VSS THE PROPER DIAG... My VSS Hertz! by Lance Wiggins Figure 1 Figure 2A Vehicle speed sensors come in different shapes, sizes and designs. But they all have one common purpose: To provide the computer with a signal to indicate road speed. In this issue of GEARS, we re going to look at some of the more common speed sensors and learn how they work. Almost every manufacturer has a sensor to measure road speed. The computer uses these sensors to determine shift points and overall transmission timing. Think of the VSS (Vehicle Speed Sensor) as the transmission s governor, and the TPS (Throttle Position Sensor) as the throttle valve cable. These sensors are crucial to proper engine and transmission operation. As the throttle signal increases, the vehicle speed will usually increase, too. When the TPS and VSS signals reach a predetermined level, the computer commands the shift. Then the process starts all over again. If a speed sensor is acting up, you can run into a number of different drivability conditions: harsh or soft shifts, erratic shifts or shift points you may even have a car that stalls the engine when you put the trans into gear. Ratio codes are another common problem that can be caused by a faulty sensor. Most vehicle speed sensors generate an AC signal, but there are some that generate DC. Some vehicles have a remote module that converts the AC signal into DC; others have that module built inside the powertrain or transmission control module. AC-generating speed sensors usually have only two wires going to them (figure 1). You can test these sensors with a scope or DMM. The AC signal voltage should rise with road speed and can reach 20 or more volts. These sensors are permanent magnet generators. In the example used here (figure 2a, 2b), the vehicle speed sensor is located on the differential housing, and it creates a signal based on movement from 4 GEARS September 2002
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VSS the proper diag My VSS Hertz! Figure 2B Some vehicles have a remote module that converts the AC signal into DC; others have that module built inside the powertrain or transmission control module. Figure 3A the exciter wheel around the differential carrier. To check this type of sensor, connect your positive meter or scope lead to one wire and the negative lead to the other wire (Fig 3a, 3b, 3c). In addition to signal voltage, you should also check the signal frequency; an example can be at 55 MPH, sensor frequency will be about 125 Hz. If the frequency or voltage appears to fluctuate erratically during the test, the sensor or pickup may be bad, or the gears driving the sensor may be loose or damaged. In either case, the erratic signal will cause a driveability problem, and the damaged component will need to be repaired or replaced. Figure 3B Figure 3C 6 GEARS September 2002
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VSS the proper diag My VSS Hertz! Figure 4B Figure 4A Figure 5A Figure 5B These DC-generating sensors can also be tested using a scope or DMM. The DC signal should rise with road speed, just as the AC sensor did. DC-generating speed sensors usually have three wires going to them (figure 4a, 4b) (see sidebar on page 12). These sensors can also be tested using a scope or DMM. The DC signal should rise with road speed, just as the AC sensor did. The difference here is the DC sensor will only increase in frequency. Voltage will always switch on and off, between zero and reference voltage (figure 5a, 5b). On today s cars, these sensors are typically Hall Effect sensors. Vehicle computers are digital devices: They have no way of reading an AC signal directly. For the computer to use an AC signal, it must be converted to a DC signal. Modules that convert AC to a DC signal can be located inside the computer itself, or they may be remote. 8 GEARS September 2002
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VSS the proper diag My VSS Hertz! Some remote modules are in the instrument cluster, such as the ones on Ford products. Ford uses a PSOM (Programmable Speedometer Odometer) (figures 6a, 6b), which converts Figure 6A Figure 6B the AC signal to DC and distributes it to other modules around the vehicle including the PCM. GM uses a DRAC (Digital Ratio Adapter Controller or Speed Buffer) (figures 7a, 7b). The DRAC module is usually mounted under the dash, but in later-model vehicles they re built into the PCM. Both the PSOM and DRAC can be reprogrammed to adjust the signal for different size tires and various differential ratios. Some import vehicles use a combination meter (speedometer) to do the conversion. These are similar to the Ford PSOM, but they can t be reprogrammed. To diagnose a VSS correctly, you must understand the conditions that are taking place. For example, we recently received a call on a 1997 Ford truck with an E4OD and a 5.4L engine. The truck would stall whenever you put it into gear. This is a fairly common call to the Helpline. Invariably the technician tells us, The converter is locking up when I put the transmission in gear. This is his interpretation of the problem, and it would be valid if there were a pump problem. So the first thing we always ask is whether there were any codes stored in memory. All too often the answer is No. Next, we ask: Does the transmission stall in reverse? Answer: Yes. Since there s no fluid going to the TCC solenoid in reverse, there are only a few things that can cause this specific condition: Worn pump Sump screen sucking air Input shaft is too long VSS signal incorrect for conditions Figure 7A Figure 7B If the pump clearance is excessive, the amount of fluid going to the release side of the converter is reduced to where it can t hold the clutch from applying. The easiest way to test this is to check the cooler flow. Remove the cooler line and start the vehicle. If cooler flow increases when you put the transmission into drive or 10 GEARS September 2002
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VSS the proper diag My VSS Hertz! reverse, the pump is causing the problem. If the screen is sucking air, the reason for the lockup is the same: There isn t enough pressure to hold the clutch off. If the input shaft is too long, it could force the clutch forward against the lid of the converter when you bolt the unit into the vehicle. But before we pull cooler lines or remove the transmission, let s check the data screen on the scan tool, and pay close attention to the VSS signal (figure 8a, 8b). On this truck, the VSS signal read 123 MPH whenever you put the trans into drive or reverse. Since this occurred while the truck was still in the shop, we have to assume this is an incorrect speed signal. The next question is, can the VSS cause the converter clutch to come on and stall the engine? Sure, but at this point with the VSS showing 123 MPH, who cares! The bigger question is, will shutting down the fuel injectors cause the vehicle to stall? Okay, I think we all know that answer to that one! The point is, there are a number of different drivability problems that will affect the transmission operation and the overall driveability. So the next time you receive a vehicle that stalls when you put it into gear, make sure the VSS isn t causing that problem and save yourself from a VSS that Hertz! Join Lance Thursday, October 10, at 6:30 PM for EXPO Kick-Off Seminar: F4E-III/FA-4AEL, Are they really that tough? Figure 8A Did You Know... Most of the DC sensors used on today s cars are Hall Effect sensors. These sensors consist of a magnet and solid state pickup to develop a signal. A trigger wheel with shutters and windows rotates between the magnet and the pickup. When a shutter is aligned between the magnet and pickup, the voltage drops to zero; when a window is aligned, the voltage jumps to reference voltage. But not all DC vehicle speed sensors are Hall Effect-style. Early GM vehicles used an optical sensor, but they were never used with computer-controlled transmissions. Figure 8B Early computerized Toyota transaxles used a reed switch sensor: As a magnet rotated past the sensor, the contacts closed, creating a pulsing DC voltage signal for the computer to use to measure vehicle speed. And recently, a new type of DC sensor began showing up on the scene called a Magneto-Resistive speed sensor. These sensors combine an AC, variable reluctance-type of sensor with the electronics to convert the signal to a DC pulse. The sensor uses a signal wheel or reluctor, just like an AC permanent magnet sensor does. But instead of an AC signal, the sensor creates a DC, on-and-off signal. One way to recognize these sensors is, with the key on, engine off, one of the sensor wires will have battery voltage to it. Depending on the system, the sensor may also have a ground wire, or it may use a case ground. And, of course, it ll have a signal wire to provide the DC signal to the computer. 12 GEARS September 2002