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Old 06-26-2009, 03:53 PM   #61
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Both inputs of the scope are set to DC, so they are showing you the absolute voltage on the line at any given time. At the left, the green 1> icon shows you the 0V point for channel 1. It is quite clear that the signal coming from the VR sensor is positive with regard to ground for half of the cycle, and negative with regard to ground for the other half.


Do you contend that the waveform shown in channel 1 is not an AC sine wave?

Joe, of course that is an AC sine wave. That is what I have been saying. You have half the signal above the 0v line and the other below.

Your contention was that if you move the 0v line to the bottom of the sine wave you still have an ac signal...which you don't because all the voltages are above 0v.

You might still get a signal with one of the probes on the negative shield wire but the proper signal is your negative probe on one signal wire and your positive probe on the other signal wire (because that is what the PCM sees).

I will be out vacationing for the next couple days, see you all later.
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Old 06-26-2009, 03:58 PM   #62
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What sensor are we talking about here. The VR sensor doesn't have a need for reference or input voltage.
Correct, but Mazda may have decided to return one side of the VR sensor output to 12V to make the input circuitry in the ECU simpler.
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Old 06-26-2009, 03:59 PM   #63
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Joe, of course that is an AC sine wave. That is what I have been saying. You have half the signal above the 0v line and the other below.
And then if you instead connect one of the signal wires to 12V, and measure at the other signal wire, you would see half the signal below 12V, and half above 12V.
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Old 06-26-2009, 03:59 PM   #64
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Your contention was that if you move the 0v line to the bottom of the sine wave you still have an ac signal...which you don't because all the voltages are above 0v.
No, it wasn't.


Let's go back and look again at what we both said:

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Originally Posted by Joe Perez
It's not until you connect the sensor to an input circuit that you really define a reference. Typically, one side of the sensor is connected to ground.
That is incorrect, if you connect one side of a VR sensor to ground it will not generate the expected ac sine wave.

The only wiring that goes to ground on a vr sensor is the shield wire, not the signal wires.

What I started out with was that the VR sensor, by itself, is a device generating a balanced output signal (one where the voltages on the two wires are always of equal but inverse magnitude) and that this voltage is not referenced to anything at all except for the other leg, until such time as one of the two legs is connected to a common point. Typically, this common point is the ground point of whatever circuit is being used to receive the signal from the other wire.

You then replied that connecting one side of a VR sensor to ground causes it not to generate "the expected ac sine wave."

I then posted a scope trace showing the output of a VR sensor which has one leg connected to ground, where you can see that the output is an AC sine wave.


What am I missing here?
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Old 06-26-2009, 04:01 PM   #65
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And then if you instead connect one of the signal wires to 12V, and measure at the other signal wire, you would see half the signal below 12V, and half above 12V.
Correct. The output of the sensor will swing above and below whatever voltage is at the common point. If you connected one side of the VR sensor to +12, then measured the other side with a scope, you would see a signal swinging above and below +12 if the scope probe's ground was connected to chassis ground, and you would see a signal swinging above and below 0v if the scope probe's ground was connected to the same +12 point as the other leg of the sensor.

IOW, all electrical potentials are relative to a reference point.
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Old 06-26-2009, 07:08 PM   #66
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Yes yes, and it's all -10,000v relative to some van degraff generator located in toledo. The important thing is that it's centered on the refernce voltage - i.e. the peak differential between on leg and the other.


Of course, little of that matters for this guy - I'm still a fan of buying a $0.50 sensor, putting it by the gear, and glueing a magnet like you would a bicycle. :-) OEMs do this, so it's not unreasonable.

