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Old 11-23-2004, 01:45 PM   #21
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And also... there is some SERIOUS misinformation in this thread (now that I've read the whole thing).

THe voltage pulses up to 9v, (at least on Parish8s truck), the reason you saw 4.5 volts might have been because the engine was running? We logged NO voltage with engine off, average voltage 4.5 when cranking, logged pulses to 9v. People, this is not "a digital signal" or "multiplexed data." There is no DAC for the factory tach, either. All the MSD tach adapter is doing is creating a pullup circuit - you can either pay $80 for this, or spend less than a dollar at radio shack and create a pullup circuit yourself.

So, take a 12v source, put a 13,000 ohm resistor on the end of it, then combine that with the tach source into your wideband, datalogger, megasquirt, whatever - and you will have the 12v square wave pulse you seek.

Different ignitions use different voltages for triggering. Some are as little as .005 volts, many are 5volts, some pulse to 9 volts, others are at 12v. The pullup circuit steps this voltage up to what's needed to trigger.

Finally, the "frequency" that's measured is the frequency of the voltage reaching it's peak. How often the voltages pulses to it's highest point is the "frequency" that's measured.

GM kept it simpler than most people think.


-scott

Last edited by dieselgeek; 11-23-2004 at 01:54 PM.
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Old 11-25-2004, 06:29 PM   #22
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geek and i got tired of not knowing for sure what was going on with that tach signal so i got ahold of a scope. on my truck with nothing else hooked to the wire other than the dash there is a 9v square wave signal. when we hooked the tach wire from the stand alone we are working with it droped the voltage down to under 2 volts and niether the tach or stand alone worked.

we added a "pull up circuit" wich is nothing more than a 12v source thru a resistor tied into the tach wire. we started droping resistance and at about 13k the stand alone started working, it was still down around 2 volts. we kept droping the resistance and got down to 380ohms, at that point we saw a 9 volt signal again and the tach started working. we have the stand alone running the fuel on my truck and are up to 10psi and over 6000rpm with no problems.
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Old 07-25-2007, 05:50 PM   #23
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Quote:
Originally Posted by dieselgeek
So, take a 12v source, put a 13,000 ohm resistor on the end of it, then combine that with the tach source into your wideband, datalogger, megasquirt, whatever - and you will have the 12v square wave pulse you seek.

-scott
What he states about dac adc and mutliplex etc is true. But in my car I would not just put in a "Pull up circuit as he defines it" That current gets sinks to the pcm with out knowing the design of the pcm future failure can occur. Use a 7406 as a buffer then use a 100k resistor connected to 12V for the output to the aftermarket tach. If necessary you can use 2 more 100k resistors to cut the output wave from the pcm in half to not burn up the 7406. If that becomes a problem for you. You will need to use 2 inverters on the 7406. To put the signal back in phase.

As for adding an external pull up as he states. Usually pcm's will have pullups in them at a relatively high value. By adding another circuit your lowering the resistance of the internal pull up. If they designed it that route which can lead to excessive current being sunk in the pcm for that output during a low output phase.

As for people with multimeters readings. Yes your reading will sit in one spot as the multimeter only samples low freq. Also the Multimeter reads the AVERAGE dc value. This would mean the if the output is a 50% duty cycle wave you will read 1/2 the peak on a dmm for a given period. I am not going to discuss how avg is achieved in a dmm.

As for the data logger guys. Wide band o2's are sampled at a relatively slow rate I would be surprised if they were even doing 500 samples per second. You will not be able to capture a signal that moves as quick as a tach signal. Thats why your missing most of the story the read out says. The signal condition in the data logger was more than likely set up for wide bands. You need an oscilloscope to see the signal correctly.

Last edited by shotty128; 08-05-2007 at 06:45 PM.
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Old 08-04-2007, 07:41 PM   #24
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Why use the P-tach adapter/Buffer.

https://ls1tech.com/forums/attachmen...d=102270&stc=1

Because the stealership is waiting for you to just add a resistor and risk burning up your ECM.

What this Buffer does. It sits between the ecm and old school tach that usually hooks to coils. By sitting between the ecm and tachometer, it protects your ecm from higher current levels through the just add a resistor mod. When you add a resistor to the wire, the ecm now has to sink more current in order to pull the line to 0Vdc. The 7406 has a fairly high impedance input.

