Electrical Mustang factory tach calibration

jrichker · Nov 25, 2010

Maryland Stang said:
Here is the info borrowed from 89ragtopgt at corral.net. All credit goes to that person...

Apparently you can use tone generation software that you can download onto your PC or laptop. Install the software and use a 1/8" plug on your sound output connected to wires that are connected to the
signal input on the back of the tach.

I have downloaded the tone generator software and setup the table of frequencies used to calibrate the tach. The next step is to use a frequency counter to see how accurate the tone frequencies genterated by the computer are.

I will keep y'all posted as to my progress...

jrichker · Nov 25, 2010

Progress Update:

I have to bust the bubble of those who planned to do calibrating their tach on the cheap. Using the downloaded NCH (NCH Corporation) software and the sound card in your PC or laptop does not work.

Here’s why:
The tach input expects a square ware with a minimum of 9 volts peak to peak. The tach doesn’t stabilize until the voltage gets up to 10 volts peak to peak or higher.

The sound card output was less than 1 volt peak to peak at maximum volume. I hooked in the speaker amplifier and it didn’t do much better. The waveform generated by the sound card was full of distortion and noise. Not enough output voltage to do the job, and terrible signal quality on top of that. The sound card output voltage is a computer industry standard, so looking for a different computer or sound card isn’t likely to give any better results.

I watched the output of the sound card and amplifier on a Tektronix 5400 series oscilloscope to verify the voltage and waveform. The Tektronix scope is old, but very reliable and stable. The calibration is reasonably close, and not a cause for concern in this case.

In case some of you are thinking that I have a junky old PC, and that’s why it didn’t work, guess again. I am using an IBM E205 Eserver, 2.8 GH Pentium 4, 2 GB memory, and WinXP Pro. It may be old but it is very good equipment.

For those of you who have more questions, PM or email me through Stangnet.

I will be doing a full scale bench test using the Tektronix 5400 series oscilloscope, Hickok function generator and Hickok frequency counter. That will insure the accuracy of the results and quality of the finished effort.

93GTStang · Nov 25, 2010

Oddly the tach in my 93 LX is within 100 rpm of my Autometer. But I know the tach in my cousin's 90 GT is WAY off.

90limited · Nov 11, 2011

bump...

25thmustang · Nov 11, 2011

NikwoaC said:
I was actually surprised to find that mine is within 100 RPM or so, even at 5500 RPM. I know some people's can be damn near 1K off at high RPM.

This would be mine. I used to have an autometer and never looked at the stocker. Now I got rid of the autometer, and the shift light. It was on the dyno I noticed when I told him to run it to 6000, and the tach went just under 7000. Now at the track 6400-6500 on the stocker seems to be where I shift (very rarely do I ever actually look down at the tach, I prefer driving by feel).

jrichker · Nov 11, 2011

I have the test equipment to check the factory tach: signal generator, frequency counter & oscilloscope. I spent some time testing a spare stock 89 tach so I could see how the process worked. This one only had 1 adjustment potentiometer: I had heard rumors of later model factory tachs having two adjustment potentiometers.

I will tell you that without modification to the circuity, you can get the factory tach to accurately read some part of the range as marked on the dial. I will also tell you that while it may be correct at that narrow band of 500- 1000 RPM, it will be off by 50-400 RPM above and below that narrow band. The further you get away from the "sweet spot", the greater the error. I haven't found a circuit diagram of the tach, so I don't know if it is possible to change some of the components to get greater accuracy.

tmoss · Nov 14, 2011

jrichker said:
I haven't found a circuit diagram of the tach

That's what we need............

Mustang5L5 · Jan 21, 2021

Bump. It's 2021

Can this $12 piece of Chinese hardware do the trick?

https://rover.ebay.com/rover/1/711-...0001&campid=5335821607&icep_item=392993715816

7991LXnSHO · Jan 21, 2021

jrichker said:
Progress Update:

I have to bust the bubble of those who planned to do calibrating their tach on the cheap. Using the downloaded NCH (NCH Corporation) software and the sound card in your PC or laptop does not work.

Here’s why:
The tach input expects a square ware with a minimum of 9 volts peak to peak. The tach doesn’t stabilize until the voltage gets up to 10 volts peak to peak or higher.

The sound card output was less than 1 volt peak to peak at maximum volume. I hooked in the speaker amplifier and it didn’t do much better. The waveform generated by the sound card was full of distortion and noise. Not enough output voltage to do the job, and terrible signal quality on top of that. The sound card output voltage is a computer industry standard, so looking for a different computer or sound card isn’t likely to give any better results.