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This is one reason why the MS1 cannot do full sequential, as its trigger inputs are edge-detecting only and require all edges to be evenly spaced. So we ignore the trailing edge, read the leading edge of CMP (which has a fixed relationship to CKP) and use it to reset the cycle counter.
Couldn't you look at falling edges and get the same information, PLU... no, I guess you need even spacing, eh?
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Old 06-26-2009, 08:04 PM   #67
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Couldn't you look at falling edges and get the same information, PLU... no, I guess you need even spacing, eh?
Correct. Both triggers on the MS1 are edge-detecting, and the trailing edge of CMP would produce uneven spacing. Actually, there's no technical reason for this limitation. The primary trigger must be edge-detecting as it's the CPU's hardware IRQ line (which by design recognizes only falling edges) however the second trigger is a general-purpose I/O line. They could have written the code to be state-detecting: IOW, when you receive a CKP and branch off to the interrupt routine, test CMP to see if it is high. If it is, then reset the wheel counter. Otherwise, increment the counter as normal.

You know, I just realized something- you could easily implement this in hardware! Put CMP and CKP into an AND gate, and do a one-shot on the gate output's falling edge. That would give you the ability to do four-channel ignition with MS1. (Still no four-channel injection.)
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Old 06-26-2009, 08:22 PM   #68
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It's all down to if the software will know what to do with it.

You're the guy who loves assembly, why not dive in there and fix it? :-)

More seriously, yes, I'm a fan of doing it in hardware, if the software can take advantage of it, that seems like a sweet, simple set up. As would be griding off all the teeth till you're down to one, which is what you are doing, no?
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Old 06-26-2009, 08:31 PM   #69
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You're the guy who loves assembly, why not dive in there and fix it? :-)
Largely because I have no CAS, and I've gotten to the point where I'm starting to think about the next car rather than the current one. If all goes to plan, that one will be running MS3.


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As would be griding off all the teeth till you're down to one, which is what you are doing, no?
Huh?
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Old 06-26-2009, 10:17 PM   #70
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Next car? Wimp! A little love and attention and that would be a quick car. Time to get serious about mechanicals. Come out to Cali and we'll have that motor out and fixed up in a jiffy. :-)

Well, you're basically masking from the CPU all but one pulse of the cam, right?
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Old 06-26-2009, 10:38 PM   #71
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Next car? Wimp! A little love and attention and that would be a quick car. Time to get serious about mechanicals. Come out to Cali and we'll have that motor out and fixed up in a jiffy. :-)
It's looking increasingly likely that this will happen, though whether it winds up being San Diego or LA is uncertain. Obviously the motor build would take place, but it would probably be fairly low-key; retain the 1.6 block and just load it with new rods & pistons. The idea would be to get in-and-out for minimum cost. At most, a slightly larger turbo would be in order. I have to be realistic however- I've reached a point of diminishing returns with this car.


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Well, you're basically masking from the CPU all but one pulse of the cam, right?
Well, yeah. Of the two pulses, one would be masked. It is, of course, irrelevant for my car since I currently have zero cam pulses.
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Old 06-28-2009, 03:23 AM   #72
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One pulse would be an infinite upgrade!

Hmmmm.... I know a certain guy who's sitting on a pile of 1.8's with nice modern heads.... :-)
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Old 06-30-2009, 10:30 AM   #73
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Guys,

Been working nights and just catching up. Some great stuff here. Can someone summarise (for a non-electrical engineer) the options for me? Simplicity and reliability are the key requirements. Got some time off coming up and getting the car sorted is one of the things I'll be doing during that time.
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Old 07-06-2009, 01:41 PM   #74
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All,

I am going the Hall route on EMs advice. It turns out that the studs on the FM adjustable cam gears are stainless steel. What I need to do is replace one of the nuts with a mild steel version and, apart from mounting the sensor, I should be good to go.

I do require a little help though. The FM cam gears don't use metric studs. I think they are UNC, but can't be sure. Can anyone on the other side of the pond tell me or find out what thread they use so I can try and source the correct nut?

I'll post up the results once the engine is running. Should be done by the end of the week.
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Old 07-06-2009, 02:21 PM   #75
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Hall? a true 'hall effect' sensor is a magnetic sensor, it looks for a voltage crossways to a current flow, and will be happy to ignore unmagnetized teeth, mild or otherwise.