Input what da heck you ask, simply it means this circuit causes the ecm to use the same amount of current or less at the white wire (tach signal).
When compared to being wired up to a stock dash.

This means that the ecm will not have to dissipate more power at 0Vdc out. You also will not be flowing current in to the 9V source in the ecm when its high. Because with the resistor if the guy that said its a 9V square out is right, which from what others state sounds true. Then your resistor will be supplying a higher voltage than normal when the signal is high causing current to flow in the ecm when the signal is high (meaning current should be flowing out). This is risky.

I built and tested this on a 2006 cadilac CTS-V ecm wired to an ls7. While attaching it to a summit gauge set link of them below.

http://store.summitracing.com/partde...5&autoview=sku

I give permission to every body to rip this off and sell it for $70+ a pop. Maybe call it a tach signal conditioner or adapter

I built this one on a radio schack PC Board part number 276-170 and then cut the board to size. The capacitor value is not that important it can be just about anything.

You are looking at about $8 worth of parts.

1. $0.25 1/4watt 560 ohm resistor green blue brown is what i used value can beanywhere between 500-700ohms
$0.25 1/4watt 100k ohm resistor brown black yellow
2. 7406 $1
3. 7805 $1
4. pc board $3.50

If you wish you can use a voltage divider to further condition the signal before the 7406 but it is not necessary.

"Powered by BOB" -champ car '05 Portland
"F-ing eddie" - champ car '05
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Who can explain how the tach signal wire works?-tach_adapter__2blazerss.gif  

Last edited by shotty128; 08-05-2007 at 06:46 PM.
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Old 08-05-2007, 01:51 AM   #25
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for the most part i have no idea what you are talking about but i can say i ran the basic pull up circuit described above for 1.5years and had no problems with the stock ecm.

the newer megasquirt(ms1 v3 board) has some kind of vr input with "opto isolator". it also has 2 adjustments right on the board that help to filter out any interferance. that is what i have been running for awhile now and it does not require a pull up circuit.
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Old 08-05-2007, 06:43 PM   #26
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Quote:
Originally Posted by parish8
for the most part i have no idea what you are talking about but i can say i ran the basic pull up circuit described above for 1.5years and had no problems with the stock ecm.

the newer megasquirt(ms1 v3 board) has some kind of vr input with "opto isolator". it also has 2 adjustments right on the board that help to filter out any interferance. that is what i have been running for awhile now and it does not require a pull up circuit.

Some things will work for a group of people but can damage stuff for others.
Different people put things in different conditions. I have seen people mount ecm's under the hood. That is a condition where the pull up circuit would be very risky.

The risk is there and there is no way around it with the just add a resistor
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Old 08-21-2007, 04:17 PM   #27
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I disagree with a pullup circuit causing damage. The processor in the LSx computer even has an option to ENABLE a built-in pullup circuit onboard. This can be found in HPT I believe. MSD products use pullup circuits all the time... I definitely understand concern for proper approach to electronics, but I think in this case, the concern above is unwarranted.
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Old 12-17-2009, 02:19 PM   #28
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I'm fairly certain I just burnt up my ECM using the pull-up circuit using a 1k ohm resistor.

After talking to a friend who is a Electrical Engineer, he wasn't surprised when I showed him the wiring diagram for the pull up circuit.

I'm now going to build the circuit as Shotty 128 posted.
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Old 12-17-2009, 02:21 PM   #29
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Quote:
Originally Posted by Ackattack1 View Post
I'm fairly certain I just burnt up my ECM using the pull-up circuit using a 1k ohm resistor.

After talking to a friend who is a Electrical Engineer, he wasn't surprised when I showed him the wiring diagram for the pull up circuit.

I'm now going to build the circuit as Shotty 128 posted.
I doubt it.

Many of the GM ECU's offer "pullup circuit" as an internal option, we found that much later than this original post.
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Old 12-18-2009, 02:42 PM   #30
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Quote:
Originally Posted by dieselgeek View Post
I doubt it.

Many of the GM ECU's offer "pullup circuit" as an internal option, we found that much later than this original post.

please explain.....I'm using an '05 Yukon ECU
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Old 05-25-2010, 07:16 AM   #31
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LS6 Miata 1999 installation. I tried the 13k pullup method and was able to make the tach work, but it was irratic in large rpm swings and it made the speedo move a little also during large rev swings.

