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Discussion Starter #1
As requested I'm going to show my tachometer repair. in steps to the best of my abilities.

History

Tachometer slowly creeps up to max. No response while engine running.

This led me to believe that my tach was working but did not have the proper signal. After some research I got a new tach filter then rewired the whole signal line from dist to tach. Put a cheap meter on it and got really funky readings. while engine was running. (didn't want to take a chance at the high voltage blowing up my fluke).

Pluged in the tach manually and still nothing.

Found this website. http://howtoalmanac.com/kevin/projects/automotive/tachfixtruck.htm

It talks about what everything is and what typically fails and how to fix it.

Now understand the 86 elky tach is not like this one. FOLLOW YOUR TRACES! huge importance there. I tested the white IC and found yes it's true pins 4 and 10 no longer read resistance nor do they have continuity. (YAY)



Now after finding this I followed my traces and decided where I could cut them.




I then soldered my 100k pots in series you will need roughly 180- 197k total. I had these laying around. Then I soldered them to the board in pin 4 and 10



I then took it outside and hooked it up all ugly like to test before i took the dash apart again. it's getting fragile so the least i take it apart the better. a video of it in action is here. yes it's in the grass.

http://s49.photobucket.com/albums/f291/immille/El Camino/?action=view&current=VIDEO0067.mp4

All that is left is to calibrate it.

Now a note about the pots. before soldering them to the board make sure they are MAXED OHM with a meter first then back it off! you don't want to fry the other older components.

As part of my calibration I'm going to take my time on this I want to try a few things first. but as of right now it works.
 

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Discussion Starter #2
Calibration

Currently I am looking at 2 options to calibrate it.

1st one is more stable it involves borrowing a known good tach and hooking it up in parallal and homing the repaired one back in. then hot glue or electric glue the pots in place!

2nd is a theory I have. With a ton of research on the subject I should be able to Use a program called audacity to create audio tones in a square wave output. 100hz 200hz 300hz and so on. I have hooked up my silly scope to the sound output on the computer and verified that yes it does create those freqs, and they are very stable I might add. I'm looking to either use a seperate power supply and a transducer to bring the audio freq signal to the tach or in a thought process while in shower build cheap pc with spare parts and have the 12volt supply power the tach and the sound will then have the same common and no other equipment is needed. BUT I do not know how well that will be accurate as a true signal from the dist is a harsh input. and this will be nice and clean. who knows. let me know what you all think about if you know another way to calibrate the repaired tach.
 

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Hey man. LOVE THIS THREAD! And I love your ingenuity for the two options of calibration.

I don't remember why I did it, I think I was testing/playing with an old 'scope that I was given, but I've also used a computer tone generator app and audio output to create waveforms for the scope to see. Great fun!

I think a good test of actual signal would be to hook up the 'scope to the new tach filter/capacitor to see how dirty the signal is to start. Then set the repaired tach a la your very elegant 12v/audio common method. Hook up the known good tach, see what it says, then hook up the repaired tach and see what it says.

In my mind, tach calibration machines are probably generating pretty clean signals. I'm in no way an expert, though but I think you should be fine. Please please please let us know what happens! This could be MAJOR DIY repair info for future members!

:You_Rock:
 

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I Will!
It will be a bit before I can resurect some old pc's laying around. I'd love to find one already working just plug it in and go but ehh i'm sure it will need alot more effort than what i'm thinking. I wanna snap my fingers and have this perfect workshop with all this already set up and just go plug it in and report back saying yes or no. so far no money tied up in this, the parts are 1.50 each x2.... heck who knows, IF this does work maybe I'll see about offering services to rebuild/test them and what not and so on.
 

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Deputy Director Region 3 NJ Participating Member
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I had to go up to 220K ohmn to calibrate a spare Tach I found. At 200K it still tracked too low.
This whole process is very close to what needs to be done to the 3rd Gen Camaros. I had to do this in my '90 IROC and used about the same value resistor (210k) to get it as closely calibrated as possible.

Very nice write up and pictures. Will have to keep this bookmarked should the tach in my '87 Choo Choo goes haywire.

Bruce
 

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Today I repaired my tachometer using this thread's explanations. Worked like a charm. I used a 220k metal film resistor - the result was spot on.

I checked the calibration of the tachometer using the output of my sound card, by generating tones at 100, 200, 300, 400 Hz (i.e. 1500, 3000, 4500 and 6000 RPM for a V8).

The tach was hooked up to a good 12V source. I connected the ground of my sound card to ground of the tach. At a 100 Hz tone (square wave) and the volume cranked up, I read 1.2V between ground and the left channel of the sound card (let's call it signal) using the VAC setting of my multimeter. That is not enough to drive the input marked "coil" on the tach (the right most spring connector in the picture below). However, cirumventing the input resistor, the signal is strong enough. Just attach your signal to either side of the diode right next to the resistor pack (the two pins right above the spring connector). Make sure the output voltage of your card is >1V, otherwise it won't work.

 
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