A DTC, A Scope and Fundamental Electrical Knowledge

A DTC, A Scope and Fundamental Electrical Knowledge

Something out of the ordinary was happening on a customer’s Ford Fusion, and it was time to scope it out.

Recently, we had a customer drop off their 2007 Ford Fusion at our shop with concerns that the transmission stops shifting, the speedometer quits working and the MIL is on. Upon the initial verification and diagnosis, a DTC scan revealed a P0720 code stored in memory.

My initial thought was a faulty output speed sensor (OSS). Failing speed sensors on these vehicles are not uncommon. In most cases and without a second thought, most technicians would simply replace the speed sensor. However, this vehicle exhibited something strange on the road test. Wanting to investigate in detail, I pulled up the PIDs for the OSS, the vehicle speed sensor (VSS) and a few others I thought would help isolate the issue.

As I started my road test, the speedometer and the speed sensor appeared to be functioning normally. Then as I let off the throttle, the speedometer quit and the speed sensor dropped to zero. Something was happening, so I decided to see if there was a pattern and if I could replicate the event. I pushed the throttle and the speedometer started working, and when I lifted the throttle, it dropped to zero (see Figure 1).

Figure 1

The output speed was also following this pattern. I was pretty sure the sensor itself was operating because there should be no effect on the sensors themselves based on acceleration and deceleration activity. Something out of the ordinary was happening, and it was time to put a scope on this sensor and see what was going on.
The Ford Fusion’s speed sensor is a three-wire style Hall Effect sensor (see Figure 2).

Figure 2

Battery power is supplied on pin 3 white/violet wire. The speed signal back to the TCM is on Pin 2 (brown/green wire) and the ground is on pin 1 (black/green), which goes to G102 at the rear of the engine compartment (behind the battery). (See Figure 3).

Figure 3

I connected the yellow scope trace to the B+, the red trace to the speed signal and the green trace to ground. As I drove the vehicle, the pattern looked fine at first. Then when I was able to manipulate the throttle, I could see the problem. The speed sensor signal and ground were both going near battery voltage. This would be a clue I could use to determine where the fault was located.

This is a simplified version of the OSS function. The sensor is supplied B+ voltage to power the circuit and ground to complete the circuit. The signal wire cycles from zero volts to 5 volts as the magnetic field passes, and the duty cycle (on/off time) is relative to the vehicle’s speed (taken from the differential, in this case).

For the circuit to work, there must be current flowing through the sensor. When current flow stops, the sensor can no longer work. I look at three-wire sensors like I would a throttle position sensor (TPS), for example, although the TPS uses a mechanical function instead of a magnetic field to pass signal voltage (see Figure 4).

Figure 4

We need power, ground and the signal return for the circuit to function in either case. If any of those circuits are compromised, the sensor fails to supply the information to the computer, codes will be set and malfunctions can occur.

While taking a closer look at the scope capture when the event occurs, we see that battery voltage stays relatively steady. The speed sensor is cycling as it should and ground is about 0.04V (see Figure 5).

Figure 5

The sensor signal back to the TCM and ground is over 10.0V (see Figure 6) during the malfunction event.

Figure 6

Why would that happen? It is a simple voltage drop test. If your circuit is complete with the ground at 0.04V and B+ supply at 12V, the signal or load will function properly. If you remove the ground, the load will no longer work and you will see B+ voltage everywhere you test along the circuit (see Figure 7).

Figure 7

Follow basic electrical principles: If voltages are high on a circuit, grounds are absent or compromised. If voltages are low or at zero, a short to ground exists or supply voltage is compromised. This is very simplified but helps us understand what is going on.

While I still had the scope connected and was driving, I decided I would jumper a ground from the green trace to the scope ground (connected to the battery), which I could do right from the driver’s seat. The result was a properly functioning circuit (see Figure 8).

Figure 8

The alternate ground cured the malfunction. I continued to drive the vehicle and verify the concern was corrected while the jumper was in place. When I returned to the shop, I performed a visual inspection and a wiggle test on the connector and the harness. I was unable to see a change in voltage.

I removed the G102 ground, cleaned the mating surfaces, fabricated a new ground wire and then spliced it into the connector at the sensor. The next road test confirmed that the speed sensor issues were corrected, all functions were normal and the code did not return.

We all want the easy fix, the silver bullet. We want to get the vehicle in, diagnosed, repaired and back on the road quickly. As we proved in this case, the process of taking these extra steps to find the root cause of the issue took less time than if a sensor was wrongfully replaced, only to find out that it did not fix the problem.

We also saved the customer money and showed that we are a shop that gets the job done right the first time. I am a firm believer that, especially with electrical/electronics, if you cannot verify the part is bad, you shouldn’t replace it. Therefore, you should always follow the steps and do it right from the beginning. TS

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While systems can and will differ, here’s a look at common ADAS features, their general configurations and calibration requirements.

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Advanced Driver Assistance Systems (ADAS) require the use of a scan tool for diagnostics, and the majority of the time, it’s required for post-repair calibration. ADAS, like any other system, requires a diagnostic routine, which begins with a base knowledge of the system. Knowing ADAS will help understand fault symptoms and scan tool data for the most efficient diagnosis.While systems can and will differ, here’s a look at common ADAS features, their general configurations and calibration requirements.Parking assist sensors, of which there can be more than one, are generally located in the front and rear bumpers. They are the inputs that affect active parking assist and parking collision warnings. Any time they are disturbed in any manner, a static calibration must be performed with a scan tool.Side object sensors, sometimes called collision avoidance sensors, are commonly located in the rear bumper. These sensors provide input for blind spot warnings, lane change alerts and rear cross traffic warnings. Static calibration with a scan tool is required when these are removed or replaced.Rear vision cameras will be located in the rear decklid, liftgate or tailgate, and act as either a backup camera alone, or part of a surround view system if the vehicle is so equipped. These cameras generally require a dynamic calibration, and no scan tool is required.A forward-looking camera is sometimes located behind the grille, and usually part of a surround view system. These too do not require a scan tool, but a dynamic calibration must be performed when they are removed or replaced.Different ADAS features may have dedicated control modules which can be located in various areas, often behind interior panels. As with most control modules, these require scan tool programming when replaced and, depending on the system, both static and dynamic calibrations may be required.The Haptic Seat Motor creates the vibration to provide a safety alert for blind spot, forward collision, lane departure, lane keep assist, parking collision and rear cross traffic warnings. These motors, sometimes called seat warning actuators, generally require no type of calibration.Cameras located in a sideview mirror are part of surround view systems. These require calibration when removed or replaced, but most of them dynamic, and no scan tool is required.The steering angle sensor located in the steering column is an input for lane keep assistance, and a static calibration is required with a scan tool any time it is removed or replaced, or any time a wheel alignment is performed.Last, but not least, is the front view, or forward-looking camera located in the windshield area. This camera is a vital part of adaptive cruise control, automatic emergency braking, automatic high beam headlights, forward collision and lane departure warnings, and lane keeping assistance. A scan tool and static and dynamic calibration are required after removal and replacement, but also after windshield removal or replacement, or any service that affects the ride height of the vehicle. TS

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