Someone explain the tach signal?
02-16-2012, 10:33 PM
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Someone explain the tach signal? Not sure if this is in the right place but I'm trying to find out where exactly the RPM signal is coming from, to be specific this is on a 5.3 truck motor 2001. Does the rpm signal come from the crank reluctor or from the cam reluctor?
I'm doing a non LS motor swap into a LS vehicle and want to keep my factory tach in place by mounting an external reluctor and sensor on the crank snout of my all mechanical engine, i would like to use factory LS sensors on my new engine (water,oil,tach).
I'm doing a non LS motor swap into a LS vehicle and want to keep my factory tach in place by mounting an external reluctor and sensor on the crank snout of my all mechanical engine, i would like to use factory LS sensors on my new engine (water,oil,tach).
02-18-2012, 11:03 PM
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I posted this info on another site. It sort of applies to your engine.
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GEN IV E38 Tach Settings for aftermarket tach (autometer)
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Correct HPTuners tach settings for E38 ecm with 58x crank sensor.
Tach Output = Crank
Resolution - High =14
Resolution - Low =15
This will make the ECM output the correct frequency for a 2 pulse per revolution tach. The Duty cycle of the frequency will be 48.3%. 14/29
Tach output is on Pin 48 of the black connector.
You need to add a resistor (about 1k ohm) between the tach output and 12V to Pull-Up the Output signal to a usable voltage for aftermarket tachs.
-----------------------------------------------------------------------------------------
Here is how I came up with these numbers, for the people who care…
I have done testing to verify everything. This is what I have found to be true. I may have made errors but I do not think that I did.
-----------------------------------------------------------------------------------------
The tachometer I am using an Autometer Phantom II Part #7588.
You can configure the tach for
8 cyl mode (4 pulses per revolution)
6 cyl mode (3 pulses per revolution)
4 cyl mode (2 pulses per revolution)
4 cyl mode (1 pulses per revolution)
A 4 stroke 8 cylinder engine only fires 4 cylinders per engine revolution, 4 are on compression/firing stroke and 4 are on exhaust stroke. This is why tachs typically look for a 4 pulses per revolution.
-----------------------------------------------------------------------------------------
HP Tuners Settings
The tach output is OFF in the factory calibration.
The two options for tach output are frequency and crank. I do not know what frequency is, I tried it, it did not work for me, I got “crank” to work and I understand “crank” so I do not care about frequency.
The other variables are
Resolution - High = (number of periods the output stays high) I will call this HIRES
Resolution - Low = (number of periods the output stay low) I will call this LORES
These values need to be whole numbers.... The ECM can not handle decimal places.
Through some investigation I have found out that when "crank" is selected the ecm is going to output a pulse (or frequency, a pulse is a frequency) based on the 58x crank sensor. The minimum vales in HIRES/LORES are 1/1. This means for every 2 crank pulses you will get 1 tach output pulse from the ECM. The highest resoulution tach output signal is 29 pulses per revolution. 58/2
To verify this I entered 1/1 in the HIRES/LORES fields and hooked up my trusty Fluke to the tach signal at idle
670 rpm = 11.17 revolutions per second
11 revs per second * 29 pulses per rev = 324 pulses per second = 324 HZ
I verified 324 HZ with my fluke at 670 rpm.
I have heard that the numbers in the HIRES and LORES needed to be equal and I have also heard that they needed to be dividable by 60. I do not think either of those statements are true.
-----------------------------------------------------------------------------------------
An Autometer tach works off of frequency. Frequency can be measured as the time between the leading edges of 2 pulses. The tach does not really care what the duty cycle is as long as it is within an acceptable range.
I set my tach in 4 cylinder mode.
I set 15/15 in HIRES/LORES
At 3000 rpm I read 97 hz with a 50% duty cycle.
I set my tach in 4 cylinder mode.
I set 10/20 in HIRES/LORES
At 3000 rpm I read 97 hz with a 33% duty cycle.
The tach still read correctly even though the duty cycle was very different. The frequency remained the same.
Ok so now I have verified duty cycle can be changed and that crank is based on the 58 tooth crank sensor.
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The highest number of pulses my tach allows for 8 cylinder mode is 4 pulses per revolution. The higher the pulses per rev the faster the tach will respond. Higher = better.
58/4 pulses per rev =14.5 total crank pulses for each ECM tach output pulse.
14.5 = 7.25 and 7.25 in HIRES/LORES
These are not acceptable numbers. Must be a whole number CRAP……
If you use 7/7 your tach will be reading 3.4% too high.
It will read 2900 when your actual engine speed is 2800.
If you use 7/8 your tach will be reading 3.4% too low.
It will read 2900 when your actual engine speed is 3000.
I switched my tach to 4 cylinder mode. The tach is now looking for 2 pulses per engine revolution. 58/2 pulses per rev = 29 total crank pulses for each ECM tach output pulse.
29 = 14 and 15 in HIRES/LORES
If you use 15/15 your tach will be reading 3.4% too low.
It will read 2900 when your actual engine speed is 3000.
If you use 14/15 your tach will be reading 100% accurate.
I should add. Being 3% off on an analog tach is not a big deal. I tried to run my engine at 3000 rpm and compare my tach to the actual scanner readout. I saw a negligible difference even when the tach was off. Hopefully I helped someone understand what the signal is and how to TUNE it…. ( this can be useful is trying to create a signal to work with a non GM gauge cluster...) Settings of 15/15 will probably not be noticable especially on an analog tach.
Now the older GEN III engines used a 24x system....
If you set the HIRES and LORES to 3/3 you would get exactly 4 pulses per engine revolution, the world is happy.
