July 31, 2008
Part One of a Two-Part Series
In the United States, the AW 55-50 is used in the:
As you can see, there are a lot of these transmissions on the street. It is important that you understand what a normal operating condition is and what is not normal for the vehicle that you are working on. Not all AW55-50 transmissions have the same operating features. As an example, Saturn has eight operating modes or shift strategies. They are normal mode, protection mode, cold mode, hot mode, uphill mode, downhill mode, high altitude mode and traffic jam mode. Each one of these modes has a different shift strategy. The customer is familiar with normal mode and may have noticed cold mode. The other modes don't show up very often and if noticed they may be seen as a very intermittent problem.
You may have a customer come in with a Saturn that has a complaint of 2nd gear starts or seems very sluggish with a lack of power from a start. The customer says that this only happens once in awhile. You could spend a lot of time trying to diagnose this normal condition, if you don't know that the computer is commanding the shifts in traffic jam mode.
Some Volvo models have a Neutral Control feature. This is when the computer turns off the C1 clutch when the vehicle comes to a complete stop for more than 2 seconds with the brake applied. This is a feature that the customer should never feel, but quite frequently you will get a harsh re-engagement and sometimes a downshift thud when the C1 clutch releases. Volvo has software updates to fix this as well as flare 2-3 shifts, harsh down shifts and harsh garage shifts. Most manufacturers have software upgrades for this transmission. Check with your local dealer for the latest upgrades available for your application.
As with almost all newer transmissions, the AW55-50 relies on adaptive strategy to adjust the shift feel for each shift. Whenever the transmission is overhauled or replaced, the valve body is replaced or the transmission control module (TCM) is replaced, the shift adapts must be cleared and relearned.
Failure to perform these procedures can result in shift feel and downshift clunk complaints, as well as reduced transmission life. Some vehicles relearn slowly and need to be relearned with a dealer equivalent scan tool. You don't want to damage your fresh overhaul waiting for the computer to relearn its adapts.
The challenge presented by having multiple manufacturers using the AW55-50 is that you need to be aware of the different operating modes and different relearn techniques. Understanding the operating system and procedures for each vehicle and knowing what bulletins and updates are available will go a long way toward fixing the car right the first time and keep you from wasting your time.
Next month we will cover the adaptives and their relearn procedures along with some known bulletins you should be aware of. For now, let's have a look at the variety of operating modes. As stated earlier, GM/Saturn strategy augments the normal operating mode with seven additional modes as outlined below.
Protection Mode – This mode is like failsafe or limp-in mode in other vehicles. When the protection mode is tripped by a code you will have 5th gear in drive and intermediate. When in manual low you will have 2nd gear. Cycling the ignition will give you normal mode again until the code is set again.
Cold Mode – When the engine coolant temperature is less than 122°F, the shifts will occur at a higher speed.
Hot Mode – When the transmission oil temperature reaches 284°F, the computer applies the converter clutch at lower than normal speeds and raises the shift points to a high speed. When the transmission oil temperature drops below 270°F, normal mode will resume.
Downhill Mode – When the throttle angle is less than 3% and the vehicle speed is increasing, the computer will command higher shift speeds to provide more engine braking.
Uphill Mode – When engine load is high and vehicle speed is decreasing, the computer will command higher shift speeds.
High Altitude Mode – When the barometric pressure is less than 12.76, the computer will command higher shift speeds.
Traffic Jam Mode – When the vehicle stops and starts with less than 10% throttle angle, the computer commands 2nd gear starts to increase fuel economy and reduce shift busyness. Normal mode resumes when the throttle angle is greater than 30% or vehicle speed is greater than 18.6 MPH.
Economy Mode – This mode is used by TCM under normal acceleration. The TCM provides the earliest possible upshifts and lockup for best economy. The TCM adjusts oil pressure to provide smooth shifts and engagements.
Sport Mode – The TCM changes from economy to sport mode if the accelerator pedal is pressed down quickly and the vehicle exceeds 31MPH. In sports mode the shift points are raised to provide the best performance and down shifting occurs at lower engine RPM. When the accelerator pedal is moved less quickly, the TCM resumes economy mode automatically.
Extreme Mode – Extreme mode is another way of saying kick-down. The TCM selects the lowest possible gear, for the vehicle speed, when the throttle is depressed to the floor.
Winter Mode – Winter mode is selected by using the "W" button on the top panel of the gear selector assembly. A warning lamp in the instrument cluster illuminates when winter mode has been selected. When winter mode is activated, the transmission will start out in 3rd gear to provide maximum traction on slippery surfaces. Depending on manual gear selection, the following shift combinations can be obtained.
At wide-open throttle in winter mode, the transaxle uses all gears for maximum performance.
Catalytic Converter Start – This function helps the engine to reach operating temperature by preventing converter lockup and delaying the 1-2 and 2-3 upshifts when the engine is cold. This is a normal function.
