You're right on all accounts. There are two types of position sensors. The crankshaft position sensor used to be a magnetic pickup. It was simply a coil of wire wrapped around a magnet. It sits next to a ring on the flex plate or flywheel. There are holes in that ring. As a hole passes under the magnet, it disturbs the magnetic field that was set up by the magnet in the sensor. A moving magnetic field induces a pulse of voltage, (electrical pressure) in the coil of wire. Those pulses of voltage are what are seen by the Engine Computer. Magnetic sensors are generally quite reliable, and can be identified by having two wires.
Sorry I didn't correlate "HE" to "Hall Effect Switch". Blame it on "oldtimer's disease". Regular transistors use a very tiny current to turn a bigger current on and off. A Hall Effect transistor uses a magnetic field instead to turn the bigger current on and off. The advantage is it can respond effectively to a much smaller disruption in the magnetic field. The sensor still incorporates a magnet. It is usually in the sensor itself, but in the case of Neons, it can be a rotating magnet that passes by the sensor. Hall Effect sensors have three wires. The ground and 5.0 volts powers the circuitry in the sensor, and the signal wire sends the voltage pulse to the computer. Unlike normal circuits, the ground wire for sensors won't have the expected 0.0 volts. The computer causes about 0.2 volts to be there. That's how it can tell the ground wires are not broken.
Magnetic sensors can be heat sensitive typically from the coil of wire expanding and tugging the ends of the wire loose from the terminals. Also, because there is so much wire length, the varnish insulation can break down and adjacent loops can touch each other. That reduces the effect of the magnetic field's ability to induce a voltage. As the voltage of the pulses go down, the point is reached where the computer no longer recognizes them as pulses. This is where cutting out and sputtering show up. Magnetic sensors do not need to have a voltage supplied to power them, although in some applications the computer will apply a voltage to one wire and watch for it to return on the other wire as a means of testing the integrity of the wires and to assist in providing information needed to recognize a defect and set the appropriate diagnostic fault code.
Hall Effect sensors have a lot of electronic circuitry which is inherently adversely affected by heat. Failures can be sudden and permanent or intermittent and will work again when the surrounding area cools down. Heat migrating up from a hot engine can take an hour or longer to dissipate.
I hope you're paying attention. There will be a test later!
When the ignition switch is turned on, the Engine Computer turns on the Automatic Shutdown (ASD) relay for one to two seconds, then it goes back off. The relay sends power to the injectors, ignition coil, alternator field, oxygen sensor heaters, and, most importantly, the fuel pump or pump relay. Fuel should remain pressurized in the supply system for weeks, but in case it bled down, that one-second is enough time for the pump to build system pressure in preparation for engine starting.
When you crank the engine, the computer knows the engine is rotating by the pulses coming from the two sensors, so it turns the ASD relay on again. To hear Chrysler tell it, one sensor synchronizes when each injector fires, and the other sensor determines when a spark plug should fire. That may be true, but if the pulses stop coming from either sensor, the computer turns off the ASD relay and the engine stops running for lack of spark, lack of power to the injectors, and lack of power to the fuel pump. There is a very important reason for this.
In the event of a crash where a fuel supply line becomes ruptured, the electric pump would pump raw fuel onto the ground creating a severe fire hazard. With the loss of fuel pressure, the injectors can't spray fuel so the engine stalls. That results in loss of sensor pulses so the computer turns the ASD relay off. Power is removed from the fuel pump, and it stops pumping fuel onto the ground. The system is very effective and relatively reliable. Ford's version of this safety system is the miserable "inertia switch. It responds rather effectively to potholes and curbs. A swinging pedulum shorts out the fuel pump wire and pops a circuit breaker.
Normally the Engine Computer has time to see which sensor stops sending pulses while the engine is coasting to a stop. It will memorize a diagnostic fault code and turn on the Check Engine light. The codes can be read easier on Chrysler products than on any other brand. Cycle the ignition switch from "off" to "run" three times within five seconds, then watch the Check Engine light. Don't crank the engine or you'll have to start over. After about five seconds, the light will flash out the digits of various two-digit codes. The first set of flashes is the first digit. There will be a little pause, then the next set of flashes will be the second digit. After a longer pause, the next two-digit code will show up the same way. The last code displayed is "55". That just means that's the end of the message. Some vehicles start out with code 12. That just means the ignition switch was turned off recently.
Once the problem is corrected, the code(s) can be erased with a scanner, (hand-held computer plugged in under the steering column), by disconnecting the battery for a minute, or the computer will erase them automatically if they don't come back after starting the engine about 50 times. Until that happens, the codes will remain, but the Check Engine light should be off. Disconnecting the batery is not the preferred method because a lot of other stored data will be lost, in particular, short and long-term fuel trim adjustment data.
New crankshaft position sensors could have a small plastic rib or strip on the end, about 1/32" high. When you push the sensor in as far as it will go and tighten the bolt, that rib sets the air gap. It will partially wear off from rubbing on the flex plate or flywheel. To reuse it after removal, use a knife or razor blade to cut the remaining part of the rib off flush, then install a paper spacer available from the dealer's parts department. It's a sticky disc the same diameter as the sensor. Many new sensors come with the spacer already stuck on instead of a plastic rib.
As a side note, some other brands, GM for instance, and some models, only use the camshaft position sensor during engine starting to determine which cylinder to fire. After that, the computer just calculates the next one according to the firing order. If the sensor fails while the engine is running, you may never know until the engine is stopped and restarted later. Some engines will not start. Some have a 33 percent chance of firing the right cylinder and will run, but they have a 66 percent chance of firing the wrong cylinder. If the engine doesn't start, simply releasing the ignition switch and trying again won't help because the computer will just keep on following the same firing order. You must turn the ignition switch all the way to "off", then, when you try again, the computer will start over with a 33 percent chance of being right.
Saturday, March 6th, 2010 AT 3:48 PM