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Look what I found, is the original 1967 United Delco Electronic Tachometer Service Bulletin in pdf. This bulletin details the testing, diagnosis, and repair of first gen tachs. At the end of the document is a catalog of resistors, capacitors, etc that are used on the tach circuit board (not the flexible printed one behind the instrument cluster, but the small board that drives the tach needle).

Feel free to sticky or archive this so that others can enjoy this document!

http://mysite.verizon.net/racerxtx/AC_Tach_Repair_Manual.pdf
 

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Re: Here it is, the original tach repair service bulletin!!

Kelly,
Want to thank you for posting this bulletin as it will be very helpful in my project.
I am trying to figure out how the factory made the 69 ZL1 transistor ignition work with the factory electric tachometer.
Thanks again.
Bob
 

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Re: Here it is, the original tach repair service bulletin!!

Very interesting. Thanks for sharing.
 

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This is like finding gold. I worked for Cadillac for twenty two years, and nver saw this. My partner was a 15-a tech, and worked on many of these cars-under warranty!!!!
He always stated that the shop manual was good, but it was these types of bulletins that really got into "heavy diag. and repair". Thanks again
 

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Link worked for me. If you need it, email me at my home address.
Too big to post here.
 

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The article addresses 67 and 68. Does anyone know if the 69 Tachometer is adjustable?

I know I can pull it out and look, but don't wish to pull it out if it isn't adjustable.
 

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Verizon servers are not so reliable up-time wise. I bet you just picked the hour a day it is down for backups.

Describe adjustable? They can be calibrated by a speedometer / gauge shop that has the equipment. Home if you have the sign wave stuff on your bench but most folk don't.

Page 4 has 69 Camaro Tach specs.
 

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The bulletin indicates a fixed resistor. My tach reads about 8 to 10% high (about 4200 when it should read 3572).

Unless my thinking is backwards (could be) wouldn't a resistor in line from the coil to the tach bring it down?

I can buy adjustable variable resistors really cheap.

Surely somone has tried this.
 

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The bulletin indicates a fixed resistor. My tach reads about 8 to 10% high (about 4200 when it should read 3572).

Unless my thinking is backwards (could be) wouldn't a resistor in line from the coil to the tach bring it down?

I can buy adjustable variable resistors really cheap.

Surely somone has tried this.
Lynn,

I'm suprised your factory tach is that accurate to be honest :)

Here's the scoop. The original "circuit board" that operates the movement (the needle on the tach) is a beryllium oxide ceramic based board in which the factory resistors are actually "etched" in and cut to specification to get the proper value and calibration. They are not the traditional resistors mounted on a through-hole design as we know. These are very prone to drift and going intermittent affecting the original design from the get-go. The solder pads are also prone to lifting and connections go intermittent.

It would be foolish to spend time trying to "tweak" a circuit that has changed from its intended original design. It would be like trying to dial-in a new ignition system on an engine that has burnt valves :(

That's why people who restore the factory tachs retrofit modern components rather then messing around with the original set-up.

Unfortunately, you really only have (2) choices, live with it as-is until a complete failure, or send it to have it rebuilt and calibrated to proper standards.
 

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My Tach worked, but after an engine rebuild, had a MSD 6AL box added. Now, my tach does not work nor does the ammeter in the console gauge cluster. Any idea what I can do to fix these issue's?
 

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I'm rewiring all the original wire under the hood of a 67 RS SS and adding a MSD!
The service bulletin could be useful please could you get me a copy?

Thanks Mike

The link dose not work?:eek:
 

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Is there a new site that works for the Tach bulliten? The Verizon site stopped working as of Sept. 30, 2014.
 

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I have a copy of it in my archive. I'll convert it to a pdf and post it here.
 
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