The circuit board conditioner worked very well and the tach operates normally. Thanks for the great thread and the wiring instructions.

More details on my LS6 miata here.
Best,
Mike
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Old 09-29-2011, 01:53 PM   #32
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Quote:
Originally Posted by shotty128 View Post
Why use the P-tach adapter/Buffer.

https://ls1tech.com/forums/attachmen...d=102270&stc=1

Because the stealership is waiting for you to just add a resistor and risk burning up your ECM.

What this Buffer does. It sits between the ecm and old school tach that usually hooks to coils. By sitting between the ecm and tachometer, it protects your ecm from higher current levels through the just add a resistor mod. When you add a resistor to the wire, the ecm now has to sink more current in order to pull the line to 0Vdc. The 7406 has a fairly high impedance input.

Input what da heck you ask, simply it means this circuit causes the ecm to use the same amount of current or less at the white wire (tach signal).
When compared to being wired up to a stock dash.

This means that the ecm will not have to dissipate more power at 0Vdc out. You also will not be flowing current in to the 9V source in the ecm when its high. Because with the resistor if the guy that said its a 9V square out is right, which from what others state sounds true. Then your resistor will be supplying a higher voltage than normal when the signal is high causing current to flow in the ecm when the signal is high (meaning current should be flowing out). This is risky.

I built and tested this on a 2006 cadilac CTS-V ecm wired to an ls7. While attaching it to a summit gauge set link of them below.

http://store.summitracing.com/partde...5&autoview=sku

I give permission to every body to rip this off and sell it for $70+ a pop. Maybe call it a tach signal conditioner or adapter

I built this one on a radio schack PC Board part number 276-170 and then cut the board to size. The capacitor value is not that important it can be just about anything.

You are looking at about $8 worth of parts.

1. $0.25 1/4watt 560 ohm resistor green blue brown is what i used value can beanywhere between 500-700ohms
$0.25 1/4watt 100k ohm resistor brown black yellow
2. 7406 $1
3. 7805 $1
4. pc board $3.50

If you wish you can use a voltage divider to further condition the signal before the 7406 but it is not necessary.

"Powered by BOB" -champ car '05 Portland
"F-ing eddie" - champ car '05
Has anybody used this workaround for the early 70's SS tachs and does the signal still need to be reprogrammed for a four cylinder output from the ECM or can that be handled with this setup as referenced-but not fully expalained- by further "conditioning" the signal?

I am about to install a 1972 SS tach gauge setup with a 2000 LS1 ECM setup. Any help would be appreciated.
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Old 10-06-2011, 08:38 PM   #33
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ok... obviously some knowledgeable people in here...
Can anyone here take pity and help out an electrical idiot hook up a simple autometer minimonster tach in an '01 Silverado?
I tried attaching it to what I thought was the trucks tach signal wire, but it does not register any rpm reading at all. Even tried setting up the tach for 4cyl signal...? It powers up, but no tach signal....?

What do I need to do? Please realize I do not understand much about technical terms...

Any help is greatly appreciated, thanks!
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Old 02-03-2013, 02:40 PM   #34
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Quote:
Originally Posted by shotty128 View Post
Why use the P-tach adapter/Buffer.

https://ls1tech.com/forums/attachmen...d=102270&stc=1

Because the stealership is waiting for you to just add a resistor and risk burning up your ECM.

What this Buffer does. It sits between the ecm and old school tach that usually hooks to coils. By sitting between the ecm and tachometer, it protects your ecm from higher current levels through the just add a resistor mod. When you add a resistor to the wire, the ecm now has to sink more current in order to pull the line to 0Vdc. The 7406 has a fairly high impedance input.

Input what da heck you ask, simply it means this circuit causes the ecm to use the same amount of current or less at the white wire (tach signal).
When compared to being wired up to a stock dash.

This means that the ecm will not have to dissipate more power at 0Vdc out. You also will not be flowing current in to the 9V source in the ecm when its high. Because with the resistor if the guy that said its a 9V square out is right, which from what others state sounds true. Then your resistor will be supplying a higher voltage than normal when the signal is high causing current to flow in the ecm when the signal is high (meaning current should be flowing out). This is risky.

