Tach to Pcm

[davepl]

But the video just says "You don't need to know why it is what it is"! I can't live like that! Granted you do give the numbers, which is great.

I think the rough answer is:

Sample values for this calibration:
For 4 pulses, use N= 3 (Note- 8 cylinder tach setting)
For 3 pulses, use N= 4 (Note- 6 cylinder tach setting)
For 2 pulses, use N= 6 (Note- LSx 8 cylinder & 4 cylinder tach setting)

Both cases we're talking about here are eRod or Connect&Cruise, which will usually be run with the factory tune, not a custom tune, so a converter box is actually CHEAPER than an HPtuners VIN license, right? I know I don't have unlimited HPTuners.

You are right, however, in my case I could have done it because I had to change the VSS output tooth count on the transmission anyway, though. But no one seemed to know that back when I asked!

[Liltroy]

Originally Posted by davepl

But the video just says "You don't need to know why it is what it is"! I can't live like that! Granted you do give the numbers, which is great.

I think the rough answer is:

Sample values for this calibration:
For 4 pulses, use N= 3 (Note- 8 cylinder tach setting)
For 3 pulses, use N= 4 (Note- 6 cylinder tach setting)
For 2 pulses, use N= 6 (Note- LSx 8 cylinder & 4 cylinder tach setting)

Both cases we're talking about here are eRod or Connect&Cruise, which will usually be run with the factory tune, not a custom tune, so a converter box is actually CHEAPER than an HPtuners VIN license, right? I know I don't have unlimited HPTuners.

You are right, however, in my case I could have done it because I had to change the VSS output tooth count on the transmission anyway, though. But no one seemed to know that back when I asked!

I apologize. I missed the part where you were using the e-rod setup. Of course that also means you're using a 58x reluctor which has different pulses altogether. I only tune the gen 3's so Im afraid I havent really dug into the correct settings for the gen 4(E38) pcms. I own an avalanche that is gen 4 and the tach output is set at 2/2 so thats all I know on that one.

[dayj1]

Originally Posted by davepl

Can anyone explain that in a way that a software guy can understand it? I get Ohm's law and all of that, but how can adding a resistor increase the voltage?

It helps to first level set a couple of concepts.

First, voltage is a measure of the potential difference between two points. Zero volts doesn't automatically equal ground. For example, put the red lead of a voltage meter on your positive battery terminal and the black lead on the ground terminal. It will read somewhere around 12 volts. Now, put both leads on the positive terminal. The meter will read zero volts. That's because there is no potential difference between the leads but they're both on the positive terminal so they are definitely not grounded.

Secondly, as a software guy, you're probably familiar with the concept of a variable that is undefined. In digital systems, there are two states:high and low, on and off, 1 and 0. Anything else is undefined. In a digital circuit, you'll often hear an undefined input/output referred to as "floating". A floating circuit is neither logically high or low.

The Gen 3 and 4 PCMs output a pulsed ground signal on the tach wire. The frequency of the pulses varies proportionally with engine RPM. Again, this is a series of ground pulses. In between the pulses, there is no voltage at all (the signal is floating). In other words, the signal is toggling between a logic low (ground) and undefined. If you looked at a graph of the signal versus time, it would be a series of dashes. Enter the pull-up resistor. Connecting a resistor from the tach signal wire to 12 volts pulls the floating portion of signal up to a logic high between pulses. If you graph the signal with the pull-up resistor in place, it's now a square wave alternating between 12 volts and ground with no floating areas in the signal.

So, we have a square wave with the resistor. Why does it mater? Tachometers are counting the transitions from high to low. Without the pull-up resistor, no transition is happening. Remember a digital signal only works with highs and lows. Undefined doesn't count and just gets ignored.

In a conventional system (think old small/big block). The tach signal wire is connected to the negative side of the coil. When the points are closed, the tach signal wire is grounded. When the points are open, the tach signal wire it connected to 12v through the coil primary winding. The coil is a pull-up resistor for the signal.

Also, in the factory configuration for a Gen 3 or 4 engine, the pull up resistor is in the gauge cluster.

Originally Posted by davepl

But the video just says "You don't need to know why it is what it is"! I can't live like that! Granted you do give the numbers, which is great.

I think the rough answer is:

Sample values for this calibration:
For 4 pulses, use N= 3 (Note- 8 cylinder tach setting)
For 3 pulses, use N= 4 (Note- 6 cylinder tach setting)
For 2 pulses, use N= 6 (Note- LSx 8 cylinder & 4 cylinder tach setting)

First, let's understand how a tach works. In a conventional system (i.e. Gen 1 small block), the tach is counting how many times the ignition coil fires. We're talking a four stroke engine, so only half the cylinders fire in one crankshaft revolution. So, for an 8 cylinder engine we get 4 pulses per engine revolution.

That's important to know because a factory tach from a typical "vintage" automobile is going to be expecting those 4 pulses per revolution for a v8 engine (or 3 pulses for a 6 cylinder or 2 pulses for a 4 cylinder).

Fast forward a few decades and a Gen 3 PCM is counting crankshaft pulses and outputing tach pulses. There are 24 teeth on the crankshaft reluctor ring which equates to 24 pulses per crankshaft revolution for a Gen 3 engine. The "resolution high" and "resolution low" values in the tune equate to how many crankshaft pulses to leave the tach signal at a logic low or logic high. Technically, "high" is really floating which is why we need a pull up resistor. At any rate, through one revolution of 24 pulses, the factory values of 6 and 6 for the resolution fields mean that the tach signal is high for 6 pulses, low for 6 pulses high for 6 pulses and low for 6 pulses. The tach is counting transitions from high to low and there were 2 transitions per crank revolution with the stock values. That's why the "stock" settings match a 4 cylinder engine. The relationship of the "high" and "low" numbers affect the duty cycle of the signal. For most tachs, the duty cycle is really irrelevant, but keeping the numbers equal results in a signal that is at 50% duty cycle.

