Don’t be alarmed if you pull an automatic transmission trouble code when diagnosing a “check engine” warning light! Since the automatic transmission operation has a major effect on grams-per-mile exhaust emissions, you’re going to see the occasional “global” P0700 to P0900-series diagnostic trouble codes (DTCs). The fact is, if you understand basic transmission operating principles, and you’re proficient with a labscope, digital multimeter and professional scan tool, it’s not a major step to diagnose and repair many electrical/electronic automatic transmission failures.
To better understand transmission terminology, the transmission control module (TCM) might be physically separate from or integrated with the engine control module (ECM). When separate, they become “TCM” and “ECM.” When integrated, the combined modules become a Powertrain Control Module (PCM). 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. With that said, let’s take a look at some automatic transmission basics.
It’s no accident that the inside of a torque converter looks like a bagel cut in half. The front half consists of an engine-driven impeller, while the back half consists of a turbine splined to the transmission input shaft. A stator is placed between the two halves to direct oil flow in a toroidal or spiraling motion through the converter during acceleration. 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. If the stator clutch fails, acceleration will become very sluggish due to poor torque multiplication. A torque converter lock-up clutch is also standard equipment on modern vehicles. The TCM monitors torque converter slippage and lock-up times by comparing the engine speed with the turbine shaft speed.
Planetary Gear Sets
Modern automatic transmissions contain multiple planetary gear sets composed of sun, planet and ring gears. As you might imagine, the sun gear is located at the center with the planetary gears revolving around it. The ring gear provides the outer gear set. 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 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. Adding planetary gear sets and hardware can increase the number of forward gear ratios.
Early 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. 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. 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. The remaining shift valves work in a similar manner. 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. As with modern transmissions, the manual control lever can be used to override the automatic shift controls to meet specific driving conditions.
Initially, the ECM was used to activate the torque converter clutch. Electronic automatic shift controls were added as ECMs became more sophisticated. 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. As with many modules, the TCM must often be initialized or programmed to fit the specific vehicle application. 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.”
Basic 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. Vehicle speed inputs are generally obtained from the anti-lock brake module and relayed through the ECM to the TCM via bus communications.
Transmission performance will be affected by engine performance. 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. The performance of a seemingly insignificant engine part, like a thermostat opening too early, can actually prevent torque converter lockup. As more gear ratios are added, engine-operating parameters become increasingly important for providing correct automatic transmission control. You should also be aware that a few high-end Euro imports might integrate transmission controls with the vehicle security system, which can interfere with parts availability for non-authorized repair shops.
Typical inputs from the TCM to the ECM include transmission fluid temperature (TFT), manual lever position (MLP), turbine shaft speed, output shaft speed and shift solenoid state. Remember that the TFT parameter must be accurate because it affects clutch application characteristics and gear ratio selection. Lastly, the transmission must receive a valid vehicle speed signal (VSS) as one of its control parameters. In some applications, the VSS sensor functions as the output shaft speed sensor while, in others, the VSS and the output shaft speed sensors operate as separate components.
The TCM monitors transmission gear selection by comparing the turbine shaft speed with the output shaft speed. When the appropriate clutches or bands are applied, the indicated input/output shaft gear ratios should be a fixed ratio that is listed in the transmission specifications. If the input/output ratio doesn’t correspond to the specified ratio due to clutch slippage or electronic failure, the TCM typically defaults to an intermediate gear range activated by a roller clutch and stores a “gear ratio error” diagnostic code.
In default mode, the transmission will typically shift 1-2 and then remain in 2nd gear. In default, forward gear operation is normally controlled by a one-way roller clutch, which prevents transmission lockup by allowing the transmission to “free wheel” during deceleration. In addition, the TCM monitors all internal electronics. If, for example, a 2-3 shift solenoid fails, a P0700-series DTC will be stored and the TCM will default to 2nd gear. A similar default mode will occur if the input or output shaft speed sensor fails.
A Word About Reprogramming
Most no-code shift quality complaints are addressed by reprogramming the software contained within the TCM or PCM. But don’t forget that while the engine might “sound good” idling in the service bay, the calculated load might be affected at speed by a clogged catalytic converter, a mistimed camshaft or a dirty mass airflow sensor.
Before jumping into reprogramming, try to determine if the latest reprogramming calibration actually addresses a shift quality complaint. Also, reprogramming can be written to address a “second-design” component replacement, like a mass airflow sensor. Nissan is a prominent example of this. In these cases, the expensive MAF sensor must be replaced before engine/transmission performance can be restored. In short, look before you leap because, once installed, reprogramming calibrations can’t be reversed.
Fortunately, many automatic transmission problems are created by shift solenoid failures, which are generally categorized as “shift scheduling errors.” Shift scheduling errors usually occur only one at a time. If, for example, operating voltage is reduced to the transmission solenoids or sensors because of a bad ignition switch, multiple shift scheduling error DTCs will be stored. In addition, inspect the weatherproof transmission connector for signs of water intrusion and corrosion, which will produce similar DTCs.
If so equipped, check the transmission dipstick for fluid level and for a catastrophic clutch failure, which will blacken the fluid. Since the transmission oil pan must be removed to access the shift solenoids, examine the oil pan for frictional or metallic residue. If such residue is apparent, the transmission should be removed for inspection and repairs. During the refill, remember that installing a non-OE spec transmission fluid can create the dreaded “no code” shift quality complaint.
Since scan tool datastream and bi-directional capability varies widely among vehicle applications and scan tools, it’s doubly important to examine all menus for data and functional tests. Bi-directional capability is especially useful for testing shift solenoids and circuits. On occasion, a repeat electronic failure might baffle even the most experienced transmission repair shop.
In one remarkable temperature-sensitive case, I found an intermittent open-circuit ground for the VSS sensor by watching for voltage variations on my scan tool graph while wiggle-testing the engine wiring harness. I found an enlarged female pin on the TCM connector, in another instance in which the vehicle owner had replaced the TCM three different times. And, most recently, I found an intermittent overdrive engagement condition caused by a nearly failed ECM driver that was randomly activating the o/d shift solenoid.
As you can see, most transmission electronics diagnostics are far from rocket science. By understanding the basics, the driveability diagnostic technician can use the same skills and tooling for transmission diagnostics as he uses in his daily diagnostic routines.