I built and tested this on a 2006 cadilac CTS-V ecm wired to an ls7. While attaching it to a summit gauge set link of them below.

http://store.summitracing.com/partde...5&autoview=sku

I give permission to every body to rip this off and sell it for $70+ a pop. Maybe call it a tach signal conditioner or adapter

I built this one on a radio schack PC Board part number 276-170 and then cut the board to size. The capacitor value is not that important it can be just about anything.

You are looking at about $8 worth of parts.

1. $0.25 1/4watt 560 ohm resistor green blue brown is what i used value can beanywhere between 500-700ohms
$0.25 1/4watt 100k ohm resistor brown black yellow
2. 7406 $1
3. 7805 $1
4. pc board $3.50

If you wish you can use a voltage divider to further condition the signal before the 7406 but it is not necessary.

"Powered by BOB" -champ car '05 Portland
"F-ing eddie" - champ car '05
This is cool tech and I am thinking about building this. Here is the 7805:

http://www.radioshack.com/product/in...ductId=2062599

But 7406 is no longer a good Radioshack PN. Which one of these would be best?

http://www.radioshack.com/search/ind...+inverter&sr=1
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Old 02-04-2013, 10:51 AM   #35
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Digikey and Mouser both carry the Texas Instruments 7406.
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Old 02-04-2013, 12:45 PM   #36
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Quote:
Originally Posted by usdmholden View Post
Digikey and Mouser both carry the Texas Instruments 7406.
They have several different version also. I'm sorry, I'm not familiar with this kind of stuff ...

http://www.mouser.com/Texas-Instrume...d=7406&FS=True
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Old 02-04-2013, 02:29 PM   #37
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In your link, look at the mounting style column. There are two options for "through hole" components. Through hole comps will be easier to solder than the SMT comps.

Of the two through hole options, p/n SN7406N and p/n SN7406NE4, I don't see the functional difference which is signified by the "E4", but I only looked for about 5 minutes. I bet they can be used interchangeably, and the E4 signifies order quantities from TI.
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Old 12-18-2013, 07:05 AM   #38
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Quote:
Originally Posted by shotty128 View Post
Why use the P-tach adapter/Buffer.

https://ls1tech.com/forums/attachmen...d=102270&stc=1

Because the stealership is waiting for you to just add a resistor and risk burning up your ECM.

What this Buffer does. It sits between the ecm and old school tach that usually hooks to coils. By sitting between the ecm and tachometer, it protects your ecm from higher current levels through the just add a resistor mod. When you add a resistor to the wire, the ecm now has to sink more current in order to pull the line to 0Vdc. The 7406 has a fairly high impedance input.

Input what da heck you ask, simply it means this circuit causes the ecm to use the same amount of current or less at the white wire (tach signal).
When compared to being wired up to a stock dash.

This means that the ecm will not have to dissipate more power at 0Vdc out. You also will not be flowing current in to the 9V source in the ecm when its high. Because with the resistor if the guy that said its a 9V square out is right, which from what others state sounds true. Then your resistor will be supplying a higher voltage than normal when the signal is high causing current to flow in the ecm when the signal is high (meaning current should be flowing out). This is risky.

I built and tested this on a 2006 cadilac CTS-V ecm wired to an ls7. While attaching it to a summit gauge set link of them below.

http://store.summitracing.com/partde...5&autoview=sku

I give permission to every body to rip this off and sell it for $70+ a pop. Maybe call it a tach signal conditioner or adapter

I built this one on a radio schack PC Board part number 276-170 and then cut the board to size. The capacitor value is not that important it can be just about anything.

You are looking at about $8 worth of parts.

1. $0.25 1/4watt 560 ohm resistor green blue brown is what i used value can beanywhere between 500-700ohms
$0.25 1/4watt 100k ohm resistor brown black yellow
2. 7406 $1
3. 7805 $1
4. pc board $3.50

If you wish you can use a voltage divider to further condition the signal before the 7406 but it is not necessary.

"Powered by BOB" -champ car '05 Portland
"F-ing eddie" - champ car '05

Trying to build this and not sure what the arrow tips go to? Does anyone have a picture of it finished.

Thanks
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Old 12-18-2013, 08:47 PM   #39
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Quote:
Originally Posted by im2bads View Post
Trying to build this and not sure what the arrow tips go to? Does anyone have a picture of it finished.

Thanks
OK

OK I looked up info and looks like that symble indicates digital ground so it looks like your grounding the one side of the resistor on the tach signal wire from ecm?
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