3+3 = 6 total crank pulses to provide 1 ECM output pulse.
24/6 = 4 pulses per revolution
Thats is what I have learned. I wanted to truly understand this info as it was not clearly stated (for me) anywhere on the internet on what the number actually mean.
7/7 can be used for a 4 pulse tach but your tach will read 3.4% high. It could possibly respond faster then having your tach set in 4 cylinder mode.
--------------------------------------------------------------------------------
GEN IV E38 Tach Settings for aftermarket tach (autometer)
--------------------------------------------------------------------------------
Correct HPTuners tach settings for E38 ecm with 58x crank sensor.
Tach Output = Crank
Resolution - High =14
Resolution - Low =15
This will make the ECM output the correct frequency for a 2 pulse per revolution tach. The Duty cycle of the frequency will be 48.3%. 14/29
Tach output is on Pin 48 of the black connector.
You need to add a resistor (about 1k ohm) between the tach output and 12V to Pull-Up the Output signal to a usable voltage for aftermarket tachs.
-----------------------------------------------------------------------------------------
Here is how I came up with these numbers, for the people who care…
I have done testing to verify everything. This is what I have found to be true. I may have made errors but I do not think that I did.
-----------------------------------------------------------------------------------------
The tachometer I am using an Autometer Phantom II Part #7588.
You can configure the tach for
8 cyl mode (4 pulses per revolution)
6 cyl mode (3 pulses per revolution)
4 cyl mode (2 pulses per revolution)
4 cyl mode (1 pulses per revolution)
A 4 stroke 8 cylinder engine only fires 4 cylinders per engine revolution, 4 are on compression/firing stroke and 4 are on exhaust stroke. This is why tachs typically look for a 4 pulses per revolution.
-----------------------------------------------------------------------------------------
HP Tuners Settings
The tach output is OFF in the factory calibration.
The two options for tach output are frequency and crank. I do not know what frequency is, I tried it, it did not work for me, I got “crank” to work and I understand “crank” so I do not care about frequency.
The other variables are
Resolution - High = (number of periods the output stays high) I will call this HIRES
Resolution - Low = (number of periods the output stay low) I will call this LORES
These values need to be whole numbers.... The ECM can not handle decimal places.
Through some investigation I have found out that when "crank" is selected the ecm is going to output a pulse (or frequency, a pulse is a frequency) based on the 58x crank sensor. The minimum vales in HIRES/LORES are 1/1. This means for every 2 crank pulses you will get 1 tach output pulse from the ECM. The highest resoulution tach output signal is 29 pulses per revolution. 58/2
To verify this I entered 1/1 in the HIRES/LORES fields and hooked up my trusty Fluke to the tach signal at idle
670 rpm = 11.17 revolutions per second
11 revs per second * 29 pulses per rev = 324 pulses per second = 324 HZ
I verified 324 HZ with my fluke at 670 rpm.
I have heard that the numbers in the HIRES and LORES needed to be equal and I have also heard that they needed to be dividable by 60. I do not think either of those statements are true.
-----------------------------------------------------------------------------------------
An Autometer tach works off of frequency. Frequency can be measured as the time between the leading edges of 2 pulses. The tach does not really care what the duty cycle is as long as it is within an acceptable range.
I set my tach in 4 cylinder mode.
I set 15/15 in HIRES/LORES
At 3000 rpm I read 97 hz with a 50% duty cycle.
I set my tach in 4 cylinder mode.
I set 10/20 in HIRES/LORES
At 3000 rpm I read 97 hz with a 33% duty cycle.
The tach still read correctly even though the duty cycle was very different. The frequency remained the same.
Ok so now I have verified duty cycle can be changed and that crank is based on the 58 tooth crank sensor.
-----------------------------------------------------------------------------------------
The highest number of pulses my tach allows for 8 cylinder mode is 4 pulses per revolution. The higher the pulses per rev the faster the tach will respond. Higher = better.
58/4 pulses per rev =14.5 total crank pulses for each ECM tach output pulse.
14.5 = 7.25 and 7.25 in HIRES/LORES
These are not acceptable numbers. Must be a whole number CRAP……
If you use 7/7 your tach will be reading 3.4% too high.
It will read 2900 when your actual engine speed is 2800.
If you use 7/8 your tach will be reading 3.4% too low.
It will read 2900 when your actual engine speed is 3000.
I switched my tach to 4 cylinder mode. The tach is now looking for 2 pulses per engine revolution. 58/2 pulses per rev = 29 total crank pulses for each ECM tach output pulse.
29 = 14 and 15 in HIRES/LORES
If you use 15/15 your tach will be reading 3.4% too low.
It will read 2900 when your actual engine speed is 3000.
If you use 14/15 your tach will be reading 100% accurate.
I should add. Being 3% off on an analog tach is not a big deal. I tried to run my engine at 3000 rpm and compare my tach to the actual scanner readout. I saw a negligible difference even when the tach was off. Hopefully I helped someone understand what the signal is and how to TUNE it…. ( this can be useful is trying to create a signal to work with a non GM gauge cluster...) Settings of 15/15 will probably not be noticable especially on an analog tach.
Now the older GEN III engines used a 24x system....
If you set the HIRES and LORES to 3/3 you would get exactly 4 pulses per engine revolution, the world is happy.
3+3 = 6 total crank pulses to provide 1 ECM output pulse.
24/6 = 4 pulses per revolution
Thats is what I have learned. I wanted to truly understand this info as it was not clearly stated (for me) anywhere on the internet on what the number actually mean.
7/7 can be used for a 4 pulse tach but your tach will read 3.4% high. It could possibly respond faster then having your tach set in 4 cylinder mode.