Temperature Controlled Lock-up – If the transmission temperature rises excessively as a result of a heavy load with high ambient temperature conditions, the torque converter clutch is applied to reduce heat generated by the torque converter. If the temperature drops below 20°C, (68°F), lock-up will be inhibited.
Slipping Lock-up – Slipping lock-up mode allows for a smoother lock-up engagement while reducing vibration and noise. The computer maintains a 50 to 200 RPM torque converter clutch slip in this mode. The following conditions must be met for this mode to activate:
Note: The friction properties of this transmission fluid are different from other fluids due to the large amount of converter clutch slip. Failure to use the correct fluid will cause transmission damage and codes.
Driving Uphill – To reduce shift business, the TCM may change the shift pattern slightly when driving uphill.
Neutral Control – (This function is not available on all models). Neutral control is when the TCM disengages the forward clutch, (C1), at a stop with the brake applied. This reduces the load on the engine, therefore reducing engine vibration and improving fuel economy. When the brake is released the forward clutch engages. The following conditions must be met before the neutral control function will be allowed:
When the shifter is moved to the Geartronic position, the transmission remains in the hydraulic "D" position. When the shifter is moved to the + or – positions, the gear selector module sends a signal to the TCM to shift the transmission up or down. The driver information module changes the symbol on the instrument cluster from D to the gear that has been selected. A signal is sent to the gear selector module to light the M and turn off the other LED's on the shifter console. The TCM determines if the shift can be carried out and activates the appropriate solenoids. The TCM will override the driver's command under certain circumstances. The following applies during Geartronic shifting:
Downslope Mode – The TCM detects downhill driving when the vehicle speed increases with a closed throttle. During Downslope mode the TCM changes the shift points for more engine braking.
Upslope Mode – The TCM detects uphill driving when the engine load is high and the vehicle speed decreases. Upslope mode raises normal shift points to avoid busy shifting.
Hot Mode Control – When the transaxle temperature gets too high, (no specification is given by Nissan), the TCM changes the shift points to reduce the transaxle temperature.
Manual Mode – When the manual shift lever is moved to the manual shift position, the driver can manually up shift and down shift the transaxle. The TCM will automatically up shift the transaxle if the engine exceeds a specified RPM. The TCM will automatically down shift the transaxle if the vehicle speed falls below a specified value to avoid transaxle damage. The TCM will control converter clutch operation at all times.
Manual Shifting Program - Manual shifting can be accomplished by moving the manual shift lever to the "M" position. The driver can then control the up shifts and down shifts using the + and – buttons on the steering wheel. The TCM will override the manual controls if the engine RPM's get too high and the shift has not been commanded by the driver, or the driver forgets to down shift when the vehicle speed drops below the threshold for the selected gear. The TCM will not allow the driver to down shift the vehicle manually until the vehicle speed drops below a predetermined point for the particular gear. The driver can start from a stop in 1st, 2nd or 3rd only. The manual feature can even be used when cruise control is on.
Temperature Program 1 - Temperature program 1 is activated when the transmission fluid temperature exceeds 125°C. Lock-up is activated in 3rd and 4th with no converter slip and the shifts are at a higher speed to cool the transmission fluid. When the transmission fluid temperature drops below 120°C the Temperature program 1 deactivates. Temperature program 1 will not activate when the TCM is in limp home.Temperature Program 2 - Temperature program 2 is activated when the transmission fluid temperature exceeds 135°C. This program forces the up shifts at an even higher speed and converter lock-up happens more often than in Temperature program 1. Lock-up is commanded on with no slippage during this program. Temperature program 2 will deactivate when the transmission fluid temperature drops below 127C and returns to Temperature program 1. If the transmission fluid temperature rises above 155°C for more than 2 seconds, code P0218 is set, the temperature is recorded, the "CHECK GEARBOX" light does NOT light, and the TCM sends a Bus message of "TRANSMISSION OVERHEATING" which is shown in the PID display. Engine torque is reduced to a maximum of 200Nm to protect the transaxle.
Special Gear Change Program - A special gear change program is automatically activated if a high load is detected. The up shifts and down shifts will occur at a higher engine speed to keep the transmission fluid from overheating. Examples of when this program would engage are: driving up long hills and trailer towing. The special gear change program will not initiate above 150 km/h.
DPS Program - Differential protection system or DPS reduces engine torque during extreme wheel spin. DPS will only activate below 80km/h. 150Nm is the maximum engine torque allowed during DPS activation.
Reverse Inhibit Program - There is a reverse inhibit feature that will turn on the S1 solenoid, which releases the B3 brake, if the TCM detects that the vehicle is moving at 7 km/h, (4.3 mph), or faster. Reading about all these modes makes it easy to see many possible intended or normal functions that could be mistaken for a functional problem. You will need to consider these modes carefully during your diagnosis, just as you will need to carefully work with relearn strategies when you complete a repair. We will cover those next month in AW 55-50 Differences between Manufacturers - Part Two.
July 31, 2008
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