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  1. #1
    6S Moderator S.Bretz's Avatar
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    How compression sensing works....for 96-02 s-series.

    Well...most of us know that changing the plugs out for bosch platinum +4's and MSD wires and/or coils will make the car run like shit. This is a graph from some one on the MSextra forums. He was able to use an scope and record the event.

    The graph show the current in the wire. First is the ignition spark then the next dip in the cumbustion happening followed shortly by the pressure event. The current generated by the explosion has a decently high current feedback due to the reaction concentration be centered around the tip of the spark plug. Later the pressure wave is lower in current because the reaction has been spent around the plug tip and the residue mixture in the chamber is gettering "wave" or "spashed" back at the spark plug as the pressure wave bounces back off of the chamber walls (head, piston, and cylinder walls).

    Since the obd2's don't have a cam angle sensor, but DO have sequential injection. So how does the PCM tell if number 4 is at TDC or if number 1 is at tdc. Normally an engine will get an pulse form a cam angle sensor before the TDC event at #1 occurs. Since the pressure wave in number #4 happens before then the next TDC for #1, the pressure wave is interpolated as virtual cam angle pulse.

    If the resistance in the ignition system is altered, the pressure wave current will be higher or lower in amplituted (more or less current is generated) at different times will be generated. With the low resistance MSD components, the PCM see's more current and thinks that pressure wave is happening too soon and that the engine has too much timing...so it will pull timing out until the point of lost power and in some cases misfires.
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    -6S Resident Mechanical Forensics member #001.
    1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
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  2. #2
    johnny95sl2's Avatar
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    What is the time scale I am assuming microseconds?
    I got nothing for that




  3. #3
    6S Moderator S.Bretz's Avatar
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    I'm not sure what the time scale is...its not really important though.

    I wish the scope would have shown the x axis as crank degrees instead of time.
    -6S Resident Mechanical Forensics member #001.
    1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
    1998 F-250 5.4L triton...stock.

  4. #4
    johnny95sl2's Avatar
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    If you knew the rpm and time scale you could calculate the crank posistion relative to spark timimg.
    I got nothing for that




  5. #5
    6S Moderator S.Bretz's Avatar
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    Yeah, you could.
    -6S Resident Mechanical Forensics member #001.
    1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
    1998 F-250 5.4L triton...stock.

  6. #6
    6S Moderator S.Bretz's Avatar
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    Let me add this before I link this to another page.

    The first post was a more advance operation description of the cam angle sensor. It is used, I think, on the 2.2L eoc's with the coil module.

    The obd2 S-Series is a little more old school. Ok...how the coils work. One tower is a positive and the other a negative....the coild fires and toggles the polarity of the coil towers so the positive coil on the last igntion event for that coil is now the negative and vice versa. So basically, you can think of the electrons as getting thrown back and forth from one tower to the other and then back again. This helps reduce the amount of energy to charge coil since you are getting some of the energy back into the coil...just on the "other side." WIth that out of the way...back to compression sesning and cam input of OBD2's


    The system still monitors the compression sensing, but not the degree of crank rotation. The main cam angle signal comes form an inductive pick up under the 1/4 coil pack. When the coil fires the inductive pickup will pull the a 5v signal low between the PCM and ignition module. That signal gets "ok'ed" by the internal logic in the ICM to pull the signal wire low (to create a signal pulse) if there was a compression sensing event. So the inductive pickup will get produce a signal every fire event on of the 1/4 coil pack...but the ICM will only let the signal get to the PCM IF there is a compression sensing event.

    The PCM throws codes for correlation between the ICM cam events and the crank events....you guys have probably seen them. P0340 and P0341.

    The PCM will see 7 notches in the crank every rotation and expects that ever time the cam signal comes through, there should be 2 crank rotations (14 crank notch counts). If the pcm see 14 crank notch and no cam input, then P0340 will set for lack of cam angle input. This cam be set for misfires or resistance changes in the primary (coils) or secondary (wires/plug) igntion circuit. If the resistance is altered, the compression sensing circuit cannot due its job correctly and may not trip the internal ignition control module logic to allow the inductive pick up to send its signal when needed.

    If the PCM see too many pulses from the cam input (more than 1 input every 14 crank notches) P0341 sets. Resistance changes in the primary and secondary igntion systems can trigger 'extra' compression seneing events and allow the cam signal to get passed to the PCM more then needed.

    When the PCM starts getting messed up from P0340's and P0341, the timing can start to get all screwed up because the PCM will start to move timing around to 'stabilize' the cam input signal. This is what happens after usually a few monthes of aftermarket "performance" coils, wires, and fancy platinum and/or +2 or +4 plugs. It may run ok at first...but it will eventually 'confuse' the PCM and the car will run like poo.
    -6S Resident Mechanical Forensics member #001.
    1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
    1998 F-250 5.4L triton...stock.

  7. #7

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    Quote Originally Posted by S.Bretz View Post
    When the PCM starts getting messed up from P0340's and P0341, the timing can start to get all screwed up because the PCM will start to move timing around to 'stabilize' the cam input signal.
    Is returning to OEM grade hardware an immediate fix for this, or will it take weeks to un-learn what it mis-learned from the aftermarket stuff?

  8. #8

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    Quote Originally Posted by cowboydren View Post
    Is returning to OEM grade hardware an immediate fix for this, or will it take weeks to un-learn what it mis-learned from the aftermarket stuff?
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  9. #9
    6S Moderator Natedogg's Avatar
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    This is cool info.

