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#1 |
Senior Member
![]() Join Date: Nov 2007
Location: Greenville, SC
Posts: 873
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Conventional Tach on LS2
Hey guys,
I bought a cheap Sun Super Tach II to run on my LS2 swap using the tach wire from the ECM (E40). It would not move the needle. I did some research on the web and several people said that you need to put a "pull-up resistor" in the circuit. The conventional tachs are looking for a 12 volt signal with the speed pulse riding on it. The ECM evidently just sends the speed pulse out on the tach wire at low voltage. So I put a 1000 ohm resistor from the tach power wire to the tach signal wire going to the ECM. So that pulls the tach signal up to 12 volts. Works like a champ. TR
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1965 C10 SWB Fleet Two owner LS2 Swap Thread http://67-72chevytrucks.com/vboard/s...d.php?t=413880 |
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#2 |
Registered User
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Location: Alta Loma, CA
Posts: 610
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Re: Conventional Tach on LS2
Interesting...My Auto Meter tach had to be set for 4 cylinder mode to read properly.
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67 5.3L Build |
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#3 |
Registered User
![]() Join Date: Aug 2010
Location: Gonzales,La
Posts: 65
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Re: Conventional Tach on LS2
Works like a dream thanks for the info.
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#4 |
Registered User
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Location: Alta Loma, CA
Posts: 610
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Re: Conventional Tach on LS2
My tach reads correctly, but the needle vibrates an awful lot. Any ideas how to make it more stable?
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67 5.3L Build |
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#5 |
1965 Chevy C10, 2005 4.8L/4l60
![]() Join Date: Aug 2004
Location: DFW Texas
Posts: 8,543
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Re: Conventional Tach on LS2
"Interesting...My Auto Meter tach had to be set for 4 cylinder mode to read properly.
__________________" 67, that is quite common on the LS motors.
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Clyde65 Rebuild of Clyde http://67-72chevytrucks.com/vboard/s...84#post8338184 69 Aristocrat Lo Liner build http://67-72chevytrucks.com/vboard/s...84#post7561684 ![]() ![]() ![]() support our troops! Last edited by Clyde65; 02-22-2011 at 06:36 PM. |
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#6 |
Senior Member
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Location: Greenville, SC
Posts: 873
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Re: Conventional Tach on LS2
67, Are you using the ECM tach wire? or do you have it wired up to one of the coils. The signal from the ECM should be very clean, and the tach should work on the 8 cylinder setting.
My tach is very stable, no noise at all. TR
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1965 C10 SWB Fleet Two owner LS2 Swap Thread http://67-72chevytrucks.com/vboard/s...d.php?t=413880 |
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#7 |
Windy Corner of a Dirty Street
Join Date: Jun 2003
Location: Pueblo West, Colorado
Posts: 2,926
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Re: Conventional Tach on LS2
I have mine (8.1L on a P59 ECM) to the ECM tach wire but also had to set it in 4 cyl mode otherwise it was reading 50%. My tach reads smooth and just as accurate as the GM Tech II scan tool.
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Current vehicle collection: 1978 Chevrolet K10, 8.1L, NV4500, NP205 1989 Chevrolet Suburban, 8.1L, NV4500, NP241 1993 Chevrolet C1500 Sportside, TBI 7.4L, 4L60E 2001 Chevrolet K2500HD, Ext Cab, SWB, 8.1L, ZF 6 speed 2014 Chevrolet Impala LTZ 3.6L Vortec 8.1L because life is too short to tolerate underpowered vehicles
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#8 |
Senior Member
![]() Join Date: Nov 2007
Location: Greenville, SC
Posts: 873
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Re: Conventional Tach on LS2
Dang DirtyLarry, Now you've got me wondering. I will have to check what setting I am using to be sure.