The stock miata "hall" sensor is not a hall sensor, it's a VR sensor. Personally, I'd take that, grind off two teeth from a stock (or stock-compatible) cam gear, and you'd be done.
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Old 07-06-2009, 05:26 PM   #76
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Hall? a true 'hall effect' sensor is a magnetic sensor, it looks for a voltage crossways to a current flow, and will be happy to ignore unmagnetized teeth, mild or otherwise.

The stock miata "hall" sensor is not a hall sensor, it's a VR sensor. Personally, I'd take that, grind off two teeth from a stock (or stock-compatible) cam gear, and you'd be done.
Yeah, I am not sure what the difference is. EM differentiate between a magnetic and hall effect sensor when you select the cam/crank sensor type. It is the only 2 options. Their suggestion is a hall effect sensor as they are less sensitive to noise. Mount the sensor, replace one nut with a mild steel version and I should be done. It should also maintain the balance of the cam gear.

Unfortunately I don't have a stock compatible cam gear to grind off any teeth. These are teeth on the stock intake cam gear?
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Old 07-06-2009, 07:19 PM   #77
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Yes, the 99+ gears all have teeth on them, two on one end, a single on the other.

They are pretty much using utterly garbage terminology. :-) A hall sensor litterally looks for a magnetic field. Some sensors have a magnet inside, then when metal goes by (mild steel) it warps the field from it's own internal magnet and reads that. Then there are "VR", or 'variable reluctance' sensors, which due a similar trick, but don't use the Hall effect to read the signal.

In practice, people often say "hall" effect for a sensor which puts out a digital, or near digital (technically an "open collector output", which grounds a wire every time the sensor is sensing something) signal. Sometimes these are processed hall signals, sometimes processed VR signals, for that matter it could very well by optical.

As to the bolt, can't you just drill/tap the gear for any bolt you have handy?

If you want a stock '99 gear, with the teeth on it, I could mail you one for the cost of shipping, no problem, but you should be able to get a junkyard one much cheaper.
-Abe.



Note the righthand gear (exhaust), there's a single tooth on bottom, and a double on the top.
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Old 07-07-2009, 09:51 AM   #78
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Yes, I see the teeth on the cam gear now. I probably have one stuffed in a box somewhere as the stock gears were removed when the head was built.

Seeing as we are talking sensors, here is the sensor I will be using.

Whether I use a small magnet (6mm diameter button) countersunk into the gear (and epoxied) or change one of the stainless steel studs/nuts for a ferrous set will depend on where I can effectively mount the sensor. I hope to drill the plastic shroud and mount through that, otherwise I'll have to fabricate something out of aluminium to mount it in front of the cut off cam cover section. I favour the magnet idea at the moment.
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Old 07-07-2009, 02:02 PM   #79
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Maybe put a stud in one and leave it longer? The sensor does look for ferrous metal, not a magnet. A magent would likely work, if the polarity is right.

DealExtreme: $2.36 Super-Strong Rare-Earth RE Magnets (8mm 20-Pack) Maybe there? They have lots of sizes, but it was the smallest I've seen.

BTW, I put a magnet into the head of a bolt on my car cause I got tires of dropping that bolt. It wasn't hard to do, just found a magnet which matched the bolt and put a drop of (probably unneeded) glue. It's the sort of like that.

McMaster-Carr I wonder if somethign like that would work. I've seen 'bungee nuts' which are just nuts with a tall metal top for hooking bungee cords on motorcycles. Anything like that might give you your pulse.
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Old 07-07-2009, 03:20 PM   #80
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You know I never considered the magnets polarity. Actually how would one check it was correct?

I have ordered a few of these magnets. EM feel a magnet might be a little strong forcing me to run a large gap, so I may go back to the idea of replacing a stud and nut with ferrous items (I think I have found a machine screw and nut that fits, no idea what the thread actually is though).
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