I watched the output of the sound card and amplifier on a Tektronix 5400 series oscilloscope to verify the voltage and waveform. The Tektronix scope is old, but very reliable and stable. The calibration is reasonably close, and not a cause for concern in this case.

In case some of you are thinking that I have a junky old PC, and that’s why it didn’t work, guess again. I am using an IBM E205 Eserver, 2.8 GH Pentium 4, 2 GB memory, and WinXP Pro. It may be old but it is very good equipment.

For those of you who have more questions, PM or email me through Stangnet.

I will be doing a full scale bench test using the Tektronix 5400 series oscilloscope, Hickok function generator and Hickok frequency counter. That will insure the accuracy of the results and quality of the finished effort.

Thank you for confirming this. Even the best Soundblaster cards I used needed an external amp for much volume, and recording from the analog outputs was often noisy. An external MIDI/synth box or synthesizer keyboard could do a pure square wave with less electrical noise, but it would still need amplified. I’m not likely to hook my guitar or keyboard amp into my car.

For a equipment less check, what RPM does the stock rev limiter activate? I generally avoid using that EEC-IV function, but ...

Mustang5L5 · Jan 21, 2021

7991LXnSHO said:
For a equipment less check, what RPM does the stock rev limiter activate? I generally avoid using that EEC-IV function, but ...

6250

Cheapskate207 · Jan 22, 2021

Looks like it could work. I doubt 2% would affect much but I’ve seen comparable units claiming 1% accuracy for around 30$.

Mustang5L5 · Jan 22, 2021

I ended up buying it. Well give it a shot.

Mustang5L5 · Jan 25, 2021

Well it works. My little $13 ebay frequency generator did the trick.

set it for 5k rpm and was slightly under.

slight tweak of the pot and spot on

it’s linked earlier, but here are the values.

Cheapskate207 · Jan 25, 2021

That’s awesome!!

General karthief · Jan 25, 2021

That could be a sticky

Mustang5L5 · Jan 25, 2021

I added it to the tech thread index

Mustang5L5 · Jan 26, 2021

BTW,

The tachs up to early 1989 only had one pot of adjustment. The late 89 and later tachs feature two pots. One for low range, and another for high range.

Here's an example I did of 4 factory tachs I had on hand.

Three are untouched from Factory, but the orange plot is one tach (with single pot) that I played around with and made some adjustments. As you can see I've gotten it better than the three untouched units.

And here's the difference in 1989 tachs. One is early, and one is late. You can see the single pot on the far side of the early tach, while there are two pots on the later model. I believe the outer pot adjusts the high end of the range, and the inner pot adjusts the low end. They affect each other, so my technique was to adjust the high end, and then go to the low end and tweak. Make small adjustments.

Deleted member 215073 · Jan 26, 2021

So how much to calibrate a tach if I send you one?

Mustang5L5 · Jan 26, 2021

JMGlasgow said:
So how much to calibrate a tach if I send you one?

Problem is, I don't know how accurate my frequency generator is. If I knew it was spot on, i'd probably be willing to do a few for a small fee. Let me see if i can find some means to verify.

Deleted member 215073 · Jan 26, 2021

I guess a person could use a photo tachometer on the balancer to verify.

I have used one on a diesel engine without a tach.

Electrical Mustang factory tach calibration

jrichker

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jrichker

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93GTStang

90limited

25thmustang

Mustang Master

jrichker

StangNet's favorite TOOL

tmoss

Gettin Wired

Mustang5L5

That is…until I whipped out my Bissell

7991LXnSHO

wanna catch the space herp

Mustang5L5

That is…until I whipped out my Bissell

Cheapskate207

The left one hangs a little lower

Mustang5L5

That is…until I whipped out my Bissell

Mustang5L5

That is…until I whipped out my Bissell

Cheapskate207

The left one hangs a little lower

General karthief

wonder how much it would cost to ship you a pair

Mustang5L5

That is…until I whipped out my Bissell

Mustang5L5

That is…until I whipped out my Bissell

Deleted member 215073

Mustang5L5

That is…until I whipped out my Bissell

Deleted member 215073

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Electrical Mustang factory tach calibration

StangNet's favorite TOOL

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Mustang Master

StangNet's favorite TOOL

Gettin Wired

That is…until I whipped out my Bissell

wanna catch the space herp

That is…until I whipped out my Bissell

The left one hangs a little lower

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

The left one hangs a little lower

wonder how much it would cost to ship you a pair

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

Deleted member 215073

That is…until I whipped out my Bissell

Deleted member 215073

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