curve, as shown by Fig.3. The 1-2 and 2-3 shift solenoids control the movements of the 1-2 and 2-3 hydraulic shift valves. These solenoids are normally-open exhaust valves that work in four combinations to shift the transmission into different gears. However, only in ‘D’ can these solenoids control shifts; in the manual positions ‘3’, ‘2’ and ‘1’ the transmission shifts under hydraulic control. The shift sole­noids are either fully open or fully closed. The 3-2 control solenoid is a pulse width modulated sole­ noid used to improve the 3-2 downshift. It controls hydraulic pressure so that the release of the 3-4 clutch and the applica­tion of the 2-4 band are smooth. The duty cycle of this valve is determined by the throttle position, vehicle speed and the gear demanded. The torque converter clutch solen­ oids are used to lock up the torque converter, giving very low slippage. The torque con­verter on/off solenoid has priority in applying and releasing the clutch. It is a normally-open exhaust valve which when earthed, causes converter feed pressure to increase and shift the torque converter clutch valve into the ‘apply’ position. The pulse width modulated torque converter solenoid is used to provide smooth engagement of the clutch. The apply rate of the torque converter clutch is determined by the duty cycle fed to the PWM solenoid. Matching Line Pressure with Engine Torque Adaptive Controls SHIFT DURATION ACTUAL SHIFT DURATION Engine Torque Hydraulically-controlled line pressure Electronically-controlled line pressure ENGINE SPEED (RPM) Fig.3: electronically controlling the line pressure in the trans­mission means that the pressure can be better correlated with engine torque output than in a conventional automatic transmis­sion. Critical to transmission control is the sensing of input and output speeds. While engine rpm gives the input speed to the torque converter, it does not give the input speed of the trans­mission, because of slippage in the torque converter. The input shaft speed of the transmission is calculated from the vehicle speed sensor data and the gear ratio that the transmission is currently in. The torque converter slip is calculated by subtracting the transmission input speed from the engine rpm. Note that the transmission slip can be either positive or negative. The torque converter slip is used in the pressure control logic, shift logic and torque converter clutch diagnostics. As an example of the latter, the PCM can determine whether or not the torque converter clutch is stuck in the engaged posi­tion due to a mechanical fault. It does this by monitoring slip when the clutch is commanded to be off. No slip means that the clutch is still engaged. The control of the torque converter lock-up clutch is de­pendent on a number of variables. The clutch will not lock up if a downshift or upshift is in progress, there is a change occur­ring 2 3 CONSECUTIVE SHIFTS 4 AS LINE PRESSURE INCREASES, SHIFT DURATION (ABOVE) DECREASES FLUID LINE PRESSURE HYDRAULIC CONTROLS ARE LESS PRECISE IN MATCHING LINE PRESSURE TO ENGINE TORQUE DESIRED RANGE FOR SHIFT DURATION 1 LINE PRESSURE (kPa) TORQUE (Nm) ELECTRONIC CONTROLS ALLOW LINE PRESSURE TO MATCH ENGINE TORQUE 1 2 3 4 CONSECUTIVE SHIFTS Fig.4: self-learning controls the shift times. Line pressure can be varied over a range so that shift times remain consistent, even as the transmission wears. Fig.5: several maps are used to control the operation of the torque converter lock-up clutch. This map is for fourth gear when in Power mode. The highlighted bar shows that the clutch will lock-up at 75 mph (121km/h) when the car is being driven with a 50% throttle opening. in the throttle position or the gear selector is in a manual range. Furthermore, the temperature of the engine coolant must be above 50°C (examples are from the VR Commodore V6), the transmission fluid temperature must be over 0°C and the transmission slip must be below August 1997  5 Fig.6: the transmission constantly monitors the 1-2 shift time, counting over-long shifts as errors. This chart shows the error count at different throttle openings. At 25% throttle, 253 errors have occurred – a substantial number! Fig.7: in response to the shift errors in Fig.6, the line pressure has been increased by 4.3 psi to shorten the shift. The transmis­sion constantly chases optimal shift times in this manner. 25 rpm. There is also a delay period before the clutch will engage, even when all the required conditions are being met. When the clutch is (finally!) being engaged, the duty cycle applied to the PWM torque converter solenoid is a calculated value. Should the 6  Silicon Chip transmission fluid temperature rise excessively, the torque converter clutch is applied in gears 2, 3 and 4. This reduces transmission slip and so also reduces the likelihood of further temperature increases. Fig.5 shows one of the two-dimensional charts controlling the torque converter lock-up clutch operation. This chart is for fourth gear, Power mode. The cursor is pointing to the bar show­ing that at a 50% throttle opening lock-up occurs at 75 mph (all units within the American derived program are Imperial). The change to a higher gear is termed an upshift. Upshift logic is performed if the current gear is 1, 2 or 3. Three tests are performed in quick succession for an upshift. If the result of any test is positive the remaining tests are skipped. The three tests are: •  A fixed upshift where the speed is greater than a preset threshold, which depends on the gear lever range and the set mode (power or economy). •  A full throttle upshift where there is a wide throttle opening (normally over 90%), speed is greater than the specified upshift speed and engine speed is greater than the specified upshift engine speed. These variables are dependent on gear, coolant temperature and barometric pressure. •  A part throttle upshift where there is less than full throt­tle, the upshift speed and/or rpm is greater than that shown by two 2-dimensional maps based on speed, throttle position and the position