JasonC SBB

Whether the factory sensor is powered by 12V or 5V, I'm pretty sure the output is 5V, which the TEC will like.

mrtonyg

iTrader: (1)

Slowmx5, here are some waveforms from a 1996 CAS vehicle. On earlier vehicles the wire color changes but the signals are the same. Hopefully they help on your quest.

The bottom signal on both screen captures is the Yellow/Blue wire and the top is the white wire from the CAS.

Both captures were at idle. The difference between them is the time scale only.


EDIT: I removed the waveforms.

JasonC SBB

That looks like a magnetic pickup output, just like the TEC's crank pikcup.

mrtonyg

iTrader: (1)

It looks similar but it's not.

A magnetic or VR sensor outputs a pure sine wave ac output. Meaning it oscillates between negative and positive. If you notice it never goes negative.

Look at the wave form, the bottom signal is triggered from battery voltage as a source. A magnetic sensor doesn't need a source voltage, it develops it's own voltage from induction ie. from the passing of a ferrous trigger across the face of the sensor.

Joe Perez

iTrader: (8)

mrtonyg, you say those captures are the two outputs of a '96 CAS? Something isn't right with those traces. The lower trace (CMP, which you call SGC) should not be a little spikey thing, but a nice squarewave with unequal duty cycle on alternating events.

Edit: here's what a healthy NA CAS looks like when both outputs have +5 pullups applied. The top trace is CMP, the next one is CKP, and the third & fourth traces are ignition outputs A and B.

(apologies for the hand-written traces, this was done on my four-channel scope when the floppy drive was broken.)

JasonC SBB

Not to be argumentative, but that's only because the output is returned to 12V instead of GND. So if you think of 12V as "ground", it *is* outputting AC.

mrtonyg

iTrader: (1)

Joe, I agree with you. I was surprised with the output of the bottom trace.

The car this was taken from runs perfect, and I double checked when I saw the output.

I will check on my other Miata.

mrtonyg

iTrader: (1)

Alternating current means negative and positive oscillations. Going from 12v to 0v is not ac. Going from +12v to -12v is ac.

You can't think of 12v as ground because it's not...that argument doesn't make sense.

mrtonyg

iTrader: (1)

Just checked on my other Miata...I have a faulty CAS sensor which led to the odd looking trace!!! Hard to believe, the car runs perfect!

Below are off my 1994 Miata.

Both captures were at idle. The difference between them is the time scale only.

I believe the top signal with the shorter pulse is the #1cyl TDC. When I get back from a 2 day vacation, I will sync this signal to the injectors and coils to see their relationship.

Joe Perez

iTrader: (8)

Yep, that's what they're supposed to look like. CMP always rises before CKP, however on one cycle it falls before CKP, and on the other it falls after. If your ECU is capable of looking at the relationship between two pulses then you can use this to determine #1 TDC.

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.

mrtonyg

iTrader: (1)

In relation to newer car terminology, essentially the top trace is CAM sensor and the bottom trace serves as the Crank sensor (CKP).

Joe Perez

iTrader: (8)

If we want to be really technical here, we need to bear in mind that a VR sensor, by itself, has no reference. It's a balanced device where the signal on one wire is always of equal but inverse magnitude to the signal on the other. You could connect it to the primary side of an isolation transformer and it'd still work just fine.

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. In such an arrangement, the voltage on the opposite wire will swing both above and below ground, having a positive potential for half the cycle, and a negative potential for the other. I'm pretty sure that this meets everyone's accepted definition of AC.

As Obi-Wan would say, it's all relative. +12 can most certainly be thought of as ground, if you are using it as the common point in a circuit. Years ago, cars had positive-ground electrical systems, where the battery's positive terminal was connected to chassis and the negative terminal was connected to the fuse block. A particle physicist might argue that this is actually the most correct usage, owing to the fact that in a DC circuit the physical flow of electrons is from negative to positive.

The fact of the matter is that in a car, there's really no such thing as ground, because the vehicle is not electrically connected to terra firma. There is only chassis, which we have all agreed to use as the common reference point for all circuits in the car.

gospeed81

iTrader: (51)

y8s

iTrader: (8)

I was LOOKING for that comic but couldn't find it in time DAHHH

mrtonyg

iTrader: (1)

AC is not ac unless the voltage goes in reverse polarity. An oscilloscope picks up reverse polarity when the voltage goes negative or below 0.

That is the definition of AC.

mrtonyg

iTrader: (1)

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.

modernbeat

iTrader: (1)

Has anyone tried using the disc out of an injected Festiva distributor in a NA6 CAS to get an isolated cam position signal for sequential injection/ignition?

JasonC SBB

mrtonyg, the sensor coil itself outputs AC, but one side is connected to 12V instead of to the chassis. Mazda may have done this in order to easily trigger on the falling edge, threshold at 5V or whatever. The giveaway is the exponential (decay) return to 12V.

Joe Perez

iTrader: (8)

Here you can see the signal coming out of my VR sensor, as well as the signal produced by the decoder.





One leg of the VR sensor is connected to ground at the same point as the shield. The other leg of the sensor is connected to the input of the zero-crossing detector. In this trace, the tip probe of the scope is connected to the input stage of the detector, and the ground clip of the probe is connected to circuit ground.

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?


I really can't think of any alternate interpretation of this. Again, this is a "pure" VR sensor, rather than the sensor which Jason is describing. However it would not matter whether the first leg of my sensor was connected to ground, to +12, or to the hot leg of one of the phases of AC power coming into my house, so long as whatever circuit was doing the zero-crossing detection was also referenced to that point.

mrtonyg

iTrader: (1)

What sensor are we talking about here. The VR sensor doesn't have a need for reference or input voltage.