A clogged catalytic converter can, for example, affect MAF and/or Baro inputs to the ECM, which, in turn, affect the calculated engine load, which is a primary TCM input. Transmission performance will be affected by engine performance. Vehicle speed inputs are generally obtained from the anti-lock brake module and relayed through the ECM to the TCM via bus communications. Keep in mind that most hydraulic torque converter and planetary clutch engagements are now pulse-modulated to more precisely control clutch engagement and shift quality or shift “feel.” ECM Inputsīasic inputs from the ECM to the TCM include engine coolant temperature (ECT), intake air temperature (IAT), throttle position (TP), engine speed (RPM) and calculated engine load as indicated by the mass airflow (MAF) sensor or manifold absolute pressure (MAP) sensor inputs. As with many modules, the TCM must often be initialized or programmed to fit the specific vehicle application. Due to modern powertrain technology becoming more complex, the separate TCM has been re-introduced in many applications and is often part of the automatic transmission valve body. Electronic automatic shift controls were added as ECMs became more sophisticated. Initially, the ECM was used to activate the torque converter clutch. Electronic Controls A voltage loss through the ignition switch can be detected by measuring the voltage drop between the underhood fuse box B+ terminal and any of the fuses activated by the ignition switch. As with modern transmissions, the manual control lever can be used to override the automatic shift controls to meet specific driving conditions. Many early-model automatic transmissions also modified shift points by adding a vacuum modulator, which uses intake manifold vacuum to fine-tune throttle and governor pressures. The remaining shift valves work in a similar manner. In contrast, opening the throttle will increase throttle valve (TV) oil pressure on the opposing side of the shift valve, which then forces the 1-2 shift valve back to the 1st gear ratio. To illustrate the basic concept, hydraulic pressure from the governor pushes the 1-2 shift valve from 1st gear to the 2nd gear ratio as vehicle speed increases. Adding planetary gear sets and hardware can increase the number of forward gear ratios.Įarly mechanical/hydraulic transmission controls use inputs from the throttle linkage and a mechanical governor mounted on the transmission output shaft to automatically select gear ratios. The hardware used for holding a specific gear set in position usually consists of a friction band, multi-disc friction clutch or one-way mechanical roller clutch. By holding the ring, planetary or sun gear in position, the planetary gear set can produce reduction, direct, overdrive, reverse and neutral gear ratios. The ring gear provides the outer gear set. As you might imagine, the sun gear is located at the center with the planetary gears revolving around it. Modern automatic transmissions contain multiple planetary gear sets composed of sun, planet and ring gears. The TCM monitors torque converter slippage and lock-up times by comparing the engine speed with the turbine shaft speed. A torque converter lock-up clutch is also standard equipment on modern vehicles. If the stator clutch fails, acceleration will become very sluggish due to poor torque multiplication. The stator is mounted on a one-way roller clutch that allows the stator to “free wheel” as fluid flow changes in both velocity and direction during deceleration. A stator is placed between the two halves to direct oil flow in a toroidal or spiraling motion through the converter during acceleration. The front half consists of an engine-driven impeller, while the back half consists of a turbine splined to the transmission input shaft. It’s no accident that the inside of a torque converter looks like a bagel cut in half. This screen displays the status of the various transmission shift solenoids. Torque Converters Scan tools offer the best bet for diagnosing automatic transmission problems. With that said, let’s take a look at some automatic transmission basics. When the modules are separate, they share data through a bus communication system that’s not complicated to diagnose if you have basic training in that area. When separate, they become “TCM” and “ECM.” When integrated, the combined modules become a Powertrain Control Module (PCM). To better understand transmission terminology, the transmission control module (TCM) might be physically separate from or integrated with the engine control module (ECM).
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