We are constantly talking about changes in the automotive world. Vehicle manufacturers are making safer, more fuel-efficient, and more responsive cars in response to customer demand and government regulations. As a result, engineers are designing systems with elaborate actuators, sensors, and computer programming that deliver the desired results. Unfortunately, the added layer of complexity leaks into the diagnostic realm. In this article, we will look at some new sensors and systems that have altered the diagnostic process when attempting to find the root cause of transmission complaints.
FUEL SAVING STRATEGIES
The challenge for each new model year is making vehicles more fuel-efficient. Stop/start and neutral idle technologies are some of the most common fuel-saving strategies used on late-model vehicles. While these strategies decrease overall fuel consumption, they also increase the possibility of drivability issues. Let’s examine these strategies and related customer complaints.
Stop/start technology was introduced to turn off the engine while the vehicle is sitting stopped in gear with the ignition on. While the concept is simple, some factors must be considered. One of them is the operation of the automatic transmission. Since most use an engine-driven oil pump to generate pressure, stopping the engine creates a problem. As a solution, most manufacturers use an electronic auxiliary pump or a mechanical accumulator to provide ‘residual’ pressure to keep drive clutches applied and ready for power flow through the gear train on demand.
When everything works as designed, the operation is seamless. However, as the vehicle ages, issues can occur. The following complaints can be directly related to problems with the operation of the stop/start system:
- A bump or a flare and bump on take-off
- Delay or no restart of the engine
- Transmission slipping or failure
The main component to check in this system is the brake switch sensor. A scan tool monitors the signal sent to the PCM. With the brake switch ON, the control module can stop the engine, provided conditions are correct according to module programming. When the brake switch is OFF, the engine is signaled to immediately restart.
Issues can arise with the stop/ start system operation if the service and/or auxiliary battery are weak, the brake lights are incorrect, the brake switch was serviced incorrectly, or the brake switch learn procedure was not performed (when applicable). Always verify that the stop/start system is working correctly before proceeding with a diagnosis involving the abovementioned complaints.
Neutral idle technology is a similar strategy; however, it controls torque transfer from the engine on the transmission side. When the vehicle comes to a stop, the forward clutches partially release, just enough so no torque is transferred into the unit. This strategy also uses the brake switch ON signal to enable neutral idle under predetermined conditions and the OFF signal to resume normal clutch operation.
A burnt or slipping transmission and a bump or flare bump on take-off could indicate the neutral idle strategy is not working correctly. Diagnose and correct these issues first before continuing with your diagnosis.
ELECTRONIC THROTTLE CONTROL
In 1988, BMW was among the first manufacturers to produce a vehicle without a mechanical-controlled throttle. By the early 2000s, most OEM vehicles had several models with electronic throttle control. Now, it is unusual to find a vehicle with a mechanical throttle.
Electronic throttle control makes sense for safety and other engineering reasons. Because throttle control is a request rather than a demand, vehicle power, and torque output can be managed ‘smartly’ relative to the driver’s request, driving conditions, and the vehicle’s operational state.
The typical electronic throttle control system consists of a throttle control actuator on the throttle body, dual redundant throttle position sensors, and a control module (Figure 1). Highspeed feedback data closely monitors the position of the throttle pedal versus the throttle plate for agreement. If a fault is detected, the system will fail-safe to low power output mode.
The most common drivability complaint related to faulty electronic throttle control is low power. The customer can mistake the fail-safe operation of this system for an issue with the transmission. Vehicles equipped with CVT transmissions may command fail-safe throttle operation to protect the unit from catastrophic failure. Always check the PCM and TCM for codes and follow the recommended diagnostic procedures.
BRAKE PEDAL POSITION SENSOR
The Brake Pedal Position sensor replaced the brake switch to provide detailed input on ‘how hard’ the driver is applying the brakes. It is like a TPS (throttle position sensor) for the brake pedal. The added information provides real-time input for responsive powertrain control. 2007 Ford equipped the Edge with a Brake Pedal Position sensor (BPP). GM started using this sensor in 2012. Stellantis (MOPAR) named theirs a Brake Pedal Travel sensor, and it is found in vehicles in 2013 and later. Currently, all auto manufacturers use this sensor. The variable output signal of the BPP sensor allows for a variety of powertrain control scenarios:
- Transmission downshifting to assist during hard braking.
