No Start Condition - Technical Discussion

new 4/15/2004

I thought our cars were waste spark systems so the plugs fired at BTDC on both the

> compression and exhaust stroke. Or is that just the V6 SHO?

from another post:
> They're not exactly random.

> The computer knows TDC for cylinder number 1, it just doesn't know
> if it's compression or exhaust. So it tries out opposite firing patterns
> until the engine starts. Then it knows which pattern to use and how to
> run the engine.

Here is my understanding of the workings of the timing. If I'm off with some of the technical details feel free to correct me but as requested please leave off the editorializing:

As previously mentioned, the V6 SHO has 3 coils for 6 cylinders, while the V8 SHO has 8 coils for 8 cylinders. This means the V6 uses waste spark but the V8 does not.

The V6 has a crank sensor that tells it when to start firing cylinder 1. It doesn't know if cylinder 1 is on compression or exhaust but since it is waste spark it really doesn't matter. However even with waste spark it still needs to know when a cylinder is up on compression so it can determine when to fire the fuel injectors on the appropriate intake stroke. It uses the cam sensor for this. As I understand it if the cam sensor isn't working then the computer will make a guess and go with it. If the guess isn't right then the car will be hard to start. So you try again, the computer guesses again, and eventually gets it right.

The V8 has a cam sensor (I have seen this). I assume it has a crank sensor although I have not actually seen it myself. With a cam sensor the engine has all it needs to know to know when the #1 cylinder is on compression. I assume it has a crank sensor to provide more accurate information on timing the spark.

bjshov8

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This is pretty close, but there's two big omission. The V-6 has something called Low Data Rate EI. The wheel on the crank has three equal windows that move thru the CPS. The CPS is a Hall Effect switch that switches from 0-12v and back as these windows pass thru it. The computer uses this information along with the information from the CPS (also a Hall Effect switch) to start the injectors and spark.
The V-8 cars and most others these days have a High Data Rate EI that uses A/C voltage generators to generate a low voltage A/C signal. On the crank, there is a wheel with 35 teeth. There's room for 36 teeth, but there is one tooth missing. [editorials omitted] The cam sensor is also an A/C voltage generator and looks at a pin on the back of the bank 2 exhaust cam.

The V-6 doesn't know where TDC is from just the CPS because the wheel on the crank does not have a signature window. It has three equal windows for reference only. The V-8 has a signature pin (missing tooth) on the crank wheel to tell it where number one is. Then there are only two guesses to get the engine to fire.

The interesting thing is that the computer only uses the cam sensor information to start the engine. Once it's running, it doesn't need that information any more. It monitors it, but doesn't use it.

Since there are 36 spaces on the crank wheel, the computer has rotational information on the crank every 10 degrees. This is also what the computer uses to indicate a misfire. There are torque impulses every 90 degrees of crank rotation. The crank actually speeds up after each cylinder fires. The computer can see this. If there is a misfire, the torque impulse is not there. It has to know cam position to know which cylinder is missing. So if you get an engine misfire, the computer uses CMP information to determine which cylinder it is. This is a function of OBD-II and something that the V-6 engine can't do.

Doug Lewis
Ford Performance Specialists Inc.

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Doug's got it right on the head {never doubted you either}.

I tired to explain this speeding up and slowing down of the crank to people a long time ago.
Caused huge problems when OBDII was coming in as it created misfire codes due to driveline shock in 5 spd cars. I have a SAE paper on the whole thing, {never did read last page as I was so bored with it}. 96-07 MY cars there was a drop I in the number of vehicles that offered MTX until the factories sorted out how to deal with the problem.

most cars do/did not use the trigger point and just fired the car evenly. now they have gone the other way and have a bigger notch, missing tooth some sort of gap in the timing wheel to indicate #1 cylinder. Most cars ignore the CPS signal once running as the PCM figures as long as the crank is turning the cams are in the right speed, and position.
IIRC ODI used about 8-10 in outs depending on the car... ODBII that number has doubled or tripled on some applications.

One thing that surprised me on this cars CPS vs my Cavalier... {not a fun car to change} no EMI Shield on it.. the Cavalier had a three wire plug..
5v Reference, signal return, and the third wire just ended up being attached to a tinfoil type sheath to combat EMI problems.

Clare

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My Saturn has a keyed crank wheel that the sensor picks up and has a ‘virtual’ cam position sensor. It has a ‘waste spark’ system and when it detects a high voltage ‘demand’ on cylinder # 1, due to the plug firing under compression, it determines that the #1 cylinder is on the compression stroke and then it looks at the crank sensor to determine TDC.
Paul L Fisher

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Yep all do it differently..
I should bring the bent Cam syncro wheel to Lapeer in may.
took it out of a 1990 crown Vic that had 2 problems..
miss all the time and the a/c cut out at part throttle and up hills.

The miss was some how one tab on the pick-up wheel had be bent inwards and it was striking the pick-up...

The a/c was a pin hole the soup can bolted to the driver's inner fender well had a small pin hole in it.. {vacuum reservoir which they now make out of plastic if they use them at all}
3
Clare

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The waste system has two spark plugs per coil. One is the positive end of the secondary coil and the other plug is the negative side of the secondary. This is why the V6 SHO needs the double platinum plugs and our V8 does not.

Paul Nimz

Right and another reason the first 4.6 crown Vic had a plug rotation schedule and a reason we went to using different plugs on the propane conversions..
I am going to stick my neck out and say that since the latest CV 4.6 has COP they have probably done away with the rotation, and the propane conversions here are all one plug type.. {have to ask my buddies at the the LPF garage as they do the conversions in house now}

Clare

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When it detects a high voltage ‘demand’ on cylinder # 1, due to the plug firing under compression, it determines that the #1 cylinder is on the compression stroke and then it looks at the crank sensor to determine TDC.

Does it have fits when your wires and/or plugs go bad? I.e. what if there is a high voltage demand (I assume high resistance) on #1 every time?

bjshov8

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Yes. As a matter of fact, wires #1 and #4 have to cross within 6 inches of the coil or a SES light comes on.

Paul L Fisher

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Where should that "missing tooth window" be located in relationship to the keyway on the crank? Since it has two notches it can be placed in one of two positions (excluding putting it on backwards) and the manual I have does not elaborate on it's placement.

Carter Fuji

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It should.. would depend on the position in degrees of the sensor in relation to the #1 at TDC compression...

follow???

Clare

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