67-72 factory tach calibration?

[SERE]

Does anyone have any experience with calibrating a factory tach? I have two of them and they work fine, but there is a ,sort of dial looking thing, on the back and I can't find anything in the manuals about it. I just want them to be indicating the correct RPM. Also, the one tach I have in my 69 with a 454 and it stops at 3200 RPM. The needle just stops like it is hitting a stopper? Any in formation on these tachs I would appreciate the help.

68 GMC 1500 4X4 short fleetside, daily driver on my 12 year resto!
69 GMC 2500 4X4 long stepside, project

[NeCrOmAnCeR]

Have any pics of the "dial thing"? I have never seen an adjustment on one before.
As the electronic components age, the electrolytic capacitors change value and usually cause the guage to read incorrectly. I have adjusted many to read correctly as well as modified other tachs (4-6 cyl) to read correctly on an 8-cyl. My favorite mod is on my truck. I have adjusted the factory tack to read up to 8K and created a new face. Here is a pic. You can PM me for more information about modifying one if you feel you can do it yourself.

[Russell]

Duude! I would VERY much like to know more about this mod! I you have a layout, I would absolutely love to get a copy of it so I can get a white faced version made! Thats very very slick!

PM or email me! ZieglerDigital@hotmail.com

[Alex Hayley]

NeCrOmAnCeR, is this something you could post for everyone?

[SERE]

Here are some pictures (I hope they make it in) of my tach. The hole at the top of the tach is the dial I was talking about. It is a cross shaped slot and looks like a brass dial. It has some numbers around it, but they are hard to read. Help!!!!

[NeCrOmAnCeR]

SERE: That looks like a zero adjust for the needle. The markings on the rear let you know it is for an 8cyl. If you are not afraid to mess with it, pay close attention to the position of the screw and turn it a little (count in small increments of a turn) to see if it moves the needle position. Then if it does not give the desired effect, return it to the starting position.


When GM manufactured these tachs, they used a frequency to voltage conversion IC (chip). To calabrate this set up for different numbers of cylinders, GM used a "resistor" chip. The IC itself uses a RC circuit calabrated to oscillate at an appropriate frequency to generate a local oscillator within the tach (trying to simplify). This frequency is compared to the input frequency (from the ignition system) and the difference is calculated and a resulting voltage is created. The movement of the guage (motor) consists of two coils of wire (sine, cosine coils). The generated voltage is applied to the two coils in varying ammounts dependant upon the difference of the before mentioned frequencies.
The first thing I do is replace all electrolytic capacitors on the tach board (bad idea to use them in the first place, say "thank you" to the bean counters at GM). As these style capacitors age, the electrolytic "gel" can leak, dry-out, or other wise cause the value of the cap to change.
The trick I use is to create very accurate input waveforms which correspond to some specific engine speeds (1K, 2K, 3K, ....) using a function generator. Then I cut a trace on the circuit board disconnecting the factory adjustment "resistor" (the chip itself contains two resistors). I then take an adjustable resistor, install it temporarily into the circuit, and create a frequency corresponding to the upper RPM limit of the tach (8K for the tach face I made) I then adjust the variable resistor to get the tach to read correctly. I then sequentually change the input frequency to another and check the linearity of the tach. I have found that the factory tachs are crude at best and have had to make adjustments so the tach reads correctly at 2 - 4K so it will read correctly at the common cruise RPM's. This usually causes the upper and lower regions to be "off" slightly. Once I am satisfied of the accuracy, I measure the resistance of the variable resistor, and replace it with a 1% tolerance resistor of the same value.
The reason I am not going to go into greater detail, and don't take this the wrong way, I don't know the ability level of everyone. I would hate someone to just start cutting the circuit boards and find that they have made a fatal (to the tach) mistake. These tachs are becoming rare.
Because most people do not have a function generator available to them, calabrating or re-calabrating a tach would be difficult if not impossible. I have discovered at least 5 different styles of circuit boards used by GM, and all of them place the components in different locations, and use different values.
I can re-calabrate a 4 or 6 cylinder tach to read correctly on an 8 cyl endine, or the other way if desired. As far as makeing the tach face, when I was cleaning up my dash, I scanned the tach face, photo shopped it with the values you can see above, and printed it on a magnetic sheet. (no permanent modification to the face, so I can return it to stock later if I choose ) I then sealed the sheet with some clear. I have had it in my truck for about 8 months and have not noticed any fading so far. You can tell upon close inspection (not evident in the picture) that it is not the factory surface, so for those trying to retain the stock appearance this may not be for you. Crossy has been working on possibly getting a silk-screen face created (7K) so that may be something to look forward to.
I would be happy to modify anyones tach for cost (about $10 + shipping). If you have an inoperative tach I would like to experiment with it to try to get it working again. For those looking to refresh their clusters at the same time, talk to Crossy, he will do a great job, and has sent me a few tachs to repair.

