Like many of you, I have the ASE A7 (Automatic Transmission/Transaxle) certification in my wallet. And, like many of you, I never have had a transmission laying in pieces on my workbench.
Let’s face it. If there is a problem with the transmission requiring more work than can be accomplished in the car, we tend to either send it out to a specialty shop or replace the failed unit with a good used or reconditioned one. But with advancements in transmission technology, the challenge is properly diagnosing the problem to begin with. Is an engine management failure, an electronic transmission control failure or an internal component failure causing the customer’s complaint?
The First Automatic
General Motors gets the credit for being the first OEM to install a fully automatic transmission in a production vehicle. The “Hydra-Matic” was a true, automatic four-speed and came as an option in the 1940 Oldsmobile line. Internally, the new transmission was very similar to a previous GM attempt at automating the powertrain called the Automatic Safety Transmission.
Shortly after its debut, GM found itself shifting from civilian production to war-time production. Among its projects was the M-5 Stuart tank, powered by two Cadillac V8s (one per track) mated to Hydra-Matic transmissions. This gave GM bragging rights after the war, advertising their transmissions as “battle tested.”
It also became apparent to GM’s competitors that not offering an automatic in their model lines was a serious competitive disadvantage. Even though the Hydra-Matic shifted hard, reduced performance and gas mileage suffered, the consumer didn’t care. And while some manufacturers developed their own automatics, many smaller companies of the times opted to buy GM’s for use in their own products. By 1952, the Detroit transmission division had built more than 2 million units, making the Hydra-Matic arguably one of the most successful transmissions of all time.
The Technology Of Automatics Beginning in the 1980s, transmissions began heading the way of
other vehicle systems, turning over their control to computers and solenoids. The goal then, as it is today, was to improve efficiency and increase fuel economy.
Gasoline engines produce maximum torque in a fairly narrow rpm range. To keep the engine in that range, engineers are adding additional gear ratios. Multispeed transmissions are now in production incorporating six-, seven- and even eight-forward speeds.
Constant variable transmissions (CVTs) are popular with many Japanese automakers, and they too continue to evolve. The Nissan XTRONIC CVT transmission, for example, has a unique supplementary two-speed planetary sub transmission that operates in series with the CVT variator. This innovation increases the ratio spread over a typical CVT, and allows lower rpm at cruising speeds while still maintaining low speed drivability. Size and mass are also reduced and the manufacturer claims a 30 percent reduction in internal friction with a corresponding increase in fuel economy of 10 percent.
A slightly different take is the Nissan 1MC2 system, a hybrid tranny
On full acceleration, both clutches are engaged and the motor adds its torque to the engine’s output. Nissan says that the absence of a conventional torque converter adds a “direct feeling driveline” that should appeal to sporty drivers. When slowing down, the front clutch disengages while the rear remains engaged to take advantage of regenerative braking. Low speed operation is electric drive, with the front clutch open to separate the engine from the driveline.
Manual transmissions also are becoming more automated. Two designs in particular blur the definition of “manual” and “automatic.” One is appropriately named the Automatic Manual transmission (AMT) and the other is a design called the Dual Clutch transmission (DCT).
Consider shifting a manual transmission without pushing in the clutch. To make this happen seamlessly, you’ll need to cover a few bases. First is the transmission control unit (TCM), mated to the network communications bus so that it not only receives the driver’s inputs but also the information it needs on how the vehicle is currently operating. Second is an electronically controlled clutch actuator to replace the driver's left foot on the clutch pedal. This actuator can be electro-mechanical (using an electric motor to move the clutch linkage) or electro-hydraulic.
Last is an electro-mechanical shift actuator that takes the place of the shift linkage. These components allow a conventional manual transmission to shift like an automatic, with no additional effort required by the driver.