If you want a formula, 24/(high + low) = number of pulses per engine revolution. So, if we need 4 pulses per rev for a standard 8 cylinder tach, then 24/6=4. 6 is the sum of the high and low resolution, so use 3 and 3 for 50% duty cycle although (1,5) or (2,4) or (3,3) or (4,2) or (5,1) would work.

Gen 4 engines are a little tougher because there are 58 teeth on the crank reluctor. 58/x=4 means that x=14.5 Since you can't get 14.5 with two integers, you have to settle for something like (7,7) or (7,8) which is 3.4% high or 3.4% low, respectively. In other words, you have to "settle" for less than perfect.

Originally Posted by davepl

But no one seemed to know that back when I asked!

I did.

[Hart_Rod]

Originally Posted by dayj1

It helps to first level set a couple of concepts.

First, voltage is a measure of the potential difference between two points. Zero volts doesn't automatically equal ground. For example, put the red lead of a voltage meter on your positive battery terminal and the black lead on the ground terminal. It will read somewhere around 12 volts. Now, put both leads on the positive terminal. The meter will read zero volts. That's because there is no potential difference between the leads but they're both on the positive terminal so they are definitely not grounded.

Secondly, as a software guy, you're probably familiar with the concept of a variable that is undefined. In digital systems, there are two states:high and low, on and off, 1 and 0. Anything else is undefined. In a digital circuit, you'll often hear an undefined input/output referred to as "floating". A floating circuit is neither logically high or low.

The Gen 3 and 4 PCMs output a pulsed ground signal on the tach wire. The frequency of the pulses varies proportionally with engine RPM. Again, this is a series of ground pulses. In between the pulses, there is no voltage at all (the signal is floating). In other words, the signal is toggling between a logic low (ground) and undefined. If you looked at a graph of the signal versus time, it would be a series of dashes. Enter the pull-up resistor. Connecting a resistor from the tach signal wire to 12 volts pulls the floating portion of signal up to a logic high between pulses. If you graph the signal with the pull-up resistor in place, it's now a square wave alternating between 12 volts and ground with no floating areas in the signal.

So, we have a square wave with the resistor. Why does it mater? Tachometers are counting the transitions from high to low. Without the pull-up resistor, no transition is happening. Remember a digital signal only works with highs and lows. Undefined doesn't count and just gets ignored.

In a conventional system (think old small/big block). The tach signal wire is connected to the negative side of the coil. When the points are closed, the tach signal wire is grounded. When the points are open, the tach signal wire it connected to 12v through the coil primary winding. The coil is a pull-up resistor for the signal.

Also, in the factory configuration for a Gen 3 or 4 engine, the pull up resistor is in the gauge cluster.



First, let's understand how a tach works. In a conventional system (i.e. Gen 1 small block), the tach is counting how many times the ignition coil fires. We're talking a four stroke engine, so only half the cylinders fire in one crankshaft revolution. So, for an 8 cylinder engine we get 4 pulses per engine revolution.

That's important to know because a factory tach from a typical "vintage" automobile is going to be expecting those 4 pulses per revolution for a v8 engine (or 3 pulses for a 6 cylinder or 2 pulses for a 4 cylinder).

Fast forward a few decades and a Gen 3 PCM is counting crankshaft pulses and outputing tach pulses. There are 24 teeth on the crankshaft reluctor ring which equates to 24 pulses per crankshaft revolution for a Gen 3 engine. The "resolution high" and "resolution low" values in the tune equate to how many crankshaft pulses to leave the tach signal at a logic low or logic high. Technically, "high" is really floating which is why we need a pull up resistor. At any rate, through one revolution of 24 pulses, the factory values of 6 and 6 for the resolution fields mean that the tach signal is high for 6 pulses, low for 6 pulses high for 6 pulses and low for 6 pulses. The tach is counting transitions from high to low and there were 2 transitions per crank revolution with the stock values. That's why the "stock" settings match a 4 cylinder engine. The relationship of the "high" and "low" numbers affect the duty cycle of the signal. For most tachs, the duty cycle is really irrelevant, but keeping the numbers equal results in a signal that is at 50% duty cycle.

If you want a formula, 24/(high + low) = number of pulses per engine revolution. So, if we need 4 pulses per rev for a standard 8 cylinder tach, then 24/6=4. 6 is the sum of the high and low resolution, so use 3 and 3 for 50% duty cycle although (1,5) or (2,4) or (3,3) or (4,2) or (5,1) would work.

Gen 4 engines are a little tougher because there are 58 teeth on the crank reluctor. 58/x=4 means that x=14.5 Since you can't get 14.5 with two integers, you have to settle for something like (7,7) or (7,8) which is 3.4% high or 3.4% low, respectively. In other words, you have to "settle" for less than perfect.




I did.

WE'RE NOT WORTHY, WE'RE NOT WORTHY!!!! In my best Wayne/Garth voice...

[dayj1]

Originally Posted by Hart_Rod

WE'RE NOT WORTHY, WE'RE NOT WORTHY!!!! In my best Wayne/Garth voice...

LOL. That's how I feel when you post progress pictures of your builds.