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).

 

OOps wrong section, can i have this moved please?

 

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.
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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.
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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.
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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.
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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.

 

kmkommes, Nice write-up on this. Thanks for posting.