Tach to Pcm

[1970 CST Short Wide]

My wiring harness says the white wire from pcm is to the tach. I hooked it up and tach works but is slow to respond. I just twisted the wires together, could a better connection fix this??

[LeesTruk]

You're gonna need what's called a "Pull-UP" resistor. Just put a 1K ohm resistor from your tach power wire to the tach signal wire. Also, if using an aftermarket tach, make sure it's set for a 4 cyl. engine. The PCM tach output is designed with that sort of signal pulse

[1970 CST Short Wide]

Originally Posted by LeesTruk

You're gonna need what's called a "Pull-UP" resistor. Just put a 1K ohm resistor from your tach power wire to the tach signal wire. Also, if using an aftermarket tach, make sure it's set for a 4 cyl. engine. The PCM tach output is designed with that sort of signal pulse

I have a in dash Factory style tach but not factory original How do I change it to 4 cyl or can I

[Mr.Chevy4x4]

If the tach is reading approximately half of what it should be, then you don't need the resistor. If the tach is barely or not moving at all then you need to put a 1K resistor from a switched power source into the tach feed wire. This will boost the signal.

Now, back to reading half of what it should. The signal from the PCM is similar to what a 4 cylinder would produce. This is why you can switch aftermarket tachs to 4 cylinder mode and they read correctly. If the tach you have isn't switchable, you can have a tuner change the tach output to make the tach read correctly.

Mike

[1970 CST Short Wide]

Originally Posted by Mr.Chevy4x4

If the tach is reading approximately half of what it should be, then you don't need the resistor. If the tach is barely or not moving at all then you need to put a 1K resistor from a switched power source into the tach feed wire. This will boost the signal.

Now, back to reading half of what it should. The signal from the PCM is similar to what a 4 cylinder would produce. This is why you can switch aftermarket tachs to 4 cylinder mode and they read correctly. If the tach you have isn't switchable, you can have a tuner change the tach output to make the tach read correctly.

Mike

Thank you, that helps a lot
Steve

[68c10airstream]

I have the 8,000 rpm aftermarket stock cluster kit installed and the pcm put out the wrong signal and after talking to avatar "critter" from "lone star dyno" i sent my pcm to him and he did the required mods and tested it on 2 different tachs before he sent it back. He said the signal needed to be changed in 2 ways, i believe from 4 cyl to 8 cyl and had to rescale it. He said this worked better than the pullup resistor, but i can't verify it. He was fair and took the time to double check it before sending it out. His name is Terry.

I did the ls1nova cluster changes and between eric (ls1nova) and terry it came out great.

[1970 CST Short Wide]

Originally Posted by 1970 CST Short Wide

Thank you, that helps a lot
Steve

Tried the 1K resister and it made my tack go backwards

[BBPanel]

Originally Posted by 1970 CST Short Wide

Tried the 1K resister and it made my tack go backwards

The purpose of the pull-up resistor is to change the +5v pcm tach signal to +12v - it worked for me on my Autometer tach. Sure you have the resistor in the correct direction - have no idea what it will do if not.

Here's a reference: http://aeromotiveinc.com/wp-content/...leShooting.pdf

[1970 CST Short Wide]

Originally Posted by BBPanel

The purpose of the pull-up resistor is to change the +5v pcm tach signal to +12v - it worked for me on my Autometer tach. Sure you have the resistor in the correct direction - have no idea what it will do if not.

Tried both power and lead to pcm. Lead did nothing power went backwards

[BBPanel]

added reference to previous post

[1970 CST Short Wide]

Originally Posted by BBPanel

added reference to previous post

Big help. It says use a 1/2 watt. I was using 1/8 & 1/4.
Thanks for the info

[Liltroy]

Common LS swap tach issues and how to fix them
https://www.youtube.com/watch?v=FS7aRfYkFAQ

[davepl]

If you're using a factory tach you'll need an adapter box. I don't know if the LS is "waste spark" but long story short there are TWO ignition impulses per firing event, so you get double the frequency.

