Using stock tach with PCM signal
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From: Gig Harbor, Wa
Using stock tach with PCM signal I currently have a cable operated tach but can easily swap it out for a mid 70's electric version. I know the PCM has a 4 cylinder signal and only 5V. My PCM has not been reprogrammed. What are my options for getting the 12V 8 cylinder tach to work with the PCM signal?
Joined: Nov 2001
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From: Anderson, Indiana
I bet you could use a Cable-X box and keep your original tach!
Otherwise the easiest way to go with the later tach would be the Dakota Digital tach adapter (SGI-8 I think). That will adjust for both the 4 > 8 cylinder as well as the voltage difference. Those sometimes cause a "twitchy needle syndrome" on some tachs though. Sometimes the voltage difference isn't a big deal and your 70's tach may work ok with the low voltage square wave of the LS1. Then you could just calibrate it with LS1 Edit or one of the other softwares.
Or you can look at the circuit on your mid 70's tach and figure out which resistor calibrates it and play with some others and get it reading right.
Otherwise the easiest way to go with the later tach would be the Dakota Digital tach adapter (SGI-8 I think). That will adjust for both the 4 > 8 cylinder as well as the voltage difference. Those sometimes cause a "twitchy needle syndrome" on some tachs though. Sometimes the voltage difference isn't a big deal and your 70's tach may work ok with the low voltage square wave of the LS1. Then you could just calibrate it with LS1 Edit or one of the other softwares.
Or you can look at the circuit on your mid 70's tach and figure out which resistor calibrates it and play with some others and get it reading right.
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91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
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91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
www.speartech.com
Thread Starter
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From: Gig Harbor, Wa
Originally Posted by Speartech
I bet you could use a Cable-X box and keep your original tach!
I'll have to look into the tach adapter.
Thanks
Joined: Sep 2004
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From: Downers Grove, IL
You can just use the tach signal and reprogram the computer so that it puts out an 8cyl signal. My tach works almost perfectly like that, it reads great except that it was recalibrated the wron direction, it now reads 1/4 RPM 
It shouldn`t be a difficult fix, but I have not had time to make it back out to indiana yet. I know that EFI Live 7 and HP Tuners have the capability to reset the tach signal but I don`t know if any of the other programs can.
It is a 5v signal but I have heard of a stepup circuit if the voltage is too low for your tach to work properly. I`ll find it and link it over here.
It shouldn`t be a difficult fix, but I have not had time to make it back out to indiana yet. I know that EFI Live 7 and HP Tuners have the capability to reset the tach signal but I don`t know if any of the other programs can.It is a 5v signal but I have heard of a stepup circuit if the voltage is too low for your tach to work properly. I`ll find it and link it over here.
Joined: Sep 2004
Posts: 3,148
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From: Downers Grove, IL
