68 Tic Toc Tach Calibration - Team Camaro Tech
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post #1 of 12 (permalink) Old Nov 9th, 18, 05:12 PM Thread Starter
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Dan
 
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68 Tic Toc Tach Calibration

I purchased a used speedo and tic-toc-tach cluster on ebay a couple of weeks ago. It wasn't cheap and came untested but they don't show up all that often and I figured I'd take a chance. I've wanted to add a tach to my car for along time and wasn't happy with the idea of strapping one to the steering column. I also wanted OEM, just because. Nothing against the reproductions....

As you'd expect from 50 year old equipment that has been kicking around for who knows how long the tach wasn't very accurate. In fact it was off by 50%.
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1968 RS/SS 350 4sp Convertible
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post #2 of 12 (permalink) Old Nov 9th, 18, 05:22 PM Thread Starter
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Re: 68 Tic Toc Tach Calibration

Fortunately I'm not the first person to have this issue and replacement parts are available. However, I couldn't find any detail threads on how, so figured I'd post one.

The first thing I needed was a way to bench test the tach before and after the modifications. Fortunately I have a Megasquirt 2 system from another project that only required a few minor add ons to be able to test an old tach. This is the Megasquirt and Stimsim connected to my laptop and wired to an amplifier and relay coil to produce the signal required to drive the tach.
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1968 RS/SS 350 4sp Convertible
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post #3 of 12 (permalink) Old Nov 9th, 18, 05:27 PM Thread Starter
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Re: 68 Tic Toc Tach Calibration

Here's a few pictures testing the tach before any changes. You can see the tach readings are 50% above the RPM's on screen for all ranges.
2,000 rpm's reads 3,000
3,000 rpm's reads 4,500
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1968 RS/SS 350 4sp Convertible
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post #4 of 12 (permalink) Old Nov 9th, 18, 05:33 PM Thread Starter
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Re: 68 Tic Toc Tach Calibration

The Cajun Tach Shop sells replacement circuit boards for these old tachs on ebay, $30 delivered. Pretty sweet! Here's a picture of the 50 year old technology alongside the new.
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1968 RS/SS 350 4sp Convertible
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post #5 of 12 (permalink) Old Nov 9th, 18, 05:39 PM Thread Starter
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Re: 68 Tic Toc Tach Calibration

Replacing the circuit board was pretty straight forward. The new board is a perfect fit. Carefully pull the spring clips off of the old board and replace them in the same locations on the new. Clip the new board into the plastic holder and your done. This is all pretty delicate stuff so obviously you want to be very gentle or you could break something. It's also very easy to lift the pot or variable resistor on the new board while applying pressure to the clips so be careful.
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1968 RS/SS 350 4sp Convertible
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post #6 of 12 (permalink) Old Nov 9th, 18, 05:43 PM Thread Starter
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Re: 68 Tic Toc Tach Calibration

Reconnect the tach to the bench test setup and nearly perfect. Initially it was off by less than 10%. A few turns of the pot and it dialed right in to be a perfect match.

Now all I need to do is replace the clock mechanism with a quarts replacement and it will be as good as new and ready for installation into the car.
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1968 RS/SS 350 4sp Convertible
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post #7 of 12 (permalink) Old Nov 9th, 18, 06:34 PM
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Re: 68 Tic Toc Tach Calibration

Very nice of you to share. Wish there were something similar for the 69 tach. Mine reads high.

Lynn
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post #8 of 12 (permalink) Old Nov 9th, 18, 06:43 PM Thread Starter
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Re: 68 Tic Toc Tach Calibration

Should also work for the 69 tach. Search ebay for "NEW! 69-77 GM, Pontiac, Tachometer Circuit Board ~ Direct Replacement!" If you want confirmation, send an email to the seller. He's very responsive.

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post #9 of 12 (permalink) Old Nov 9th, 18, 08:35 PM
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Re: 68 Tic Toc Tach Calibration

Very cool. With your test equipment and expertise, I think you'd do well by offering your services !

  • '68 RS, AA, 749, L30, M(uncie)20 (originally a 3spd), 15yr backburner resto........
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post #10 of 12 (permalink) Old Nov 10th, 18, 01:28 AM
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Re: 68 Tic Toc Tach Calibration

Nice job walking us through it!

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post #11 of 12 (permalink) Old Nov 10th, 18, 01:37 PM Thread Starter
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Re: 68 Tic Toc Tach Calibration

Happy to share something useful for a change

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post #12 of 12 (permalink) Old May 8th, 19, 06:38 PM
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Re: 68 Tic Toc Tach Calibration

So, I have a 68 with electronic ignition and an OER tic toc tac. It seems to read pretty accurately at idle, then is way off (LOW) as the rpms go up. Anybody know if a tach filter would help with accuracy? Id also consider sending it out (to whoever?) to have it reworked? Any help or ideas would be appreciated. Thanx in advance.
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