Thoughts on factory replacement tachometer

[chev-obsession]

Hey Guys, I am in the process of rebuilding my gauge cluster for my ls swap. I have always liked the factory tach look. Mostly because it maintains factory looks and style, and seems to even the dash out and not have a blank spot front and center. However I've also heard they are more of an ornament and not very accurate.

So my questions:

Is it worth it to put in a factory replacement tach while I have the cluster apart to use the factory style tach to read engine rpm accuratly?

Will the factory style work with the ls engine since the ls engine is read digitaly?

Guys that have installed the factory style tach with an LS motor suggest going with a factory style tach or use something like an autometer?

[tj1970cst]

My factory tach seems to work well. I have never checked it against anything. I agree I prefer the look. Dunno about LS. I'd bet there is a simple solution already figured out. That's a popular swap.

[TBONE1964]

If you are running an LS with a factory tach, the tach has to be converted to run as a 4 cyl tach.

I can make this happen if interested. Send me a PM.

Thank you,
Tom

[Richard]

Should be able to use the Dakota digital interface https://www.dakotadigital.com/index....rod/prd127.htm You can also get an interface that has the tach interface and allows modification of the VSS signal for the speedo if you are running a cable drive or a have a speedo that requires a different input signal than what the ecm puts out.

[LockDoc]

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On my '67 Panel Truck with a stock tach dash & 2001 LS1 with the 411 PCM I just hooked the tach wire from the PCM to the tach output post on the back of the tach and changed the PCM resolution from 6 to 3 in the PCM tach settings. (that changes it from 8 cyl to 4 cyl) Seems to work OK.... No resistor needed and no changes to the tach.

LockDoc



Here is what lsnova said in another post:

Depends on what year your PCM is, but generally the 2003 and newer will need the pull up resistor. Also you'll have to change the tach resolution from 6 to 3 in the tune to make it read correctly since LS motors use a 4 cylinder signal, otherwise it will read half of what it's running (on a V8 tach). Other than that, just hook up the ignition wire like it was originally and hook the signal wire, the one that went to the coil, to the tach output out of the PCM, generally a white wire at C2 10, but that may depend on year and application.

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I can't remember who posted this.

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 in the PCM to make the tach read correctly.
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[custom10nut]

All this Hubbub about factory Tachos being “ Terribly Inaccurate” is BS.
The two that I’ve had (old and replacement) have both been within 25 RPM.
If you’re racing? Go aftermarket. If not enjoy the factory stuff.
How accurate are these “Factory” Tachs in the Muscle cars?

[chev-obsession]

Originally Posted by custom10nut

All this Hubbub about factory Tachos being “ Terribly Inaccurate” is BS.
The two that I’ve had (old and replacement) have both been within 25 RPM.
If you’re racing? Go aftermarket. If not enjoy the factory stuff.
How accurate are these “Factory” Tachs in the Muscle cars?

Precisely the type of answer I was looking looking for.... wanting to know if they are accurate, if not I would look at an auto meter. Trying to keep things looking stock.

[tj1970cst]

Even if you were racing, who cares about up to 100rpm error? The only use for aftermarket would be recall or shift light.


I'd bet mine is accurate within 50 or 25, based on sound.

[custom10nut]

Originally Posted by tj1970cst

Even if you were racing, who cares about up to 100rpm error? The only use for aftermarket would be recall or shift light.


I'd bet mine is accurate within 50 or 25, based on sound.

Agreed. I’ve calibrated both of mine with my multimeter, and the tach stays pretty true all the way up through the range. Mind you, My engine is pretty much stock (warmed up a little), and I run a Points Distributor, with a Mallory Electronic conversion. I’ve not had a single problem in 20+ years