- Throttle control override (reduced engine power).
- Emergency braking assist initialization.
- Vehicle stability control initialization. Overall, the BPP sensor creates a ‘quick trigger’ input to allow the vehicle to stay in control. A problem with BPP can cause any of the following symptoms:
- Unwanted grade braking.
- Shift hunt, upshifts/downshifts, even when the transmission should not be shifting.
- The ECM can command reduced engine power due to a brake pedal override software feature.
- No DTCs or DIC messages or possible DTCs.
- Any symptoms in any combination may occur and may be intermittent.
Most manufacturers use a dual redundant system that feeds separate signals to separate modules (Figure 2). Refer to appropriate wiring diagrams to identify the system you are working on. BPP sensors require scanner-driven initialization procedures whenever replaced. Use scan data in graphing mode to diagnose faults and verify proper operation.
G-SENSOR/DYNAMIC SENSOR
G-sensors, also called dynamic sensors, inclination sensors, or several other names, are found in most vehicles built after 2015. They were first used in car alarm systems, and now they provide dynamic feedback for powertrain management. This sensor is a dynamic mechanical element that generates an electrical signal relative to forces acting on the sensor (Figure 3). It is usually located at the vehicle’s center of gravity, where it is not easily disturbed. The signal produced by the G-sensor is used for, but not limited to:
- Preventing gear changes in tight turns at higher speeds.
- Initializing engine braking during heavy deceleration.
- Altering shift strategy relative to vehicle ascending or descending.
- Hill hold assist to prevent vehicle rollback when starting from a stop on an uphill grade.
If a G-sensor is dropped or subjected to a severe jolt, do not use it. So, the G-sensor must be replaced if a vehicle was in an accident.
A bad or miscalibrated G-sensor can cause the following:
- Erratic or no-shift complaints
- Unwanted engine braking, aggressive downshift while stopping
- Excessive vehicle roll on level ground at a stop
- No hill hold assist; vehicle rolls back when starting from a stop uphill
Note that whenever the G-sensor is replaced, or module programming or replacement is performed, the G-sensor must be checked and calibrated as needed.
ADAS (ADVANCED DRIVER ASSISTANCE SYSTEM)
ADAS is a system that has been added to vehicles to help the driver avoid accidents by overriding some driver inputs based on conditions assessed by optical and radar sensor inputs related to the current vehicle operating state. ADAS systems incorporate radar, optical, speed, and other sensors for operation. Radar and optical sensors are usually embedded in the bumper, grill, and windshield.
LOOK AT THE CARFAX!
Now that we have examined some of these new sensors and strategies, we can better understand how powertrain issues can be caused by concerns outside the box. To add insult to injury, if the vehicle was in an accident, something as simple as a miscalibrated sensor could create an intermittent fault that would be difficult to find. Here’s where your detective skills need to kick in!
If a vehicle was involved in an accident, some sensors or systems may have been compromised if not replaced or properly serviced. Items to look for when checking out a vehicle you are working on include the following:
- Windshield replacement.
- Front and rear bumper repair (including the grille).
- Frame or unibody repair.
- Airbag deployment.
Special equipment is required to service and calibrate ADAS-related components. Failure to follow proper initialization procedures after repair can cause the following:
- Erratic up and downshifting.
- No shift complaint, vehicle sticking in gears.
- Cruise control inoperative.
- Excessive engine braking during normal stopping.
Suppose you are experiencing any of these issues as a customer complaint, and the vehicle was in an accident. In that case, I recommend contacting an ADAS-certified collision facility to validate and repair the system. Note that most ADAS calibrations require more than just a capable scan tool!
New technology requires us to look at powertrain management systems more closely to determine how they may affect our diagnosis of a customer’s drivability complaint. ATRA is positioned to help you connect the dots between transmission function and technology so you can confidently diagnose and deliver the goods to your customers!