of the power/economy switch. If none of the upshift tests command an upshift or if the car is already in fourth, downshift logic is performed. The downshift logic again comprises the fixed downshift, full throt­tle downshift and part throttle downshift approach. The level of the hydraulic line pressure helps to determine clamping pressures of clutches and bands and the harshness of the changes. The program calculates a desired pressure based on the temperature of the transmission fluid, throttle position and road speed. This is converted to solenoid current. The PCM then meas­ures the amount of current flowing through the line pressure control solenoid and compares this with the calculated current. If the difference is greater than a calibrated value, a trouble code is set. The PCM controls the time taken for each gear change, with the desired shift times for the 1-2 and 2-3 gear changes included in the program. The times taken for the shifts are measured and compared with the desired or reference times. If the times are incorrect, the shift time is altered by changing the line pres­sure via its control solenoid. Fig.4 shows the approach taken. Incorrect shift times can be caused by transmission wear (although there is compensation for wear) and fluid temperature variations. In practice, the transmission seems to spend a lot of time chasing its programmed shift times. Corrections to the shift time are held in an adaptive memo­ry. Dubbed the Pressure Adapt Modifier (PAM), the values are arranged in an array of 17 learning cells, which are referred to according to throttle position. The values in this table are modified on the basis of the correction factor needed to cause shifts to occur within the desired times but only when certain conditions of throttle position and change in road speed are being met. The PAM is very similar to the Long Term Fuel Trim memory used to correct engine air/fuel mixtures and is held in non-volatile memory. Fig.6 shows an actual logged PAM chart. At a throttle posi­tion of 25% (horizontal axis), 253 shift errors have been counted, indicating that the 1-2 change at 25% throttle openings is frequently taking too long. To correct this, hydrau­ lic pressure has been increased, as Fig.7 shows. At a 25% throttle opening, line pressure has been increased by 4.3 psi. At other throttle positions, the opposite is occurring – the shifts are too quick and so pressure is being dropped (at 50% throttle, for example). These charts are constantly changing as the program chases optimal shift times. When regulating a 3-2 downshift, the duty cycle for the 3-2 downshift valve is a function of throttle position and road speed. However, its duty cycle is also corrected if the air conditioning is on, if the range selector is in D1 or D2 and for transmission fluid temperature. The final figure is then checked against the programmed maximum and minimum duty cycles permitted for this solenoid. Conclusion In the same way in which the Holden V6 and V8 engines are relatively simple mechanical designs made competitive by very advanced engine management, the old-fashioned hydraulically controlled Hydra-Matic has been effectively updated by the addi­tion of sophisticated electronics. It’s possible to sit in a moving car Transmission Data Log Record The Kalmaker software allows the logging of transmission factors in real time. While the system logs at 10Hz, the accompa­nying graph has been simplified, with data shown at 0.5s intervals. The graph shows the behaviour of engine rpm, torque converter slip, throttle opening, vehicle speed and Pressure Control Solenoid current for a 16.5s period. During this time the Awesome Automotive Commodore V6 was accelerated from a standstill to a speed of 108km/h. The throttle was then closed and the car gradually slowed to a speed of 54km/h. The car was left in ‘Drive’ during this manoeuvre. The pink line shows the car’s speed and the black line shows throttle position, both being referenced against the right-hand axis. Engine revs are shown by the yellow line, while torque converter slip is shown by the aqua line. It can be seen that when the car is stationary, engine speed and torque converter slip are of a similar magnitude. This is because at an idle speed of 700 rpm, the slip must be 700 rpm if the car is not moving! With a throttle opening of 100% the car accelerates rapidly with the amount of slip decreasing. The first-second gear change occurs at and see on a plugged-in laptop PC the continual monitoring of shift-times, the result­ing changes in PCS current and the locking and unlocking of the torque converter clutch. Watching the live screen really brings home the complexity of the calculations 13 on the Y-axis and with the newly-applied load, the slip within the torque converter rises. It slips by up to 1700 rpm before the value drops back to about 300 rpm after about 1.5 seconds. In second gear, Leon Vincenzi has kept his foot flat to the floor until the engine speed reaches 4750 rpm, upon which he has lifted his foot entirely (23 on the horizontal axis). The trans­mission immediately changes from second to third to fourth, with this transition taking only 0.2 seconds. Again there is a major increase in slip through the torque converter, which then goes into negative numbers as the engine brakes the car. During these processes the Pressure Control Solenoid is being varied in its duty cycle, controlled by the current flow through it (shown by the blue line). A high current flow results in a low fluid pressure, while a low current flow increases fluid pressure and thus clamping forces. With the application of full throttle the current rapidly reduces, staying at 156mA for the 1-2 full throttle gear change. It rises to 1074mA as the throttle is lifted, responding to the reduced torque load on the transmis­ sion. Even when engine braking, the PCS keeps pressures low. continually occurring in the silver box behind the kick-panel! Contacts (1) KAL Software (Brad Host) 0412 266 758; (2) Awesome Automotive SC (08) 8277 3927 August 1997  7