Tach signal using 5V Ref and 10k resistor???

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Dec 12, 2011 | 09:09 PM

Dave-1970

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Tach signal using 5V Ref and 10k resistor???

2006 LQ9 into 1970 Chevelle...Autometer tach isn't getting a signal from the PCM. I called autometer and the tech told me that I could use their Tach Adapter #9117 "OR" wire in a 10k resistor onto the tach signal wire from PCM and wire a 5V reference from PCM on the other end of the resistor.... Does choice 2 sound like it would work? Really don't want to damage my PCM... I'm not super versed in electronics though I managed to create a nice looking stand-alone harness that works set aside from the tach. Any help is greatly appreciated. Thanks

Dec 13, 2011 | 02:25 AM

garys 68

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From: Camdenton, MO

Pretty common problem.
Do a seach for "tachometer resistor". I used a 680 ohm, works fine.

Dec 13, 2011 | 10:51 PM

Dave-1970

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Yeah, I've searched... I've read of plenty of people using a 12v ignition source and a "pull-up" resistor... My only worry is putting too much current back into the pcm...Maybe I worry too much... Anyway, AutoMeter told me to use a 5v reference wire from the pcm (instead of 12v ignition) and a 10k resistor... I have not seen anybody take this route using a 5v ref pcm wire. Anyone think this would work?

Dec 14, 2011 | 11:59 AM

Old Geezer

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The 06 PCM should have a vss output. I ran that wire to the sig in post on the Autometer, and then jumped the 12v power post to it with the resistor that Gary mentioned. Works like a charm.[DON'T put the resistor in backwards! radio Shack has them for about no money for a pack.]

Dec 14, 2011 | 03:38 PM

gMAG

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From: CT

http://www.buyrcars.com/jag.htm
scroll down about 80%.

Dec 14, 2011 | 05:58 PM

Haggar

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Quote:

Originally Posted by Dave-1970

Well, if it makes you feel better, in the factory applications that GM does use a discrete tach signal to the cluster, we use a 680 ohm resistor (on the instrument cluster circuit board) pulled to switched battery signal, just like people are suggesting. Most GM applications drive the tach via the GM LAN bus, though. With a 680 ohm resistor to battery, the current is about 20 milliamps, which is pretty minor.

I designed the cluster electronics in the new Camaro, I had to design in the tach input circuit, but its not used in production.

Now, if autometer is saying to use 5v, then, that's probably what I'd use. It all depends on their circuit, what it can handle. Its just counting pulses from high to low. The resistor is what pulls the line high, and the PCM is pulsing it low.

Dec 27, 2011 | 12:57 AM

ShredSled

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so where are you all hooking up to a fuse protected 12v keyed power source?

Then once that is found, all you do is connect that to a 680ohm resistor from radio shack, then splice it onto the tach signal wire going from pin 10 back to the tach, and then we're good???