Hi guys. If this is a three-wire sensor, those can't be tested with an ohm meter. There is circuitry inside. It doesn't have a resistor element in it. Most speed sensors today use a "Hall-Effect" transistor which turns on in the presence of a magnetic field. The magnet in this case is spun on a shaft by the drive gear. Other styles don't have any moving parts. They have a toothed wheel that moves past a magnetized coil of wire. On older versions you COULD read the resistance because that coil and magnet were all that was in there. Those will be two-wire sensors. Three-wire sensors are the ones with the additional circuitry inside. One wire is the power supply. That is usually 5.0 volts, but it could be 8.0 or 10.0 volts. One wire is the ground wire, but it will have 0.2 volts, not 0.0 like you would expect. The third wire is the signal wire.
You won't get a signal from these sensors if the ground or the 5.0 volts is missing. Testing from the Engine Computer connectors is fine if you're doing this in the classroom as a learning experience, but when you're paying by the hour, you want your mechanic doing it the fastest and most efficient way. With the ignition switch turned on, back-probe the wires through their rubber weather-pack seals right at the sensor's connector. For the readings to be valid, they have to be taken with the plug connected. If you have 5.0 volts on one wire and near 0.0 volts on another, the choices are the sensor is defective, the signal wire is open, (cut), or grounded, or a connector terminal is corroded or stretched.
Mechanics don't like to throw random parts at a problem. We start with those quick voltage tests, THEN order the replacement part if that's what it looks like is needed. In the absence of these tests, it's common for a do-it-yourselfer to just get a replacement part to "try". If it solves the problem, fine. If it doesn't, you're out a lot less dollars than it would cost for an hour of diagnostic time. You can head to the repair shop with the confidence to know you've already eliminated one of the possibilities, and the mechanic can concentrate his time on those other areas.
As for those resistance measurements, it's important to understand that values listed in the service manual are just approximate and are just for reference. When specifications say "1600 ohms", some people get all excited if they come up with 1400 or 1800 ohms. With a coil of wire, your reading will be infinite, meaning the coil is open, as in a corroded-off wire, 0 ohms, meaning the ends of the coil are shorted together, or correct. 0 ohms is extremely unlikely simply due to how they're constructed. That is more likely to be caused by the connecting wires outside the actual sensor being shorted together. Infinite is the most common failure and is caused by a tight wire contracting and breaking of the point it's soldered to, or it's corroded off. It is also the typical failure on sensors that have wires that flex, like ABS wheel speed sensors, and breaker points and pickup coils on a moving plate in a distributor.
When you get a good resistance reading, that coil of wire can't go up in resistance from what it was because that requires the piece of wire to become longer or thinner. Neither of those can change. The resistance would go down if many loops in the coil were shorted together, but that only occurs from excessive heat buildup in high-power coils, then the varnish insulation burns off the wires. Examples of high-power coils are power lock solenoids, transmission shift solenoids, starter solenoids, and things like that. Sensor coils dissipate almost no power, so there's no heat. They need a real lot of loops of wire to be able to generate a usable signal, and that's why their resistance is so high. The point of this story is if you have a sensor you can do a resistance test on, it's going to be open, shorted, or okay. It's not going to have a good resistance reading that's wrong.
I never let my students use reference charts in service manuals when testing sensors. We know they're good or bad by their function and design. If a sensor is working properly, how can you say it's defective because its resistance isn't exactly as listed on some chart? Also, we don't test sensors on the job except to verify other test results and to understand how their failure caused the customer's complaint. Customers want their cars fixed quickly. They don't care about parts autopsies. We only test parts in the classroom to understand how they work and why they cause the symptoms they do. You aren't under the same time constraints mechanics are so how you test the circuit is not that important. What concerns me more is Engine Computer connector terminals are very small and delicate and can be easily damaged. The most common problem comes from shoving the test probe into them and spreading the female terminal so it makes no or intermittent contact. Sensor connector terminals are beefier. They can be damaged the same way, but it's much easier to take readings on them without causing damage.
Based on your original post, throw a speed sensor in, erase the fault code, and see if the problem is solved. Being unfamiliar with the circuit, that is the fastest way. The voltage readings I mentioned are a mechanic's starting point because that takes them only a minute or two. There's a 99 percent chance that will be the end of the story. In the one percent chance it is not solved, THEN dig deeper into it or get your mechanic involved.
Saturday, December 15th, 2018 AT 11:30 AM
(Merged)
