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(More info:
“Signal” could in theory be that sensor is alive (which could be detected before cranking) or is detecting slots or ECM has detected meaningful sequence of slots sufficient to determine positions.
The only knowledge the ECM has to know the engine is being cranked comes from the
cam/crank shaft sensors. It does not provide fuel until the engine is being cranked. But it has to know the engine is being cranked, which can only come from the position sensors. Guessing cranking speed of 200rpm, or 3.33 rps, it is conceivable it can determine position within one second which is about 3 revolutions. So the period of opening all injectors is short. Recall these computer-controlled engines start quite quickly compared to carburetted engines.
Somewhere in there is ignition, which also depends on knowing crankshaft position. Ignition timing is more critical than fuel timing, since it determines when the push of combustion occurs thus which way the engine turns.
The ECM needs both camshaft and crankshaft position as this is a four-cycle engine with two TDC physical events per crank revolution for each cylinder, so camshaft position relates to timing of the power stroke that the ignition and fuel are timed for (camshaft rotates at half crankshaft speed).
The ECM uses the camshaft position to know which cylinder is near TDC for injection and ignition, and the crankshaft sensor for precise location.
(The sophisticated critter even uses the number of revolutions since cranking was initiated as one of several parameters used to vary fuel delivery.)
(Details of the no-start:
The vehicle had begun to stumble on occasion, not taking load unless the rpm was above 2000 rpm. It would suddenly start running normally.
Finally it would not start.
Then when start attempted it would fire and appear to be accelerating to idling on its own, but die. Immediate repeat attempts to start, including continued cranking, would give only sporadic firing.
But if left for a while the same sequence would occur on attempting to start.
This seems like lack of fuel.
Despite normal fuel px and indications that the ASD relay is working (and obviously the fuel pump relay is working – they are closed by the ECM using IGN ON power).
Fuel for the initial response might be from the initial opening of all injectors, or from a slow leak from an injector (px reduces very slowly, which could be leak through check valve in pump).
My current suspicions are the crankshaft sensor (given its location that could result in wiring damage or metal sticking on its magnet, and that the transmission was re and re several years ago), and the MAP (which is used by the ECM to set initial fuel strategy, along with coolant temp sensor and TPS – both replaced due condition).
Nevertheless I am crawling through all wiring, doing what sensor tests I can, checking relevant fuses and relays, and checking other parameters.
“Signal” could in theory be that sensor is alive (which could be detected before cranking) or is detecting slots or ECM has detected meaningful sequence of slots sufficient to determine positions.
The only knowledge the ECM has to know the engine is being cranked comes from the
cam/crank shaft sensors. It does not provide fuel until the engine is being cranked. But it has to know the engine is being cranked, which can only come from the position sensors. Guessing cranking speed of 200rpm, or 3.33 rps, it is conceivable it can determine position within one second which is about 3 revolutions. So the period of opening all injectors is short. Recall these computer-controlled engines start quite quickly compared to carburetted engines.
Somewhere in there is ignition, which also depends on knowing crankshaft position. Ignition timing is more critical than fuel timing, since it determines when the push of combustion occurs thus which way the engine turns.
The ECM needs both camshaft and crankshaft position as this is a four-cycle engine with two TDC physical events per crank revolution for each cylinder, so camshaft position relates to timing of the power stroke that the ignition and fuel are timed for (camshaft rotates at half crankshaft speed).
The ECM uses the camshaft position to know which cylinder is near TDC for injection and ignition, and the crankshaft sensor for precise location.
(The sophisticated critter even uses the number of revolutions since cranking was initiated as one of several parameters used to vary fuel delivery.)
(Details of the no-start:
The vehicle had begun to stumble on occasion, not taking load unless the rpm was above 2000 rpm. It would suddenly start running normally.
Finally it would not start.
Then when start attempted it would fire and appear to be accelerating to idling on its own, but die. Immediate repeat attempts to start, including continued cranking, would give only sporadic firing.
But if left for a while the same sequence would occur on attempting to start.
This seems like lack of fuel.
Despite normal fuel px and indications that the ASD relay is working (and obviously the fuel pump relay is working – they are closed by the ECM using IGN ON power).
Fuel for the initial response might be from the initial opening of all injectors, or from a slow leak from an injector (px reduces very slowly, which could be leak through check valve in pump).
My current suspicions are the crankshaft sensor (given its location that could result in wiring damage or metal sticking on its magnet, and that the transmission was re and re several years ago), and the MAP (which is used by the ECM to set initial fuel strategy, along with coolant temp sensor and TPS – both replaced due condition).
Nevertheless I am crawling through all wiring, doing what sensor tests I can, checking relevant fuses and relays, and checking other parameters.