Transmission sensor sensibility

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.

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.”)

A simple illustration about how a signal finally reaches the controlling computer is CAN BUS. There are several manufacturers whose transmissions do not have an output/vehicle speed sensor (OSS/VSS). Instead, they use the ABS, which can broadcast a VSS signal over the network.

With an OSS/VSS in the transmission, earlier year Fords used a PSOM in the instrument cluster (Figure 5) to condition the signal to the PCM while GM used a DRAC/Speed Buffer (Figure 6).

Many other manufacturers to this day like to run vehicle speed signals generated from the transmission into the instrument cluster to be conditioned for the PCM.

Knowing how a signal is being generated and the route it takes to reach the controlling computer goes a long way when diagnosing incorrect signals. This also lends towards being intuitive regarding possible tailored computer strategies.

As mentioned before, there are several transmissions that do not have an OSS/VSS signal so the system relies on the ABS signal for a vehicle speed. There are also systems that use both the OSS/VSS and ABS. This is important to note, do not limit switch/sensor signals to just those on the transmission. When you see possible redundant signals you should think why; what if any is the related computer strategy? A redundant signal is not always just for error detection/correction, it can also be used for comparison as well. This is what I was referring to by being intuitive toward possible tailored computer strategies.

With some high-end vehicles, tire size can affect shift scheduling due to curve recognition/lateral motion programs. This type of program is designed to prevent shift business during turns, which it recognizes via the wheel speed signals. If there are a couple of underinflated, overinflated or worn tires and a beefy treaded tire such as a spare, this will alter a wheel speed signal enough to initiate the program while driving straight. The result is late shifting, no shifting, or a loss of high gears and it may seem to be related to engine load when it is not as the heavier the throttle, the more pronounced the problem appears.

Many BMW (Figure 7) and Land Rover vehicles (Figure 8) are the most sensitive to this type of programming. While writing this article, technical field advisors Jim Dial here at ATSG solved an intermittent late shift under medium to hard acceleration with a 2006 Dodge Charger using the NAG 1 transmission. One tire was approximately ½” taller than all the rest. Once this discrepancy was resolved the complaint was resolved as well. Nissan’s Maxima using the RE5F22A (AW55-50SN) transmission experiences a slightly different problem. Downshift clunks and bumps especially on turns is experienced with unequal sized or unequally inflated tires (Figure 9), especially low profile tires.

As many times as this sensor sensibility mindset has served my diagnostic skills well, I do get caught off guard temporarily every now and again. When I finally get the problem nailed down, that which was lacking becomes a “school of hard knocks” dimension added to my sensor sensibility thinking. No pun intended, it was quite a learning experience to discover that a malfunctioning knock sensor will cause a loss of high gear with Toyota and Lexus vehicles.

Most recently, I dealt with a 2010 Ford Flex 3.5L with a 6F35 FWD transmission having an intermittent torque converter clutch cycling on and off concern. My immediate approach with a scan tool was to check the run monitors, for any stored codes or freeze frame data. While there I checked the transmission range sensor signal, the transmission control switch, the brake on/off command, engine coolant and transmission fluid temperature sensor readings. In doing so I discovered a code P1288 for an out of test range for the Cylinder Head Temperature (CHT) Sensor (Figure 10).

Because I could not find an actual Engine Coolant Temperature Sensor PID, I looked into this a bit further and discovered that this vehicle does not use an ECT sensor.

Ford provides the following CHT information:

The Cylinder Head Temperature (CHT) sensor is a thermistor device in which resistance changes with the temperature. The electrical resistance of a thermistor decreases as temperature increases, and the resistance increases as the temperature decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature. The CHT sensor is used to control TCC operation.

The CHT Sensor is located in the cylinder head and measures the metal temperature. The CHT sensor provides complete engine temperature information and is used to infer coolant temperature.

If the CHT sensor conveys an overheating condition to the PCM, the PCM initiates a fail-safe cooling strategy based on information from the CHT sensor. A cooling system concern, such as low coolant or coolant loss, could cause an overheating condition. As a result, damage to major engine components could occur. Using both the CHT sensor and fail-safe cooling strategy, the PCM prevents damage by allowing air cooling of the engine and limp home capability.

The codes assigned to the CHT:

P1288 - Cylinder Head Temperature (CHT) Sensor Out of Self-Test Range

P1289 - Cylinder Head Temperature (CHT) Sensor Circuit High

P1290 - Cylinder Head Temperature (CHT) Sensor Circuit Low

I found this to be yet another good example as to how a sensor signal is generated. What a great way to provide an Engine Coolant Temp signal but by measuring metal temperature. There is definite sensor sensibility to it for isn’t that the goal in mind of the coolant system? 

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