Originally posted by GNVenom
>>Cam sensor tells the ECM when the number one piston is at TDC on compression. Crank sensor can recognize TDC, but doesn't know about the valves, so cam sensor is necessary for proper spark. <<
This isn't true. Cam sensor is strictly there for fuel sequence timing, not spark timing. During sequential operation, the ECM needs the cam sensor signal to initiate it. It has nothing to do with spark sequence...that's the crank sensor's job. Once the engine starts, ECM spark advance tables take over.
Also, there is no provision to shut the engine down when oil pressure drops. The switch is there only to duplicate the fuel pump relay contacts in the event the relay fails. If the engine quit, it may be because the cam sensor "slipped", causing an improper sync to the fuel, stalling the engine.
-John Spina
http://www.casperselectronics.com
Oh Geez, I hate doing this, but I gotta call ya on this one John. I know you're the electronics expert on these cars but, here's how I've seen it explained:
I've bolded the section that is particularly interesting about how the cam sensor plays into the spark game:
The DIS system used in the Turbo Buicks is a 3x system, meaning that 3 reference pulses occur from the crank sensor for every crankshaft revolution. These reference pulses occur 120 crank degrees apart, with 60 degrees between the rising and falling edge. The falling edge of the crank signal occurs exactly 70 degrees BTDC, meaning the rising edge will occur exactly 10 degrees BTDC.
There are two modes of operation, bypass and Electronic Spark Timing (EST). Bypass mode is used during cranking and in limp-home operation. In bypass mode the coil dwell time is defaulted to 60 degrees, as the coil begins charging on the falling edge of the 3x signal and fires on the rising edge. Therefore spark advance in bypass is always defaulted to 10 degrees BTDC, a reasonable number for cranking and limp-home operation. Fuel delivery during cranking is batch mode instead of sequential. Once RPM reaches 400, the system switches to sequential fuel delivery and EST mode, where spark advance and dwell are controlled by the ECM. In EST mode, spark advance is triggered on the falling edge of the crank sensor. Basically, the EST software algorithm looks at desired spark advance and RPM and computes the length of time from the crank sensor falling edge (knowing that this falling edge is at 70 degrees BTDC) that would be required to “wait” in order that spark delivery occurs at that desired spark advance. Additionally, the start if injection occurs on the falling edge of the crank signal as well. Therefore it is obvious that the crank sensor alone is responsible for ignition and injector timing. However, it is also evident that the 3x trigger wheel mounted on the harmonic balancer must be positioned correctly; likewise, the balancer must be keyed correctly – if either was off, the crank sensor rising and falling edges would no longer occur when they should, therefore causing ignition and fuel delivery timing to be inaccurate. Although I haven’t seen it, I have heard reports of some balancers having these problems.
So, what use is the cam sensor you ask? Well, if you only had this uniformly spaced crank sensor signal, you would never be able to tell exactly which cylinder each crank pulse corresponds to. This is where the cam signal is needed, as it occurs once every two crankshaft revs, that is, once every six crank signals. The cam sensor should be adjusted so that the falling edge occurs at 25 degrees ATDC of cylinder #1. Now we are able to sync each crank pulse to its respective cylinder, because after detecting a high to low transition of the cam signal, the ECM knows that the next hi-lo transition of the crank signal will be cylinder #6, which will be the first cylinder to fire. Thus, it’s evident that adjusting the cam sensor so that the falling edge no longer arrives at 25 degrees ATDC does absolutely nothing, until it is adjusted so far off spec that the next low going crank signal is no longer cylinder #6, but #1 or #5. The problem is that the ECM still thinks that pulse should be #6, and it will fire the 3-6 coil right in the middle of the intake stroke.
The cam sensor resyncs every 2 crank revolutions. If you happen to lose the cam signal while the engine is running, a malf code will set, and the system operation will default to simultaneous double fire operation instead, but the engine will remain running until you shut it off. Once you kill the engine, you will not be able to restart.
Ever wonder why cranking times vary so much from start to start? It all depends on how the cam sensor is positioned when the engine finally stops after a key-down. If it stops immediately before the cam sensor transition, then the next start-up will be very quick as everything can sync up in less than ½ a crankshaft rev. However, if the engine stops after the cam sensor transition, a full two engine revs may be required before everything is able to sync.
Now, it is very possible (and often happens) to install the cam sensor 180 degrees out of phase and still have a running car! It makes no difference to the ignition system that the cam sensor is 180 out, since it’s a waste spark system. You’re still firing the same coil pack - from the ignition standpoint if the cam sensor is 180 out there’s absolutely no difference as the current still runs out the same terminal, thru one plug from center electrode to ground electrode, thru the block and back from the ground electrode to the center electrode on the other plug and returns to the other coil terminal. This pattern occurs the same way regardless of whether the cylinder is under compression or exhaust. The plug that fires the “usual” way does this every compression and exhaust stroke, and the plug firing “backwards” always fires backwards on its compression and exhaust stroke.
What changes is the fueling. With the cam sensor 180 out you’ll fuel the cylinder on the wrong stroke, and the fuel will sit around puddling on a hot intake valve for 1 engine rev until the valve opens and it’s pulled(forced) in.