The DCT offers the efficiency of a manual shift tranny with the convenience of an automatic. DCTs essentially are two manual transmissions in one. One side handles the even gears, and the other side takes care of the odd gears, with some DCTs having seven total gear combinations. The heart is a dual, multi-plate, wet or dry clutch separated into an inner and outer assembly. One clutch transmits power to one side of the gearbox, and the other to the opposite side. An electronic control unit anticipates which gear will be needed next and engages that gear to its shaft, but no power flows until the corresponding clutch is engaged. The result is a seamless shift from one gear to the next.
Before You Condemn the Trans Determining the need for transmission repair or replacement is akin
Always begin with the fundamentals. Take the customer for a test drive so you can feel first hand what they are complaining of. Often the fault lies in the engine management system and the bump and jerk the driver is feeling is not tranny related at all. Be sure to check the OE Technical Service Bulletins (TSBs) for information related to the complaint. If it is in the tranny, there may even be an extended warranty period offered that will save your customer money and make you look like a hero.
Once you have the car back at the shop, check the fluid level and condition. Like most maintenance, customers hesitate spending money on routine fluid changes. And like any other fluid, transmission fluid has a finite life. Once surpassed, it can no longer do its job and in some cases, catastrophic transmission damage can occur shortly thereafter. Don’t expect it to be fresh out of the bottle pink, but if the fluid is black and has a burnt odor to it, it’s a good bet that the fluid needs changing.
While we’re on the topic, simply dropping the pan to drain the old
| Fluid Couplings and Torque Converters |
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Fluid couplings are a means of transmitting energy hydraulically. Two ring tori (plural for torus, a geometric shape that looks something like a doughnut) face each other in a closed container filled with a hydraulic fluid. One ring is the impeller, driven by the engine. As it spins, it throws fluid toward the other ring, the turbine. The turbine begins to spin and the energy is transferred from one to the other. Want an idea of how this works? Take two electric fans and face them toward each other. Turn only one on and see what happens. A torque converter adds a third component called a stator, which resides between the impeller and the turbine. The stator is a bladed disc, mounted on a one-way clutch on a fixed shaft surrounding the transmission input shaft. At low speeds, oil leaving the turbine hits the blades of the stator, reversing its direction before it get back to the impeller. That change of direction increases the efficiency of the impeller. At lower speeds, the impeller actually multiplies the torque from the engine. As the impeller and turbine speeds increase, the multiplication factor fades. Eventually, the stator begins to freewheel on its one-way clutch so that it won't impede the return of oil from the turbine to the impeller. The inherent slippage of the fluid coupling is both a good thing and a bad thing. Bad, in that some energy is lost to heat and not available to drive the wheels. Good, in that the low speed slippage makes it easier to get the vehicle rolling and keeps the engine from stalling when it comes to a stop. To get back some of that lost energy, mechanical clutches are incorporated that allow a direct lock up between the engine and the tranny’s input shaft at cruising speed. |
fluid is an exercise in futility. Most of the fluid is still up in the hydraulic passages and the torque converter. The only way to get it all out is to flush the system with appropriate equipment and the right type of fluid. You wouldn’t do a “drain and fill” on the cooling system, would you? Or just change the engine’s oil filter and not drain the crankcase? Same thing!
Be sure to follow the published service procedures for checking fluid level, too. Some cars need to be checked with the fluid in a specified temperature range, others specify fluid markings based on varying temperature and some have no dipstick to check the fluid with. Proper fluid level is critical, just as it is in the engine, and has to be neither under- nor over-filled. Often, transmission shifting complaints can be cured simply be servicing the fluid.
While not as common as it used to be, don’t overlook the possibility of a misadjusted cable. Check the service information for any routine adjustments the transmission may need. Once you’ve determined that the basics are in place, take out your scan tool and check the Engine Control Module (ECM) for stored trouble codes. Some ECMs do double-duty, acting as both engine and transmission controller.
That’s the difference between the generic ECM and the more precise term Powertrain Control Module (PCM). A PCM will have specific transmission fault codes stored, while the ECM may only give you a P0700 (transmission fault detected) as a clue there’s an electronic problem to solve. Certainly, if you discover engine-related Diagnostic Trouble Codes (DTCs), correct them first and retest for the customer’s original complaint.
