First, understand a lot of people confuse the issue when they say, "the engine does not start". That can mean the starter cranks the engine just fine, but the engine does not start and run. "Does not start" can also mean the starter does not crank the engine, which is totally different. You included a perfect description when you said, "Will not crank at all, like no power going to the starter". That is strictly a starter circuit problem, and the crankshaft position sensor is not related to that in any way. To suggest it can cause this problem is the same as suggesting your flat tire is the cause of the inoperative wipers. (The logic is the car cannot be driven with a flat tire, so there is no need to turn on the wipers).
It is not until the starter is spinning the engine that the crankshaft position sensor and the camshaft position sensor begin to generate signal pulses that are sent to the engine computer. When the computer sees those signals from both sensors, it knows the crankshaft is rotating. In response, it turns on the automatic shutdown, (ASD) relay. That relay sends current to the ignition coil pack, injectors, alternator field, oxygen sensor heaters, and fuel pump or fuel pump relay. This is a very effective safety system. If a fuel line gets ruptured in a crash, raw gas would be dumped on the ground and would be a serious fire hazard. Instead, there cannot be any pressure in a ruptured fuel line. With no fuel pressure, no gas would spray from the injectors, so the engine would stall. Once stalled, there will be no signal pulses from the two sensors. The computer sees that as the crankshaft and camshaft are not rotating. In response, it turns the ASD relay off. That turns off the electric fuel pump so it stops pumping gas onto the ground.
The engine computer can set diagnostic fault codes for those two sensors, but time is a factor. Often a code will not be set just by cranking the engine. The problem is more likely to be detected, and a code set when a stalled engine is coasting to a stop. For that reason, do not assume a sensor is okay just because there is no fault code related to it. It is also very important to not disconnect the battery or let it run drained. That will erase all the fault codes, then that valuable information will be lost.
As for the transmission range sensor, fault codes can be set for it, but it depends on the type of failure. Some sensors are a set of switches that turn on and off at various times. If one of those has an arced or pitted contact and does not turn on at the right time, it will send confusing data, but it sends that to the transmission computer, not the engine computer. You may see that as all the gear position lights on the instrument cluster light up at the same time. The computer will not know for sure which gear you have selected, so it will not shift properly if the engine does start and run, and it may inhibit starter operation.
Some sensors are a single variable resistor, similar to a throttle position sensor. The signal voltage varies from a low of around 0.5 volts at one extreme to a high of 4.5 volts at the other end. The transmission computer is programmed to know that 3.2 volts, for example, equates to "neutral". If that resistor element becomes worn or a little chip of carbon breaks off and gets stuck under the movable contact, the signal voltage might change just enough that the computer interprets it as the transmission is in "reverse" or somewhere in between two gears. If it interprets it as being in "reverse", it will not allow starter operation. If it interprets it as between two gears, that is a legitimate signal voltage, so it will not set a fault code. Typically a signal voltage has to go outside the acceptable range of 0.5 to 4.5 volts to set a fault code.
Typically you are going to need a scanner to start the diagnosis. The first step is to read the fault codes in the transmission computer. Simple code readers usually only can read codes in the engine computer. Regardless if there is a code or not for the range sensor, the next step is to view live data and see what the computer is interpreting the signal voltage as. I have seen loose movable contacts on sensors I have autopsied. Those do not move right away when the shift lever is moved, so they will indicate the wrong gear. You can watch the positions change as you run the shift lever back and forth.
As for the check engine light, that only turns on when a fault code that is set could relate to something that could adversely affect emissions. A defective range sensor that prevents the starter from working will not turn the check engine light on. An engine that will not start and run cannot pollute. There are over 2,000 defects that can be detected by the engine computer. About half of them refer to things that can increase emissions, and those turn the check engine light on. There are other defects that can be detected by the transmission computer. Many of them put the system into "limp mode" that allows you to drive slowly to a repair shop without needing a tow truck. In that mode, the transmission stays in second gear as the only forward gear. That raises engine speed which does increase tail pipe emissions. That being the case, the transmission computer sends a message to the engine computer which sets code 700. Code 700 just means there are other codes in the transmission computer. It also triggers the engine computer to turn on the check engine light because of those higher emissions.
Tuesday, March 20th, 2018 AT 2:12 PM
