Transmission Diagnostics using
These days, nearly all the automatic transmissions
on the road are controlled with two basic types of
systems. Hydraulically-controlled shifting or
Electronic/hydraulically controlled shifting. In either
case, they shift gears in much the same manner but
use different components to control the shift
points and shift feel.
For clarification, a Transmission is any device that
solely converts torque into speed which in turn sends that energy to the final drive and then onto the drive wheels. A Transaxle on the other hand, is similar to a transmission, but contains the final drive and differential that is connected directly to the drive wheels with a constant velocity joint (CV axle). The electronics are fairly similar and for the most part can be diagnosed in the same manner, it’s just good practice to call em’ what they are.
The type of transmission fluid used is a very important as well. Using the wrong type of fluid, can have devastating effects on the longevity and performance to most transmissions. Fluids such as GM Dexron VI, Ford type F, Ford Mercon V and SP, and most CVT transmission fluids all have very specific properties designed to be used in specific types of transmissions. It wouldn’t be the first time that I’ve had to diagnose a transmission shift problem that wasn’t necessarily a fault of the transmission but a fault of the operator putting in the wrong fluid.
How they work
Let’s look at a basic 5 speed transmission, it has 5 forward speeds, one reverse, and a TCC (torque converter clutch). There are generally 4 valves inside the transmission labeled as 1-2, 2-3, 3-4, 4-5 shift valves. The gear selector moves a manual valve in the transmission, and uncovers oil passages to the desired shift valve which allows oil pressure to pass through that specified point. The spool valve is situated so that it cannot be in two positions at once. For example, if you wanted reverse, the spool valve would uncover the passage from the oil pump, and to the reverse bands/clutches. But if drive is selected, the spool valve moves to cover up the reverse passages, and uncovers the drive passages. For the transmission to shift 1-5 automatically, we need devices that will tell it when to shift, since it is not electronic. A Throttle Valve or Governor Valve is used.
The Governor valve is centrifugal valve that is driven like a speedometer gear, which has weights attached to it that fly out as it goes faster. This allows a greater amount of oil pressure to pass through it. The Throttle valve type works similarly like a gas pedal. By stepping on the gas pedal, the valve moves, and increases oil pressure in this track. This pressure is fed each shift valve, and assists in keeping the respective valve closed, until governor (speed) is too great, and the valve must move to the open position, allowing the oil pump to apply pressure to the next clutch.
Each shift valve stays open to apply oil pressure to the next shift valve when it opens. When the car shifts to 4th and 5th gears, oil pressure is also routed to the TCC (or PWM) solenoid, allowing the TCC to be applied when needed. This allows converter lockup and a direct connection between the engine and transmission. Once the brake pedal has been engaged or in some setups, the gas pedal is moved significantly, the converter clutch will normally disengage.
In an electronic-shift transmission, hydraulically controlled throttle pressure and governor pressure are eliminated entirely. Electronic solenoid valves are used to create throttle pressure, governor pressure. A TCM or PCM is used as the store house for the transmission matrix which monitors and dictates shift timing, based on vehicle speed, engine load, and any number of additional inputs such as MAP or MAF input, CTS, TPS, and RPM for example.
This allows for a much simpler valve body design, and diagnostics can be greatly improved with the addition of monitoring sensors and solenoids. Basically, the functions of the inner workings of the transmission remain basically the same with the exceptions of electronic solenoids that move the fluid in the transmission instead of how the hydraulic transmission moves the transmission fluid. Now all of the inner movements as well as the input and outputs of the transmission or transaxle can be monitored, tested, and diagnosed with a scanner without removal and tearing the transmission down.
Transmission problems can be divided into electrical, hydraulic or mechanical problems. Each issue can produce an error code by means of the sensors that monitor the inner workings of the transmission, but using the literal meaning of the codes definition can inadvertently lead you to the wrong conclusion or a complete misdiagnosis if that information isn’t complete.
Although, a sensor failure code can mean the sensor has failed it’s not always true. In some instances the connector or wire leads may actually be the fault and not the sensor itself. Code definitions should never be taken at face value and as with any code, a more in-depth diagnostics needs to be performed.
When it comes to aftermarket type scan tools the first thing to be aware of is that they are not all the same. An aftermarket scanner will offer coverage over a broader range of manufacturers, while the factory scan tool is designed for that specific manufacturer. The trade-offs between the factory scanner and the aftermarket scanner only become apparent when you have the chance to compare the two.
Since most shops work on different makes and models, a generic scan tool becomes a necessity due to cost. Having factory scanners would be ideal, but they change too. Which means at some point in time that factory scanner may become obsolete in regards to the year of car that is coming into your service bays now. Some of the aftermarket scanners have solved some of those issues by making their scanners updatable.
One of the challenges for the aftermarket scan tool manufacturer engineers is deciding what information to put in and what to leave out. When it comes to automatic transmission data, it seems to be the one area that is left more to chance than to necessity. I’ve found some aftermarket scanners that instead of giving a percentage value for such things as the torque converter lock up all they’ll gave was a Yes or No. It’s sort of helpful, but not really. Especially when the diagnostic tree asked for a percentage and not a true or false answer.
But it’s not just the capabilities or the way they read that makes it more of a challenge for the aftermarket engineers to input the data into their scanners, it’s also the amount of changes the electronic transmissions have gone through.But it’s not just the capabilities or the way they read that makes it more of a challenge for the aftermarket engineers to input the data into their scanners, it’s also the amount of changes the electronic transmissions have gone through. For example, the GM 4L60-E transmission which began in 1993 is not the same transmission as it was by 2009. Solenoids were changed from on and off types to PWM (Pulse Width Modulated) type from year to year. Some solenoids were even entirely eliminated or their actual functional name was changed.
As these changes were occurring, computer strategies were changing as well, which in turn affected the data parameters being offered on aftermarket scan tools. Some parameters and sensor PID’s were overlooked while others carried over as if they belonged in that year vehicle and led to confusion as to what data was actually being read. Not to say it is impossible to use an aftermarket scanner, you just have to sort through the information and PID’s and make sure the information you have in front of you is viable and accurate for the vehicle you’re working on.
Shift BIAS control
A neat feature I find more useful than most others is the BIAS control features. When I look at an aftermarket scanner and its capabilities it’s almost always the first feature I’m most concerned with. The ability to snap a shift solenoid on or off either in the service bay or on a drive test can simplify the diagnostic procedure immensely. It not only tells me the condition of the shift solenoid but wire integrity all at the same time. Saves me the time of tracing out a length of wire for a bad spot if I already know the leads are in good shape and the solenoid is clicking away in the transmission.
One of the other nice aftermarket tools to have is a Schaffer Shifter or Super Shifter. This an in-vehicle tester that allows you to shift the transmissions just as the vehicle's computer does. Test solenoids, PRNDL, and much more. A very handy tool for electronic transmission diagnosis.
For the most part, an aftermarket scanner can perform nearly all the functions you’ll need to do while performing any diagnostic work on today’s transmissions. Code reading is available on all of them, but it’s those “extras” that will make the difference. As with any diagnostics, knowing the inner workings of the system you’re testing will greatly enhance your ability to test it properly.