TR
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1965 C10 SWB Fleet Two owner LS2 Swap Thread http://67-72chevytrucks.com/vboard/s...d.php?t=413880 |
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#9 | |
Windy Corner of a Dirty Street
Join Date: Jun 2003
Location: Pueblo West, Colorado
Posts: 2,926
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Re: Conventional Tach on LS2
Quote:
BTW….I love you truck! Man, your swap turned out great! ![]()
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Current vehicle collection: 1978 Chevrolet K10, 8.1L, NV4500, NP205 1989 Chevrolet Suburban, 8.1L, NV4500, NP241 1993 Chevrolet C1500 Sportside, TBI 7.4L, 4L60E 2001 Chevrolet K2500HD, Ext Cab, SWB, 8.1L, ZF 6 speed 2014 Chevrolet Impala LTZ 3.6L Vortec 8.1L because life is too short to tolerate underpowered vehicles
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#10 |
Registered User
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Location: Alta Loma, CA
Posts: 610
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Re: Conventional Tach on LS2
Yes, I am using the tach wire, but my harness was done by Speartech, so I'm not sure which wire it is. If everyone else is getting a smooth signal, maybe I'll contact John and see if he has any ideas.
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67 5.3L Build |
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#11 |
Senior Member
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Location: Greenville, SC
Posts: 873
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Re: Conventional Tach on LS2
DirtyLarry,
Thanks for the kind words. I love driving the truck with this swap. Yep, I am using the 4 cyl setting!! Sorry, I read the switch wrong. If I get a chance this weekend I will put a scope on that signal and see what it really looks like. 67, Maybe the signal is just marginal and the gage is dropping "tops" making the needle erratic. Does your harness use a pullup resistor? Maybe that is the issue. TR
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1965 C10 SWB Fleet Two owner LS2 Swap Thread http://67-72chevytrucks.com/vboard/s...d.php?t=413880 |
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#12 | |
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Location: Alta Loma, CA
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Re: Conventional Tach on LS2
Quote:
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67 5.3L Build |
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#13 |
BMW & ASE Master Certified
Join Date: Feb 2004
Location: Houston, Texas
Posts: 1,262
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Re: Conventional Tach on LS2
Ive got an autogauge (by autometer) and its set on 4cyl mode and it reads about 200 rpm more compared to HPtuners reading.
clint
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1987 Silverado - L83 5.3 w/ 6L80e ,A/C,P/W,P/L,TILT, HID projectors, Wilwood C-10 Pro Spindles w/ 2018 silverado front brakes & C-5 Corvette Rear Disc Brakes 1999 BMW 528i - 5.7 LS1 w/ 4l60e, Ford 8.8 IRS w31 spline posi & FX-r projector retrofit New project: 2006 BMW x5 6cyl AWD to L83 6l80e 4wd to 2WD / RWD A poor man buy's it twice finally got my domain name back, 87chevy.com.... site rework in progress |
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#14 |
Registered User
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Location: Alta Loma, CA
Posts: 610
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Re: Conventional Tach on LS2
Wow, that is some great info. Thanks for posting. I was just curious how you did this. Was it done with an oscilloscope?
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67 5.3L Build |
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#15 | |
Senior Member
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Location: Greenville, SC
Posts: 873
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Re: Conventional Tach on LS2
Quote:
I read up on your swap. Very nice work. Very nice truck. Let me know if you get your cruise control to work. I took the data on the tach line with a cheap USB data aquisition unit. http://www.mccdaq.com/usb-data-acqui...08-Series.aspx I have the 1208FS. It has an oscilloscope mode and a data logger mode. Very handy. TR
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1965 C10 SWB Fleet Two owner LS2 Swap Thread http://67-72chevytrucks.com/vboard/s...d.php?t=413880 |
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#16 | |
Registered User
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Location: Alta Loma, CA
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Re: Conventional Tach on LS2
Quote:
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67 5.3L Build |
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#17 | |
Senior Member
Join Date: Jul 2005
Location: Winters Ca. 95694
Posts: 4,843
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Re: Conventional Tach on LS2
Quote:
I just did an 05 5.3L swap and I am using the stock style 70 tach and it does not read at all from the ECM's tach wire. So if I install the same 1000 ohm resistor like you did it should all work our fine? Can I just put the resistor right at the back of the gauge under the spade connections? Is their a power and a signal side of the resistor? Thanks for any further help on this. Kevin LFD Inc. |
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#18 | |
Senior Member
Join Date: Jun 2007
Location: Pine Ridge Florida
Posts: 4,125
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Re: Conventional Tach on LS2
Quote:
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Click on my Not really a build thread, yet NOT SO RUST FREE RUST FREE BLAZER LS ENGINE SWAP INFORMATION THREAD BOLT RESTORATIONS 4l60 to an NP205 |
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#19 |
Senior Member
Join Date: Jul 2005
Location: Winters Ca. 95694
Posts: 4,843
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Re: Conventional Tach on LS2
Is this a radio shack resister or? Would you have a part number or description so I can tell them what I am needing? Now I do not have a program for my ECU as I had Street and performance do the flash, will a hand held programer do anything for this?
