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

Re: Conventional Tach on LS2
 

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

Re: Conventional Tach on LS2
 

Works like a dream thanks for the info.
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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?

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, that is quite common on the LS motors.

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

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.

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

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

Re: Conventional Tach on LS2
 

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!

http://farm6.static.flickr.com/5254/...2b9c827e_z.jpg

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.

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

Re: Conventional Tach on LS2
 

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.

Re: Conventional Tach on LS2
 

I took a look at the tach signal that the ECM is putting out on the tach wire. This is for the E40 ECM so I don't know about other ECMs. The tach wire is used to drive the OE tach in the vehicle.

The ECM puts out a low voltage square wave that is two cycles or "tops" per engine revolution. That is the same as the number of times a 4 cylinder fires per revolution. So that is why we have to use the 4 cylinder setting to get the correct RPM. (V8 fires 4 times per rev.)

The first picture is the signal from the ECM. So the (-1.5 V) voltage is probably too low to fire the traditional tachs that are looking for around 12 volts.

The second picture is the signal with the pullup resistor installed. So now the voltage is up to 10V at the high. So the tach wroks fine.

The engine was idling at about 680 RPM or 11.3 rev per second. The frequency of the signal is twice that, around 22 cycles per second.

TR

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?

Re: Conventional Tach on LS2
 

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

Re: Conventional Tach on LS2
 

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.

Re: Conventional Tach on LS2
 

TR65,


I believe the 1.4 V signal was not from the ECM. The MCCDAQ USB1208FS has a low level output signal at 1.4V. Essentially you were using the DAQ as a pullup for the ECM Hall Effect Signal.

I made the same mistake, thinking my O2 sensor was outputting 1.4V. 100$ sensor later, I have confirmation from MCC that Single Ended readings on the DAQ output a 1.4V signal. This can be confirmed by reading the pins directly on the DAQ with a DMM. Mine reads 1.4VDC, 11 micro Amps.

I found this stray voltage does not happen when you use differential measurements.

Typically Hall Effect sensors ground when the magnet is lined up. If you aren't getting a reading, check the power being applied (pullup circuit). There may be a blown fuse or loose connection

Carl

Re: Conventional Tach on LS2
 

Carl,

1. I am not reading the sensors. I am reading the tach signal output from the ECM. Neither of the engine rotation sensors would be suitable for a tach. The crank position sensor is 24 tops per rev and the different tooth lengths give a non constant frequency. The cam position sensor is one cycle per two revs.

2. I was using a single end connection. I went back and measured using a differential setup as you suggested. There is then no signal recorded from the ECM.

So, The ECM is grounding the tach signal line at a frequency of 2 times per engine revolution. So to get any signal at all the tach has to supply a voltage on the signal line. With the pullup resistor you then get a 0-12 volt square wave that the tach likes. (As in the graph above just shifted up)

Thanks for the tip.

TR

PS

I found this recording of the two engine speed sensors.

http://www.ls1tech.com/forums/pcm-di...going-bad.html

Re: Conventional Tach on LS2
 

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.

Re: Conventional Tach on LS2
 

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.

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.

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

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.

Re: Conventional Tach on LS2
 

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