NB8A - Tach Signal characteristic at Diagnostic Port?

wleung · Postby **wleung** » Wed Sep 23, 2015 11:36 am

Hi All,

I've a little project whereby I'm installing a cruise control into my 98' NB8A.
I'm a cheap bastard, so I've gotten my hands on an AP60 cruise control kit (the cheapest), and want to try hooking the magnetic sensor line via some smart-ish electronics to the Tach Signal at the diagnostic port.

I've been trawling the various mx5 forums last night, and couldn't figure out what type of signal the NB8A diag connector puts out via the IG- pin

Is it:

- a Pulse train (-12V - 0) where the pulse is given out at -12V corresponding to the ignition coils.
i.e. frequency of pulses is 2 * rpm.

- a PWM signal where the width of the pulse is proportional to the rpm.
(A friend has confirmed that his NB8B outputs a PWM signal, but they don't remember the PWM characteristics)

Can anyone on this forum enlighten me on this subject? (I don't have a CRO at home to check it out).
i.e. if its PWM, anyone know the period/freq of the PWM signal and what the equation is to convert it back to rpm?

(Maybe there's variance in tach signals in different countries on the same make of mx5?)

W

93_Clubman · Postby **93_Clubman** » Wed Sep 23, 2015 11:51 am

timk on here might know as he's done a number of electronic projects on his MX5s over the years, some involving tach signal.

Magpie · Postby **Magpie** » Wed Sep 23, 2015 4:26 pm

This is the wave from a stock NA6 that I had at home. The pulse was taken from the diagnostic port.

This is from the PS1000 on my car

wleung · Postby **wleung** » Wed Sep 23, 2015 8:43 pm

Those are some awesome shots magpie!

I'm surprised the signal was only 28.21 Hz. I'm actually seeing something of similar order in my NB8A diagnostic port.

I only had a cheapo VC97 multimeter at home, and this is what I saw from the IG and GND pins
(I forgot which polarity I put in, but let's not worry about the sign.)

- DC Mode: Voltage was 5V at idle speed (Magnitude as I forgot the sign).
- DC Mode: When the engine was revved slightly the DC voltage didn't change
- Hz Mode: The signal had a frequency of about 28-30Hz at idle speed.
- Hz Mode: When the engine was revved slightly I saw the frequency go up to roughly 65Hz.

So I'm still not sure if I've got a PWM or some other signal. I would think the DC voltage would increase/decrease as the pulse width changes, but I guess I didn't spot it.
The VC97 spec indicates a sampling rate of 3Hz, but I'm thinking that may be the display and not the signal sampling?
Otherwise I don't see how it can detect a 30Mhz signal on the Frequency setting - but I'm not a multimeter expert.

Any other thoughts? Ideas?

W

ndragun · Postby **ndragun** » Wed Sep 23, 2015 9:08 pm

What is your idle speed?

28-30Hz is between 1680-1800RPM

(0.0166666667Hz = 1RPM, 1Hz = 60RPM)

If you revved it to 3900RPM - you would have seen 65Hz.

Does that sound right?

Magpie · Postby **Magpie** » Wed Sep 23, 2015 9:13 pm

I took those screen shots a while ago when trying to figure out why the OEM dash would not show RPM from the Haltech. Turns out the PS1000 needs to be set on 12v and the signal inverted to work.

If you are in Brisbane I could lend you the osciliscope to diagnose the signal.

wleung · Postby **wleung** » Wed Sep 23, 2015 9:32 pm

I did another measurement with AC mode out of curiosity & clarified polarity:
+ve probe was on IG,
-ve probe was on GND
- DC Mode: Voltage was +5V at idle speed (Magnitude as I forgot the sign).
- DC Mode: When the engine was revved slightly the DC voltage didn't change.
- AC Mode: Voltage was +6.7V (suspiciously close to Sqrt(2) * 5?) at idle.
- AC Mode: When the engine was revved, the AC voltage reading didn't change.
- Hz Mode: The signal had a frequency of about 28-30Hz at idle speed.
- Hz Mode: When the engine was revved slightly I saw the frequency go up and down with the rpm (hit 80Hz, but I didn't want to rev too hard, and I didn't have a 2nd person to watch the tacho).

What is your idle speed?

According to my tacho its 800rpm. Maybe my scaling factor is different?
I might check again tomorrow morning as I was pumping noxious gases into my garage

.

That said, for the AP60 it shouldn't matter. It looks like I probably could hook the IG & GND up to a voltage follower and offset bias the magnetic sensor line to make the signal swing +ve & -ve like a magnet sensor.
I'm not yet sure if the frequency is too high for the AP60 yet - will test it out and see if it works.
If its too high I might have to make some sort of frequency divider for the signal.

If you are in Brisbane I could lend you the osciliscope to diagnose the signal.

Thanks for the offer, but I'm in Sydney.
I actually have access to some CROs at work. Maybe I can borrow one during lunch and test it out.
If I do, I'll try to post up what I get so this may help some other people later on.

W

ndragun · Postby **ndragun** » Wed Sep 23, 2015 9:36 pm

wleung wrote:If I do, I'll try to post up what I get so this may help some other people later on.

Please do! I'd like to know out of sheer curiosity, since I went through all this not long ago trying to diagnose a tacho issue.

Caffeine · Postby **Caffeine** » Thu Sep 24, 2015 4:51 pm

ndragun wrote:What is your idle speed?

28-30Hz is between 1680-1800RPM

(0.0166666667Hz = 1RPM, 1Hz = 60RPM)

If you revved it to 3900RPM - you would have seen 65Hz.

Does that sound right?

Don't forget that there are 2 pulses per revolution, so your rpm is half that. An idle of 830 - 900 rpm makes more sense.

ndragun · Postby **ndragun** » Thu Sep 24, 2015 4:54 pm

Caffeine wrote:Don't forget that there are 2 pulses per revolution, so your rpm is half that. An idle of 830 - 900 rpm makes more sense.

Who let this man with a brain in here?

wleung · Postby **wleung** » Thu Sep 24, 2015 8:36 pm

Don't forget that there are 2 pulses per revolution, so your rpm is half that. An idle of 830 - 900 rpm makes more sense.

Yep, Caffeine's a genius.

Put a CRO on the IG (+) & GND (-)

Short Answer
The signal out of the diagnostic box on the IG- is a 0V-13V Frequency-Modulated Square Wave with a fixed duty cycle of roughly 38.5%, whereby the frequency of the square wave signal corresponds to the two ignition coils igniting (i.e. 2 pulses per revolution)

So it was actually a pulse train, not a PWM.

Extended Answer
This is why:
- DC Mode on multimeter we see 5V => 13V * 0.385 duty = 5V
- DC Magnitude never changes on increase in rpm => Fixed Duty Cycle at all frequencies
- Frequency goes up with acceleration => ECU sends out duty-cycled pulses at the frequency of the two ignition coil pulses.

I took three measurement points this morning with the multimeter before the CRO:
860-ish rpm => 28Hz
2000-ish rpm => 66Hz
3000-ish rpm => 102Hz

Excel worked out an equation RPM = 29.9(Freq) + 61
Notice the gradient is almost 30?
30 corresponds to converting Hz to Rpm by multiplying by 60, and then dividing by 2 for the 2-pulses per revolution.
(The 60 just means I'm off by an average of 2Hz in my readings)

Here are the CRO images:
860-ish rpm:

860rpm.JPG

2000-ish rpm:

2000rpm.JPG