    Autolite coppers and Bosch wires for me!
    Former modded N/A 97 SL2, unmolested 96 SC2, and turbo 95 SL2 owner.

    99 SL2 Turbo - Parted out the turbo bits and putting it back to stock. It's already sold in it's stock form.
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  10. #10
    6S Moderator S.Bretz's Avatar
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    Quote Originally Posted by cowboydren View Post
    Is returning to OEM grade hardware an immediate fix for this, or will it take weeks to un-learn what it mis-learned from the aftermarket stuff?
    Most of the time its pretty fast. I have seen a few cases where the spark modifiers are really out of whack and need to be reset. There are two ways to do it.

    The first way to reset the table is to touch the battery cables together. (pulling the pcm fuses and cycling power on for about 30secs works to. just remember to turn the power off before you put the fuses back in)

    The second way is to look at a tech2 and constantly cycle the power to the PCM and wait for the tech 2 to update, then cycle power off and repeat. About every 3-4 power cycles, the spark modifier table will come down a little bit. You need to repeat until the modifier gets back to zero to start "fresh."
    -6S Resident Mechanical Forensics member #001.
    1995 SC2 Turbo 3.6L DOHC, 6sp manual, Ford 8.8 rearend running on MS3x.
    1998 F-250 5.4L triton...stock.

  11. #11
    6S Moderator Natedogg's Avatar
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    By touching the batter cables together, he means disconnect them from the battery first. I don't want anyone going out there and touching them together while connected to the battery and frying themselves.
    Former modded N/A 97 SL2, unmolested 96 SC2, and turbo 95 SL2 owner.

    99 SL2 Turbo - Parted out the turbo bits and putting it back to stock. It's already sold in it's stock form.
    00' SC2 DD - SOLD
    2015 Fiesta ST - Recaro package. More fun to drive than a turbo Saturn and more reliable.
    Loaded and cushy 2014 Ford Explorer as the family hauler.

  12. #12

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    Ok, you lost me about halfway through your post... Are you saying that the PCM senses a combustion event through the spark plug wires instead of having a cam angle sensor (and that messing with resistance of anything will make things go wonky)?

    I have had MSD wires and copper plugs on my car for probably a couple of years, now, and it has been running fine with no SES light. I just scanned for codes and I have neither a P0340 nor a P0341. I have a "service wrench" light on, but that has been on pretty much ever since I swapped PCMs to get cruise control. There was also a "P0141" code, which is apparently an oxygen sensor heater circuit malfunction, which makes no sense at all...

  13. #13

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    duration, scale, burn time

    The burn time/duration will be between 400 microseconds and 1.7 ms, on the primary (non-waste) spark.

  14. #14
    rubensw2's Avatar
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    So basically keep the OEM cables and ngk Sparks?
    Buy it, Build it, Drive it, Break it, Fix it

  15. #15

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    Old post:
    Quote Originally Posted by Insomniac View Post
    Ok, you lost me about halfway through your post... Are you saying that the PCM senses a combustion event through the spark plug wires instead of having a cam angle sensor (and that messing with resistance of anything will make things go wonky)?
    So this is my understanding so far (please correct me if I got it wrong):

    It is sensed through the spark plug wires. In a wasted spark system, you have two plugs attached to opposite ends of the coil secondary. When the coil fires, the sparks are of opposite polarity... one ignites the fuel/air mix in the cylinder that undergoing compression, the other is the 'waste' spark. The pressure, gas mixture and probably a whole bunch of other stuff influences the timing of the two sparks... one has an earlier breakdown voltage and fires first -- this is key.

    By looking at polarity and the timing relationship of the two spark events, it can determine the cam position. (Because it knows crank position, what plugs it fired, and it can see which is in compression). With this information, it can then adjust spark timing.

    So any properties of the wire (resistance, inductance, capacitance to ground, etc) will influence the amplitude (height) of the sensed voltage, and I can imagine it could influence the arrival time of the signal where it's sensed. (Remember, this is not just some steady DC voltage influenced solely by resistance, it is a short transient pulse so all of this matters.) I could certainly imagine aftermarket wires messing with detection.

    Quote Originally Posted by rubensw2 View Post
    So basically keep the OEM cables and ngk Sparks?
    Or ACDelco sparks, or any copper plugs probably. No doubt you could experiment and find other stuff that works, but probably not easily without a lot of test time, a scope, and a tech II. And if this basic setup works, why mess with it.

    The patent is here: http://www.google.com/patents/US5410253 The language is EE stuff, but the summary of the invention provides a good description and you don't need an advanced degree to read and understand it.

    I'll try to dig up some other references. Need to learn about this myself.
    93 SW1, 95 SW2, 98 SW2, 97 SW1 -- all stick. Wagon hoarder.

  16. #16

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    Quote Originally Posted by 85HP View Post
    So any properties of the wire (resistance, inductance, capacitance to ground, etc) will influence the amplitude (height) of the sensed voltage, and I can imagine it could influence the arrival time of the signal where it's sensed.
    I'm no EE, but I believe the first half of your statement to be true. I don't think that it affects the arrival *time* of the signal, however. I think it confuses the ICM because the amplitude of the signals is all wrong, and depending on how the circuit was implemented, it could be filtering 'out of bounds' signals to the point that it can't tell which event was 1 vs 4.

    But yes, OEM-ish cables and good, cheap copper plugs (NGK seem to be the favorites) are the way to go.

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