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Diagnosing transmission control

How do you begin to diagnose any problem today’s complex transmissions could throw at you? It might not be as hard as you think.
Friday, August 1, 2014 - 07:00
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Automotive transmission shifting problems have been around for many years. In early automobiles, the transmission was shifted manually from one gear step to the next using cogwheels or gears. In most modern automobiles this is accomplished automatically. The automatic transmission started as a hydro-mechanical unit where hydraulic pressure acting on different valve areas allows hydraulic pressure to be channeled in order to lock or unlock epicyclic gear sets (planetary gears). This allows gear step changes to occur without manually shifting the transmission. In these types of transmissions, shifting problems would be located within the transmission itself or the shifting linkage.

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The transmission is a device that changes the rotational speed from the input to the output thus adapting speed to torque. This speed change from the input to the output in the automobile is where the input speed from the internal combustion engine is high in comparison to the wheel speed of the vehicle. The engine speed ranging from 600 rpm to around 6,000 rpm must be adapted to the rotational speed of the wheel, which could be zero rpm to around 2,000 rpm. The transmission accomplishes this with the use of gear changes. These gears use mechanical advantage or leverage to multiply torque. Torque is the twisting action on an object and is the product of the applied force, the lever arm length and the angle between the force and the lever arm. The gears allow the engine to produce horsepower (the rate at which work is done) that is produced at higher rpms and converts this into torque or the twisting action which is produced at lower rpms. In order for the engine’s rpm band to be correctly converted into torque multiple gear ratios must be used.

With the introduction of electronic control to the modern automobile the gear step changes are now controlled with a microprocessor. The microprocessor gathers data from input sensors and then commands solenoids on or off to control hydraulic pressure in order to accomplish gear step changes. If a shifting issue occurs in a hydro-mechanical unit the problem is in the transmission, throttle valve adjustment or shift linkage. However, with the modern computer controlled transmission this is not the case, shifting issues have become far more complicated and might be in the transmission or in the electronic control system. It will be important to separate the electronic control system from the mechanical transmission in order to properly diagnose these units.

The question at hand is how does one diagnose these modern transmissions? When looking at a wiring diagram there are so many components and wires it first appears to be over whelming. This is actually much easier than the wiring diagram would indicate. The first thing to do is to locate the shift control solenoids in the wiring diagram and the wiring color for each of the solenoids; additionally the wiring diagram will give the location of the transmission components. You also will need a shift solenoid command chart that indicates which solenoids are on and which solenoids are off for each gear application (Figure 1).

In order to check the operation of the electronic control transmission solenoids, it will be necessary to use an oscilloscope. You may ask why you should use an oscilloscope when a scan tool allows the shift solenoid data to be displayed? There are several very important reasons to use an oscilloscope. One problem with the scan tool is how the data is transmitted from the vehicle to the scan tool. With all data bus systems, there is a delay between when the scan tool requests the data and when the data is actually received and displayed on the scan tool. No scan tool data is ever live, there is always a delay. This delay will vary depending on which communications bus type is used and how much traffic (data transmissions) is currently on the bus. This delay can be large and will create real problems when trying to diagnosing electronic systems. These electronic systems can fail in a millisecond (1/1,000 of a second); however the scan tools loop read speed can be greater than 250 milliseconds (1/4 of a second). If you are using a scan tool to diagnose the transmission, you might feel the transmission fail but be unable to determine what caused the failure.

It is also be imperative to understand that the scan tool data is only as good has the engineering of the Transmission Control Module (TCM). When monitoring the transmission shift solenoids, the scan tool display will indicate the command status of each solenoid. This will indicate whether the solenoid is on or off so you can determine what gear step is commanded.

The problem with a command status on the scan tool is this is exactly what this is: a command. This command does not mean that the solenoid was actually turned on or off. The command is accomplished by an algorithm running in the microprocessor. This base shift algorithm will equate the engine load, engine rpm, engine temperature, transmission temperature, transmission pressure and vehicle speed in order to select the correct gear for the current operating conditions for the vehicle. This algorithm will then command the solenoids on or off, which will be enabled by the TCM’s hardware.

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