Conventional Tach on LS2

[TR65]

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

[67C10BUB]

Interesting...My Auto Meter tach had to be set for 4 cylinder mode to read properly.

[1tonjimmy]

Works like a dream thanks for the info.
Posted via Mobile Device

[67C10BUB]

My tach reads correctly, but the needle vibrates an awful lot. Any ideas how to make it more stable?

[Clyde65]

"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.

[TR65]

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

[DirtyLarry]

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.

[TR65]

Dang DirtyLarry, Now you've got me wondering. I will have to check what setting I am using to be sure.

TR

[DirtyLarry]

Originally Posted by TR65

Dang DirtyLarry, Now you've got me wondering. I will have to check what setting I am using to be sure.

TR

I am even running the same Tach as you but an older version. I wonder if there is any difference between E40 and P50 ECM’s with respect to the Tach operation? Hmmm.

BTW….I love you truck! Man, your swap turned out great!

[67C10BUB]

Originally Posted by TR65

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

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.

[TR65]

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

[67C10BUB]

Originally Posted by TR65

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

I honestly have no idea how Speartech handles the tach output wiring. They do not include schematics with their harnesses. Perhaps I will forward your suggestion on to John (Speartech) and see what he thinks. Thanks for the input.

[87chevy.com]

Ive got an autogauge (by autometer) and its set on 4cyl mode and it reads about 200 rpm more compared to HPtuners reading.
clint

[67C10BUB]

Wow, that is some great info. Thanks for posting. I was just curious how you did this. Was it done with an oscilloscope?

[TR65]

Originally Posted by 67C10BUB

Wow, that is some great info. Thanks for posting. I was just curious how you did this. Was it done with an oscilloscope?

Thanks 67.

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

[67C10BUB]

Originally Posted by TR65

Thanks 67.

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

Very interesting stuff. A little over my head, but interesting. You are obviously a very valuable member of this LSX swap community with knowledge like that.

[Rokcrln]

Originally Posted by TR65

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

TR,
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.

[Thealien]

Originally Posted by Rokcrln

TR,
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.

I am using the factory Tach in my 1972 Jimmy with my 5.3 swap. I needed the pull up resistor and did install it right between the wires at the connector that plugs into my Tach. My tuner had to change the output in EFI live (think the same can be done in HP tuners) as our Tachs don't have a 4/8 cylinder switch. Mine is very close to the readings in EFI live and an aeroforce guage I use.

[Rokcrln]

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.

[TR65]

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

[Critter]

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.

[1972K20]

Critter, when the computer is flashed, who would you recommend to have it done correctly?

[Critter]

Originally Posted by 1972K20

Critter, when the computer is flashed, who would you recommend to have it done correctly?

Me!

[mooseknuckles]

I have an autometer elec tach and it's reading about 1000rpm too low....psi harness.

[Billett]

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?