The transmission game has changed. Solenoids, sensors and computers have replaced vacuum lines, governors and kickdown cables on modern automatic transmissions. The tools have also changed: scan tools, scopes and meters have replaced pressure and vacuum gauges.
Even with codes and datastream information available, you must be able to think like the transmission to find the correct diagnosis, which typically doesn’t involve replacing the entire transmission.
Thinking like a transmission means understanding what information the transmission has, how it interprets that information and how it uses the information to select the correct forward gearing. You also have to understand how the transmission prioritizes performance when it’s compromised.
Rule 1: The modern transmission is information hungry.
On earlier models, once the throttle cable was pulled so far, or a switch under the gas pedal was activated, the transmission would kick down to the next, lower gear.
The modern transmission is much more complex. When the driver presses the gas pedal, the request goes to both the engine and transmission control modules. A transmission control module/software may look at 30 different parameters to determine gearing and torque converter settings.
On most late-model vehicles, the transmission is on one of the faster serial data buses or an exclusive bus that ties it to the engine controller, and maybe one other controller. Some use one module to control both the engine and transmission.
The control module craves information, and it uses this information to make the best possible shifts — shifts that are efficient and smooth and cause the least possible damage to the wear components inside the transmission.
It uses information beyond the calculated engine load, throttle position and input/output speeds. For example, some control modules will look at mass airflow (MAF) sensor and engine temperature information to determine the correct shift points and line pressures.
Relying on this extra information helps the transmission sense additional problems, such as malfunctioning crankshaft position sensors, high engine temperature and even clogged air filters.
If any of this information is missing or outside of operating parameters, the control module will throw up a red flag and set a code. Depending on the discrepancy, the control module might conclude that the problem could damage the transmission and, in turn, reduce power or go into a limp mode as a response.
Rule 2: Transmissions are programmed to save themselves.
One condition a driver can’t ignore is a transmission that has gone into a limp, failsafe or reduced-power mode. To prevent a transmission from destroying itself due to a malfunctioning component, a controller will put the engine and transmission into a safety mode so that the wear components won’t be compromised.
Customers will notice that their speed is limited or their engine is not allowed to rev beyond a preset rpm limit. Internally, the transmission may increase line pressure and change shift points, with some transmissions skipping gears to prevent further damage to the clutch pack.
A multitude of conditions cause a failsafe mode — a failure to communicate on a data bus, a failed sensor or a bad shift. The code is just the starting point, not a final diagnosis.
Rule 3: Transmissions adapt to wear; a driver does not.
A transmission is programmed to deliver seamless shifting. The goal is to engage and disengage clutch packs smoothly, but, as a transmission wears, the engagement behavior can change. Having more gears helps, but “how” those gears are controlled makes the biggest difference.
The way a transmission determines a shift point is similar to a closed-loop feedback emissions system that still uses short-term and long-term fuel trims. When an automatic transmission shifts gears, it looks at the input and output speeds to determine the quality of the shifts.
While changing gears, the transmission may detect a flare in engine rpm that does not produce a change in vehicle speed. The control module will register this as a bad shift and recognize that the clutches are slipping with the current shift program. The control module will then make changes to the shift timing or line pressure in order to eliminate the gap during the shift. This will prevent slipping in the clutch and potential damage.
If a clutch is gripping too harshly, the control module may see a sudden drop in rpm, with little change in vehicle speed. The control module will then change how the clutches are engaged. Most modern transmissions will set codes if they can’t compensate anymore, or if there is a sudden change in clutch performance.
This adaptation mechanism can also adapt based on the condition of the transmission fluid. If the fluid is the wrong specification or has degraded to the point that it’s changing the friction levels of the clutch, the computer will adapt and change the shift behavior.
But this adaptation feature can mask the problem for only so long, and it can create other problems. For instance, if the module controlling the transmission loses the shift information because the battery was disconnected or the module was replaced, then the transmission will revert to factory settings that do not include the adjustments for clutch wear. So, a minor battery replacement could leave customers accusing you of damaging their transmission.
There are a few methods for recalibrating the transmission control module. First, drive the car or let it run with the wheels off the ground (however, some ABS/traction control systems will not allow you to do this). Second, some transmissions have calibration protocols that require a scan tool.
Rule 4: Pressure is now on demand.
On early models, a transmission had only one-line pressure. The pump was always supplying enough fluid and pressure for the highest demand situations, even if it wasn’t needed, which resulted in a lot of wasted fuel.
Most modern transmissions now use a variable displacement pump that produces pressure on demand, or an electronic actuator controls them. Modern transmissions may also have multiple-line pressures to further increase efficiency. These are reasons why a late-model automatic transmission can equal or beat the fuel efficiency rating of a manual transmission.
Years ago, when a driver came to your shop with a malfunctioning transmission, the typical repair path involved replacement with a rebuilt unit. The transmission stood alone in the vehicle, and any problems were confined to mechanical items inside the case. Today, transmissions can save themselves until they can be serviced, and, oftentimes, the problem will exist with a component outside the transmission. You’ll just have to think like a transmission to find it.