Kevin LFD Inc. |
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#20 |
Senior Member
![]() Join Date: Nov 2007
Location: Greenville, SC
Posts: 873
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Re: Conventional Tach on LS2
Rokcrln,
The current draw is very low so a common 1/4 watt 1000 ohm resistor from Radio Shack will work fine. Should be less than $1. There is no + or - to the resistor, mount it either way. It just needs to go across the power and signal wires. Shouldn't matter where. Like Thealien said. With a stock tach from a V8 you will probably need to change the tach calibration with tuner software. TR
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1965 C10 SWB Fleet Two owner LS2 Swap Thread http://67-72chevytrucks.com/vboard/s...d.php?t=413880 Last edited by TR65; 03-27-2011 at 10:36 PM. |
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#21 |
Future TOTM Winner
Join Date: Aug 2006
Location: Bulverde, TX
Posts: 2,692
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Re: Conventional Tach on LS2
This FAQ has been up for a while, but for the sake of information for the masses I felt the need to add to it.
With EFI Live I simply tell the ECM to "pull up" the output of the factory harness wire. Also I correct the pulse calculation so that the wire puts out the correct signal for a standard 8 cyl tach. The reason that people are having a variety of things happen here is most likely due to how the person that flashed the computer set up (or didn't set up) the tach signal.
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#22 |
Registered User
Join Date: Nov 2010
Location: Alabama
Posts: 739
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Re: Conventional Tach on LS2
Critter, when the computer is flashed, who would you recommend to have it done correctly?
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#23 | |
Future TOTM Winner
Join Date: Aug 2006
Location: Bulverde, TX
Posts: 2,692
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Re: Conventional Tach on LS2
Quote:
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72 Chevy CST K5 HIGHLANDER BLAZER Getting a 6.0,NV4500,NP205,14BFF (Currently laying all over the place in little pieces) 72 Chevy swb step-side "LS" 6.0 Here's a build thread of sorts 2002 HD2500 Crew Cab, 8.1L Allison 4x4 Daily Driver Check it out www.lsdyno.com |
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#24 |
Registered User
Join Date: Sep 2009
Location: ottawa,canada
Posts: 4,550
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Re: Conventional Tach on LS2
I have an autometer elec tach and it's reading about 1000rpm too low....psi harness.
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#25 |
Registered User
Join Date: Jun 2012
Location: Pittsgrove, NJ
Posts: 1,502
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Re: Conventional Tach on LS2
Bringing up a slightly old thread. Can't seem to get my autometer tach to read right. Ive tried connecting the signal in all 4 modes my tach is capable of and it doesnt want to read at all. It just sits at about 3k and if i really get into it it will go up to 4k or so. Ive tried the 1/4w 680 ohm resistor mod from the lt1 site but that didnt change anything either. The pcm is a 2003 and hasn't been tuned at all yet. Should the tuner be able to change the output in the pcm to fix the signal to the tach and get it working, or should I keep trying to fix it before bringing it over there?
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