4848 Blog 4 Electronic Transimission

Background:

GM used semi-automatic transmission in automative in 1937, however a real automatic transmission system was firstly used on the Oldsmobile back in 1948 also by GM.

An automatic transmission also known as auto gearbox is a type of motor vehicle transmission that can automatically change gear ratios when the vehicle moves, so the driver does not have to shift the gears manually and a clutch pedal is not needed. There are more types of automatic transmissions like the older type semi-automatic transmission and the more modern and better Continuous Variable Transmissions(CVTs) and Tiptronic Transmissions.

Comparing the automatic transmissions to the manual gearbox, vehicles equipped with automatic transmissions are less complex to drive, it became a lot more popular after the 1990s. In a lot countries, some driver licenses only allows people to drive automatic vehicles but a manual license can allow the driver to drive both manual and automatic vehicles. Since automative cars are easy to drive and very reliable, a lot people do not even know how to drive a manual car.

In the automatic transmission, there is a torque convertor which takes the place of  a clutch in a manual transmission which you can see is the big blue/green thing on the left to the transmission shown in the above picture. But Honda automatic transmissions use gears like a manual gearbox and apply the correct gear using a clutch, this is different to other manufacturers but all the other automatic transmissions use the same principle.



Theory:




Some common modes for an automatic transmission system:

• P(Park)-- the output shaft is lock for stopping the vehicle to go any directions, often used when the engine is off or the car does not need to be driven.
• R(Reverse)-- the reverse gears are engaged and the vehicle will go backwards
• N(Neutral)--all the gears are disengaged and the wheels are free from transmission control, the vehicle will move forward or backward if the car is not parked on a flat surface, PNP(Park Neutral Position) switch will lock the engine away from starting up for most of the vehicle for safety reason.
• D(Drive)--All the gears can be engaged depends on how fast the vehicle goes, often used for driving
• 2 or S(2nd gear or Second)--only the 1st and 2nd gear can be engaged
• 1or L(1st gear or Low)-- only the 1st gear can be engaged.

A automatic transmission is a system which is way more complicated than a manual gearbox system because all the jobs are done by electrical components rather than shifting gear manually.


An automatic transmission system is running by a PCM( Powertrain Control Module) similar to an engine ECU(Electrical Control Unit), all the electrical components is energized and controlled by the PCM


Here is a diagram how a PCM controls the whole system:

All the solenoids are controlled by the control coils, when the PCM energizes each coils the solenoids will generate magnetic field and do its job.Let me explain this using the shift solenoids with a picture shown below:

We will just assume a vehicle is running on a automatic transmission with 2 shift solenoids. If we put on Drive, and drive the car,at the 1st gear the 1st solenoid will be and 2nd solenoid will be off, when the PCM detects the gear needs to be changed to 2nd gear then it will turn off the 1st solenoids so both solenoids are not working and turn on the 2nd solenoid if it needs transmission to go into 3rd gear,at last if the vehicle is faster enough then the PCM will turn on both solenoids for going into 4th gear.


And how do PCM know when to shift gears?
The PCM gets signal from few sensors to decide if it needs the gear changed and they are:

• VSS( Vehicle Speed Sensor) for how fast vehicle is going
• TPS( Throttle Position Sensor) for engine load
• CKS( Crankshaft Position Sensor) for engine RPM
• ECT( Engine Coolant Temperature Sensor) for engine temperature
• BPS( Brake Pedal Switch) for driver intent

When the vehicle is going faster enough or engine is having enough load or engine RPM is higher enough, the PCM detects the signals from the above sensors and shift up the gears. When the vehicle is slowing down or engine load is decreasing or engine RPM is going low, the PCM also detects the signals from the above sensors and shift down the gears.
Another condition is, when the engine is cold, the PCM always shift up gears a little later than when the engine is hot.
Some unique sensors for the automatic transmission system such as ATF( Automatic Transmission Fluid)sensor and PNP(Park Neutral Position) sensor will also affect on PCM deciding when to change gears.


Another very important thing in an automatic transmission system is the fluid, not just to cool down the temperature of the transmission, it is also used in the valve body to change the gears.The fluid will get pumped to the gear that the PCM wants to change.








Experiment:




To find out the problems of a automatic transmission system, simply use the scan tool in the  OBDII plug of the car for diagnosis.
When the PCM detects there's a problem it will write a fault code in the memory and can be picked up by the scan tool in diagnosis.


The vehicle we used to diagnosis was a 1998 Mitsubishi Lancer

And the gear changing status is shown in the picture below:

From the test, I noticed the TCC pulse width modulation solenoid got turned on at 31km/h,the duty cycle of the solenoid was about 40% and increased to 97% after the vehicle reaches 77km/h.


Here is a list of common OBDII fault code which can help fault finding an automatic transmission system:

These are only a part of the codes, there are a lot more


Let's assume if we find a fault code of P0710 from a vehicle which the list above shows us there is a problem in the Transmission Fluid Temperature Sensor Circuit.However it does not mean the sensor needs to be changed, if the sensor is broken, it must have some reason that caused the problem, maybe the resistor in the circuit is shorted? maybe the vehicle is using wrong type of fluid and broke the sensor by over heating? there could be a lot possibilities. Therefore the fault code can only narrow down the problem to the particular circuit but further investigation needs to be done to get the problem fixed.








Reflection:


The results showing at the test indicated that particular transmission was working good.
The solenoids was in good order as the TCC pulse width modulation solenoid should be off at low speed and turns on with speed increases.


A quite common symptom happens to an automatic transmission system is the gear won't change or get stuck in some gears. This is usually caused by broken shift solenoids.
To fix this, we will need to know how the shift solenoids work from the engine manual and using an oscilloscope to test the sensors. It will be the same way as testing other sensors in the engine, get the waveform and compare it to each solenoid.


There is one more important thing to maintain a automatic transmission is the fluid. The fluid is used to cool down the temperature and also change gears, use the proper fluid is very IMPORTANT for all the automatic vehicles and make sure there is enough fluid and no leak from the transmission and the transmission oil cooler.

References:

Unitec moodle

Wikipedia

Vijay Naidu