Tach issues

[XLexusTech]

HI I have an LS3 PSI harness and an auto-meter electronic tach. The tach is a special addition 2006 Gm Performance parts issue that I need to keep.
I mention this because tachs pre 2007 are known to have a problem with low voltage signals.

Tach is inoperative but is fine on a bench test. Found lots of info about using a step up resistor inline. Then found so much conflicting info.. Mus use 12v.. not must use 5V... must use 1Kohm.. no us 5K ohm. that I got really worried.

So I went out and bought autometer 5117 https://www.autometer.com/tach-adapter.html?gclid=EAIaIQobChMInYW48L-n8gIVysizCh0U4Q7nEAQYAyABEgIJqPD_BwE&gclsrc=aw.ds

question has used this tach adapter? Instructions say tap into each coil bank.. which I am not doing..
I want to use the tach signal (white wire) out of PSI harness

Alternatively If someone could set me straight on the step up resistor that would be helpful as well

[dhutton]

It’s a pull-up resistor. Try 4.7k ohms to 12 volts. If that doesn’t work then try 1k ohms to 12 volts.

If the pull-up doesn’t work then Dakota Digital has a tach interface. I’ve got one of their old versions sitting on a shelf. Sgi-8e. It worked for me on an early build. But they have a new one that includes a speedometer adapter too.

That Autometer box looks like a kludge to me. I wouldn’t mess with it unless I had exhausted all other options.

Don

[dhutton]

Does your tach have the ability to change the number of cylinders? If not you may have it reading off by a factor of 2. The older LS engines required that you select the 4 cylinder mode on the tach. I can’t say for sure if that is the case with the LS3.

If you do have that problem let me know and I’ll send you my Dakota Digital box since it will let you modify the tach output to drive an 8 cylinder tach.

Don

[XLexusTech]

Does your tach have the ability to change the number of cylinders? If not you may have it reading off by a factor of 2. The older LS engines required that you select the 4 cylinder mode on the tach. I can’t say for sure if that is the case with the LS3.

If you do have that problem let me know and I’ll send you my Dakota Digital box since it will let you modify the tach output to drive an 8 cylinder tach.

Don

It does per the directions I have it set to 4 Cyl Dual pulse. (left toggle up Right toggle down)

[dhutton]

It does per the directions I have it set to 4 Cyl Dual pulse. (left toggle up Right toggle down)

OK so try the pull-up resistor.

Don

[XLexusTech]

OK so try the pull-up resistor.

Don

Will do.. so to be clear.. take 12v ignition on power lead and connect it to resistor then connect the other end of the resistor to the the 5V output form the ECM the then connects to signal connection of the tach?

[dhutton]

Will do.. so to be clear.. take 12v ignition on power lead and connect it to resistor then connect the other end of the resistor to the the 5V output form the ECM the then connects to signal connection of the tach?
Yes.

Don

[XLexusTech]

Worked like a charm, 1K Ohm resistor to sep up the signal. Returned the autometer adapter and I am good to go.