If you have an aftermarket tach, just set it to 4 cylinder more, add the resistor, and you're done. For some reason now that I type that it sounds completely backwards, and you're really need a V16 setting, but I'll ignore my instincts as that's what the docs said.

If you can't set your tach to 4 cylinder, you need an adapter box. I got this one from Dakota Digital via Amazon:

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

Works great, simple to connect. My factory tach works perfectly.

The purpose of the pull-up resistor is to change the +5v pcm tach signal to +12v

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?

[Liltroy]

Originally Posted by davepl

If you're using a factory tach you'll need an adapter box. I don't know if the LS is "waste spark" but long story short there are TWO ignition impulses per firing event, so you get double the frequency.

If you have an aftermarket tach, just set it to 4 cylinder more, add the resistor, and you're done. For some reason now that I type that it sounds completely backwards, and you're really need a V16 setting, but I'll ignore my instincts as that's what the docs said.

If you can't set your tach to 4 cylinder, you need an adapter box. I got this one from Dakota Digital via Amazon:

https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1

Works great, simple to connect. My factory tach works perfectly.



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?

Please watch the video I posted. If you're doing a swap then theres no reason why you cant just change the signal in the pcm when you have your pcm flashed. Converter box is a completely unnecessary cost.

Also the resistor isnt there to increase voltage. As I explained in the video, youre pulling 12 volts from a switched 12v source and adding it to the tach signal, the resistor is there to DECREASE the voltage and to a safe level and prevent backfeeding and damaging the pcm I believe.

[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.

[Willie Makeit]

i see what you did there dayj.

[davepl]

So a pull-up resistor is just the positive side of a pulsed ground system for when it doesn't have ground potential? And the ECM value for Tach Output is 12/resolution value?

Seems shorter when I say it ;-)

Thanks though, your explanation did clarify things! Still wondering what'll happen if I hook a logic probe up to a floating circuit (seriously!).

[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 davepl

So a pull-up resistor is just the positive side of a pulsed ground system for when it doesn't have ground potential? And the ECM value for Tach Output is 12/resolution value?

Seems shorter when I say it ;-)

I'd think that's a fair description in this example. The pull-up resistor is used to make the "default" value of the signal high.

Also, there is such a thing as a "pull-down" resistor that does just the opposite (defaults the signal to low).

Originally Posted by davepl

Thanks though, your explanation did clarify things! Still wondering what'll happen if I hook a logic probe up to a floating circuit (seriously!).

If you have a fancy logic probe that graphs an output it wouldn't graph a floating signal at all as it's undefined (i.e. neither high nor low).

I use an older logic probe with just red (high) and and green (low) LEDs. The raw tach signal without a pull-up would show as a flashing red light. In other words periods where neither LED is lit.) Adding the pull-up resistor would cause the signal to be represented as alternating red and green lights with one or the other LED being lit at all times.

[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.

[Liltroy]

And now you know why I say in the video " it doesn't matter why, you just need to know what numbers to put in the box to make it work". Lol
Posted via Mobile Device

[65stinger]

Dayj1...........Let me know if you need any help wiring up that truck.....I know a guy

[aknovaman]

Hate to rain on someones parade but your theory on the PCM tach output is incorrect. First the older PCM red/blue actually puts out a positive pulse from ground to about 8 volts. This transition is sufficient to trigger most aftermarket tachometers. The newer ecu don't work the same way. They actually ground the tach output wire for each tach event. The purpose of the pull up resistor is to do that, pull up the voltage to 12 volts between each tach event. Then with each tach event, the pcm output is grounded, thus transitioning the voltage across the external tach threshold. This can be verified using a simple oscilloscope to see the waveform. If you don't have the pull-up resistor connects to 12 volts, the newer PCM output will float anywhere from 1-8 volts erratically. There needs to be a solid/consistent voltage transition across the external tach in order for it to function reliably.

Respectfully, Aknovaman