Nice writeup - great job. As an engineer, I wholly approve of this! (I've got the analog dash with a working tach so it's not relevent to me, but still very well done -- thanks & somebody should sticky this 
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Digital Cluster - Not accurate Tach - Repair guide
Hi guys,
I'm fairly new on this site and i already took a lot form it. But today it is time to give back!!!
My 1988 vert Z24 had a Tach problem in it's Digital Cluster. It was indicating a RPM way over what it should be. So i finally desided to take care of it.
Here is my solution to those not accurate Tach in our Digital cluster. And it is really easy if you have some electronic skill.
You need:
1 Oscillioscope
1 frequency generator
1 Multimeter
1 500K Ohms, 12 turn Potentiometer
First, some basic info about how this tach is working.
It gets a triangle signal from the coil that has 12 volt of amplitude and a frequecy between 0 and 350 Hz. This signal goes in the circuit and it is transfert to the needle.
Car RPM = { Coil frequency(Hz) x 60(seconds) } / {1/2 # of cylinders}
So for a V6 engine it's simply {frequency(Hz) x 20}
therefore on a V6 100Hz is 2000 RPM
It is a Chip " White Chip with no part number " that has a bad resistor in it. When GM made this tach, they have use a laser to cut the resitor in this chip to calibrate the tachometer. Over the time the value of the trimed resitor have change and made the needle on the tach not accurate.
You're gone have to cut the circuit that use this resistor and replace it by a potentionmeter to make it work and fine tune it.
There are multiple ways to do this, fortunately due to my electronics background I have some fancy equipment.
either way you will need something that will measure frequency.
I used a frequency generator and a scope to make sure the frequency was accurate.
They were nice enough to label B+( 12 volt ), G ( Ground ), C ( coil ) and ILL ( light ) on the back of the tach board.
Simply cut trace on the board and solder your potentiometer.
Hook up Ground and 12 volts to the power pins (bolts). now hook up your frequency generator to the coil pin (bolt).
Set the frequency of the generator to 100Hz, now adjust the amplitude of the generator until you get a stable RPM measurement on the tach (approx 12-15v).
Now just adjust the potentiometer until the tach reads 2000 RPM.
Feel free to check the calibration in multiple places, i.e. 200Hz = 4000, 350Hz = 7000 RPM and others just to make sure it is mostly linear.
Once your done simply reassemble everything together .
**** I did not have a 500K Ohms pot, so i made a 204K Ohms resistor in serial with a 47K Ohms 12 Turns Pot. ******
**** My final Resistor result after calibration was 248.7K Ohms
For those who have a dead Tach, it's gone a be the other black chip"9555A" that it is dead. You're gone have to completly replace it by a replacement chip.
Use a NTE1670 or CS289GN14 or LM1819 Integrated Circuit Air Core Meter Driver to repair it.
I'm fairly new on this site and i already took a lot form it. But today it is time to give back!!!
My 1988 vert Z24 had a Tach problem in it's Digital Cluster. It was indicating a RPM way over what it should be. So i finally desided to take care of it.
Here is my solution to those not accurate Tach in our Digital cluster. And it is really easy if you have some electronic skill.
You need:
1 Oscillioscope
1 frequency generator
1 Multimeter
1 500K Ohms, 12 turn Potentiometer
First, some basic info about how this tach is working.
It gets a triangle signal from the coil that has 12 volt of amplitude and a frequecy between 0 and 350 Hz. This signal goes in the circuit and it is transfert to the needle.
Car RPM = { Coil frequency(Hz) x 60(seconds) } / {1/2 # of cylinders}
So for a V6 engine it's simply {frequency(Hz) x 20}
therefore on a V6 100Hz is 2000 RPM
It is a Chip " White Chip with no part number " that has a bad resistor in it. When GM made this tach, they have use a laser to cut the resitor in this chip to calibrate the tachometer. Over the time the value of the trimed resitor have change and made the needle on the tach not accurate.
You're gone have to cut the circuit that use this resistor and replace it by a potentionmeter to make it work and fine tune it.
There are multiple ways to do this, fortunately due to my electronics background I have some fancy equipment.
either way you will need something that will measure frequency.
I used a frequency generator and a scope to make sure the frequency was accurate.
They were nice enough to label B+( 12 volt ), G ( Ground ), C ( coil ) and ILL ( light ) on the back of the tach board.
Simply cut trace on the board and solder your potentiometer.
Hook up Ground and 12 volts to the power pins (bolts). now hook up your frequency generator to the coil pin (bolt).
Set the frequency of the generator to 100Hz, now adjust the amplitude of the generator until you get a stable RPM measurement on the tach (approx 12-15v).
Now just adjust the potentiometer until the tach reads 2000 RPM.
Feel free to check the calibration in multiple places, i.e. 200Hz = 4000, 350Hz = 7000 RPM and others just to make sure it is mostly linear.
Once your done simply reassemble everything together .
**** I did not have a 500K Ohms pot, so i made a 204K Ohms resistor in serial with a 47K Ohms 12 Turns Pot. ******
**** My final Resistor result after calibration was 248.7K Ohms
For those who have a dead Tach, it's gone a be the other black chip"9555A" that it is dead. You're gone have to completly replace it by a replacement chip.
Use a NTE1670 or CS289GN14 or LM1819 Integrated Circuit Air Core Meter Driver to repair it.
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Fantastic information. Makes me want to kick myself for not buying a digital dash a while back. Could have gotten it for $35.00 from a junk yard. I passed on it knowing that many of them were having tach problems and were hard (or very expensive) to get working again.
Still while your write up doesn't apply to me, I very much appreciate the time you took putting the post together for others that will make good use of it
Still while your write up doesn't apply to me, I very much appreciate the time you took putting the post together for others that will make good use of it
Thanks a lot for your nice comments! It is been a pleasure writing this post to help other members
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