I searched google and the forums but I was not able to find the answer I am looking for. I know the Tach signal coming out of the HEI distributor has 4 pulses per crankshaft rotation in the case of a V8 engine, but what I don't know is what those pulses look like. What voltage does the signal spike to when a pulse occurs? I am trying to build a digital tach, but I doubt I can plug the tach signal directly to the microchip I am using. It would probably fry it considering I get shocked when I touch the tach wire. I dont have an oscilloscope so I need your help to figure out how I may need to filter the signal to TTL 5V levels.

I have actually scoped an MSD tach signal, but not an HEI.

The period will be the same, but the signal shape will not because the
HEI is hall effect.

Here is a quick drawing of what I think the signal will look like based on
a 12 volt input:






I believe there will be some residual voltage as the time to drain off will
decrease as RPM increases.

I would also expect the peak voltage to drop as RPM increases.

You will have to design some sort of buffer to convert the signal down to
a voltage suitable for the IC you plan to use (TTL?).

I saw the coil signal on my stock distributor (on a scope) and it was pretty close to a square wave signal. The frequency will of course change with the RPMs but the width of the pulse and duty cycle won't be constant, count on that.

Watch out for the negative spikes, they can go to hundreds of volts. They went off-screen on the scope so I have no idea how big they really were.

A good beefy filter and voltage divider on the input of your chip ar a must.

Hope this helps.
Lou

I believe his circuit input will be direct, and not an inductive type as he's
triggering from the tach signal.

I plan on using the signal going to the stock tach. I found a circuit online that uses an opto-isolator and some diodes. I'm going to ask my professor about it and post any info I find out. I think the diodes should take care of the negative voltage spikes.

Thanks for the info so far.

Some of the HEIs were hall-effect, most were variable-reluctance. That doesn't matter though unless you're interested in the signal from the pickup itself. The tach signal is taken directly from the coil primary so it could have considerable ringing and the duty cycle will vary with dwell.

Good point Aperion.

I scoped the pre-module signal from the reluctor. The diagram is indicative
of the reluctor signal (due to installing the MSD w/ external module).

Anything that is probed post module is likely going to be squared up and a
lower voltage which will make it easier to trigger from.

Look what I found!

This is a reluctor signal without a diode:

That still looks more like a reluctor signal.

Yes, you're correct. I keep jumping ahead of myself because I didn't have
the module installed when I scoped the signal. What I'm showing comes
right off the pickup.

Anything post module will be squared up. The MSD tach signal is 5 V square,
I do not know what the HEI modules produce, but I would think based on
these new diagrams, the voltage will be 5 volts as well. It might just work
with his TTL IC logic.

No matter what voltage, you can just use a pull-up on the chip input and diode (cathode on the signal side) and you'll be detecting the falling edge with no problems.

Lou

I know the thread is dead, but considering how many views this thing has, people are obviously interested. I thought I'd post up some working pictures of the tach signal coming straight from the tach diagnostic connector on the ignition coil of a '92 305 TBI 'Maro (L03). This is at idle, btw.

You can see the whole waveform here.

A bit zoomed in.

And you can see how high the voltage spikes up.

That looks like what I'd have expected.

And is that an analog O-scope? I haven't seen one of those in years.

Haha. Yep. It's my new toy. It's from 1993. I picked it from ebay for $250 including shipping and a 100x passive probe.

1993? Puts my HP 120AR to shame.

That reminds me, I really need to find a new tube for it someday.

Just looked that up. Wow.

While I've got ya, do you know why there is that sort-of-high part right before the spike? So the signal goes: big spike, high, low, sort-of-high, repeat.

You'll see that on electronic ignitions when the module detects that the coil is fully saturated and begins limiting current to prevent coil damage. It's still part of the dwell period as far as the coil is concerned.

Ok, but then why is it after the signal goes low but before the signal goes fully high? I was under the impression that it would go high than a bit lower and then all the way low. I'm certainly not too informed in this area, though.

The signal starts out high because the transistor in the module connecting the coil to ground is off, and you're seeing the +12V side. Then it goes low when the transistor turns on and the coil is grounded and essentially shorts out the +12V as far as the scope can see. Then when the coil is saturated, the current limiter switches in and the effective resistance to ground increases, so you see an increase in voltage on the scope. Then the transistor switches off, the magnetic field in the coil collapses rapidly and induces the secondary voltage, and you see the initial spike as the secondary builds up breakdown voltage to overcome the gap and the plug fires. The voltage persists at a lower level briefly while the spark is firing. Then when the voltage in the secondary falls below what's required to maintain the spark, you see the ringing as the fields bounce back and forth and dissipate the remaining energy internally.

Here, we'll label it.

A. Idle period ends, dwell period begins. Transistor turns on, coil begins charging
B. Coil saturated, current limiting begins
C. Dwell period ends, firing period begins. Transistor turns off, coil discharges, secondary reaches breakdown voltage, plug sparks.
D. Secondary voltage falls, spark ends, ringing begins, firing period ends.
E. Ringing ends, idle period begins.

Ahh. That was a truly fantastic explanation. Thank you.

I own a small technology company that's building some electronics to interface with pre-OBD vehicle instruments and reporting it to software that runs on a touchscreen interface in the cockpit. We've completed bench testing everything but the signal from the tach. In fact, we're confused about how this even works.

Is it a set number of pulses per RPM? We've heard 4 pulses, 8 pulses and even variable pulses. The voltage shouldnt matter to us, as long as we step it down sufficiently and protect the circuits. We just need to be able to count pulses. Our processors can sample the analog input many times in a second, so I'm not worried about not being able to keep up.

Any information regarding the number of pulses per RPM, average voltage or high voltage amount, etc. is great appreciated!

-T