[SERE]

Thanks I will work on that needle adjustment and go from there. Excellent stuff!!

Steve

[-Greg72]

Originally Posted by NeCrOmAnCeR

SERE: That looks like a zero adjust for the needle. The markings on the rear let you know it is for an 8cyl. If you are not afraid to mess with it, pay close attention to the position of the screw and turn it a little (count in small increments of a turn) to see if it moves the needle position. Then if it does not give the desired effect, return it to the starting position.


When GM manufactured these tachs, they used a frequency to voltage conversion IC (chip). To calabrate this set up for different numbers of cylinders, GM used a "resistor" chip. The IC itself uses a RC circuit calabrated to oscillate at an appropriate frequency to generate a local oscillator within the tach (trying to simplify). This frequency is compared to the input frequency (from the ignition system) and the difference is calculated and a resulting voltage is created. The movement of the guage (motor) consists of two coils of wire (sine, cosine coils). The generated voltage is applied to the two coils in varying ammounts dependant upon the difference of the before mentioned frequencies.
The first thing I do is replace all electrolytic capacitors on the tach board (bad idea to use them in the first place, say "thank you" to the bean counters at GM). As these style capacitors age, the electrolytic "gel" can leak, dry-out, or other wise cause the value of the cap to change.
The trick I use is to create very accurate input waveforms which correspond to some specific engine speeds (1K, 2K, 3K, ....) using a function generator. Then I cut a trace on the circuit board disconnecting the factory adjustment "resistor" (the chip itself contains two resistors). I then take an adjustable resistor, install it temporarily into the circuit, and create a frequency corresponding to the upper RPM limit of the tach (8K for the tach face I made) I then adjust the variable resistor to get the tach to read correctly. I then sequentually change the input frequency to another and check the linearity of the tach. I have found that the factory tachs are crude at best and have had to make adjustments so the tach reads correctly at 2 - 4K so it will read correctly at the common cruise RPM's. This usually causes the upper and lower regions to be "off" slightly. Once I am satisfied of the accuracy, I measure the resistance of the variable resistor, and replace it with a 1% tolerance resistor of the same value.
The reason I am not going to go into greater detail, and don't take this the wrong way, I don't know the ability level of everyone. I would hate someone to just start cutting the circuit boards and find that they have made a fatal (to the tach) mistake. These tachs are becoming rare.
Because most people do not have a function generator available to them, calabrating or re-calabrating a tach would be difficult if not impossible. I have discovered at least 5 different styles of circuit boards used by GM, and all of them place the components in different locations, and use different values.
I can re-calabrate a 4 or 6 cylinder tach to read correctly on an 8 cyl endine, or the other way if desired. As far as makeing the tach face, when I was cleaning up my dash, I scanned the tach face, photo shopped it with the values you can see above, and printed it on a magnetic sheet. (no permanent modification to the face, so I can return it to stock later if I choose ) I then sealed the sheet with some clear. I have had it in my truck for about 8 months and have not noticed any fading so far. You can tell upon close inspection (not evident in the picture) that it is not the factory surface, so for those trying to retain the stock appearance this may not be for you. Crossy has been working on possibly getting a silk-screen face created (7K) so that may be something to look forward to.
I would be happy to modify anyones tach for cost (about $10 + shipping). If you have an inoperative tach I would like to experiment with it to try to get it working again. For those looking to refresh their clusters at the same time, talk to Crossy, he will do a great job, and has sent me a few tachs to repair.

That is the most complete and detailed description I've ever found describing the "way off tach" phenomenon!!! Do you mind if I post your comments (unedited) directly over in the 1st Gen Forum of CK5? This information could really be useful to the guys there as well....

Thanks!

Greg Blanchette
aka "Greg72"
Forum Moderator
ColoradoK5.com

[NeCrOmAnCeR]

Sure! No problem.

[Dude68]

Necromancer, are you interested in adjusting a V-8 tach to an L-6?