Autometer tach and a pull-up resistor?

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Apr 1, 2008 | 11:19 PM

jw33

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From: McKinney, TX.

Autometer tach and a pull-up resistor?

So my Autometer tach is not working and I have the tach signal coming in from my bulkhead connector. I am using one of the new GM swap harness kits and it says,

"A tachometer signal is included in the bulkhead connector. This is a 2 pulse/rev output which may correspond to a 4-cylinder setup in some tachometers. Note the signal is a low voltage square wave and some older style of tachometers may need a pull-up resistor in order to read the signal."

I'm about 99.9% sure I have the tach wired up correctly, but I get nothing no matter if it's in 4, 6, 8 cylinder mode. This is a brand new tach also. It says on Autometer's website that the tach uses a 12v square wave and I have no idea what GM's "low voltage square wave" is.

If I need to use a pull-up resistor I am assuming I will attach one side of the resistor to my switched/ignition 12v source and the other side of the resistor will got to my tach signal wire before it goes into the tach? How do I determine the resistor size? I have a scope I can see the signal with if I need to.

Apr 1, 2008 | 11:51 PM

MEFISTUFF

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We use a 680 ohm, 1/2 watt resistor for the pull up. you are correct on how to connect it up. steve

Apr 2, 2008 | 07:16 PM

68shortstep

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From: tulsa OK

i used a 1000ohm, 1/2 watt resister..... the 680ohm one was a little bouncy below 1000RPM the 1000ohm one made it better...

Apr 2, 2008 | 10:15 PM

jw33

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Quote:

Originally Posted by 68shortstep

i used a 1000ohm, 1/2 watt resister..... the 680ohm one was a little bouncy below 1000RPM the 1000ohm one made it better...

ha, I just got done messing around with it using a 680 ohm resistor and I was going to ask if there was a way to smooth the needle out at below 1500 rpms. Thanks....