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Transmission sensor sensibility

Making sense of these parts are important as you rebuild or service transmissions.
Wednesday, May 29, 2013 - 13:58
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My definition of sensor sensibility is to have an awareness of sensors present, their range of influence, how their signals are generated, how these signals reach the controlling computer and what affects they have on the system.

The number of sensors used, and their range of influence, will be considerably different from a Toyota Yaris when compared to a Lexus LS 600h L, particularly in regard to powertrain management as it affects both engine and transmission operation. Being sensitive toward sensor sensibility is one aspect of diagnostics that I have included in my own personal approach to diagnosing automatic transmission concerns.

From an electronic standpoint, when I begin to diagnose transmission problems, I am concerned about power, grounds and connector connections (fretting issues, cross connected issues, corrosion and/or bent, missing or pushed out terminals).

I then consider three basic areas; ABS/Traction Control, engine management and automatic transmission control along with the sensors they exclusively have and the ones they share. Within these three basic areas I have four main sensor/switch signal groups I focus on: transmission range select sensor, temperature, engine load and speed.

Figures 1 and 2

As part of this sensor sensibility, I consider how a signal is being generated and how it finally reaches the controlling computer. These are critical points to sensor sensibility thinking. A good example of knowing how a signal is being generated is the crank signal on many trucks using an Allison transmission (Figure 1). This signal originates from an AC pulse generator reading raised hash marks on the torque converter as seen in Figure 2 (Editors note: The converter industry refers to these marks as dimples. However a dimple is an inward recession so for clarification I called it a raised hash mark).

If during a rebuild service, a new or rebuilt converter is installed with a deformed hash mark (Figure 3), a rhythmic drop out of the signal will occur which in turn causes the converter clutch to cycle on and off. I have even seen a converter drilled to be drained and then spot weld closed making an additional pulse in the signal also causing TCC cycle complaints (Figure 4). The rhythmic addition or lack to the signal as seen in a scope is a sure give-away of a problem with the way a signal is being generated. (For another example of how a sensor signal is generated and its effects when compromised, refer back to the March 4, 2013, Powertrain Pro newsletter, “Fixing an Output Speed Sensor Fault on Ford and Mazda.”)

Figures 3 and 4
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