communique article Bench Test and Calibration of your Tachometer

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BobWitt
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communique article Bench Test and Calibration of your Tachometer

Unread post by BobWitt » Sat Feb 21, 2015 11:50 am

2/28/15 - I just edited the title of this thread from “Looking for May 1999 issue of the Communique” to “Bench Test and Calibration of your Tachometer”.


I am a CORSA member - looking for "Rad Davis' article for tach repair in the May 1999 issue of the Communique" - I read of this on some other forum...

But, on the CORSA site, there is only 78-79 and 2009-2015 Communique's available in PDF...

Anyone have a copy they could shoot me? I have seen an artical on the Corsa speedo repair - anyone know of any others out there?

Thanks!
Last edited by BobWitt on Sat Feb 28, 2015 4:15 am, edited 2 times in total.
BobWitt - South East Michigan
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Re: Looking for May 1999 issue of the Communique

Unread post by T34C12UH60 » Mon Feb 23, 2015 5:34 pm

Never posted here before but will give it a try.
Bill
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BobWitt
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Re: Looking for May 1999 issue of the Communique

Unread post by BobWitt » Mon Feb 23, 2015 6:08 pm

Well, Bill, you've done well...

Thanks!
BobWitt - South East Michigan
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Re: Bench Test and Calibration of your Tachometer

Unread post by BobWitt » Sat Feb 28, 2015 4:22 am

I just edited the title of this thread from “” to “Bench Test and Calibration of your Tachometer”.

First a note about safety - If you are not comfortable and confident working around 120 volts AC – DO NOT TRY THIS! Without the proper care you could shock or electrocute yourself.

I wanted to share that I used my 12-volt battery charger to test and calibrate my Tach. Per the above article you can use the 12-volt AC output from the transformer to ‘simulate’ a tach signal. So I opened my trusty old charger, looked around and made some voltage measurements – easily finding a 12volt AC source. I connected the Green wire to this and to the “Coil +” input on the back of the Tach. I then wired the DC 12 volts to the “12 Volt” input with the red wire, and connected the black to the ground.

This is my set-up.
tach1.JPG
Powered on the battery charger and sure enough the Tach came to life…
20150227_191840.jpg
But, as you can see, it is reading 1350 RPM. Per the above article, it should read 1200 RPM. I then turned the “R6” (see Red Arrow Above) to dial it into 1200 rpm.
20150227_192657.jpg
All worked as the article stated, but, I’m trying to understand electrically what is going on…

If I calculate the number of “point closings” the coil and tach should see per second at 1200 RPM I come up with: ( 3 point closings / crank rev) X ( 1200 rev / minute) X ( 1/60 minutes/sec) = 60 point closings/sec or 60 cycles/second or 60 Hz.

But the 12VAC signal coming from the transformer in the battery charger is 120 Hz (cycles/sec) – twice as much. Why does the tach not read 2400 RPM? It all worked as stated in the article, but I do not understand why!

Can you explain?
BobWitt - South East Michigan
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Re: Bench Test and Calibration of your Tachometer

Unread post by bbodie52 » Sat Feb 28, 2015 6:45 am

Since the distributor rotates at ½ crankshaft speed, at 1200 rpm the distributor is rotating at 600 rpm. 600 rpm X 6 (cylinders) = 3600 ppm (pulses per minute) = 1200 rpm at the crankshaft.

The AC line voltage frequency at the wall outlet is 60 Hz (cycles per second) X 60 (seconds per minute) = 3600 cycles per minute = 3600 ppm, which, again, translates to 1200 rpm at the crankshaft.

How do you figure the line voltage frequency to be 120 Hz? Are you counting both the positive and negative peaks of the sine wave? One complete AC cycle includes both one positive and one negative transition, and using line frequency to calibrate your tachometer, the tachometer is only going to "see" the negative voltage transition of the sine wave as a negative voltage spike. The positive voltage sine wave transition will be invisible to the tachometer circuit.

Most Corvair owners have access to a dwell/tachometer for tuning the engine. When calibrating the instrument panel tachometer why not simply connect it, along with a dwell/tachometer, to an operating engine ignition coil, and use that as your bench test calibration setup? Doing so will utilize the engine to provide the "pulse generator", and the tachometer simply has to be set to match the reading on the dwell/tachometer for any given engine speed.
Brad Bodie
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azdave
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Re: Bench Test and Calibration of your Tachometer

Unread post by azdave » Sat Feb 28, 2015 6:49 am

BobWitt wrote:But the 12VAC signal coming from the transformer in the battery charger is 120 Hz (cycles/sec) – twice as much. Why does the tach not read 2400 RPM? It all worked as stated in the article, but I do not understand why!

Can you explain?
I'm sick and just woke up so forgive me if I'm not thinking straight but all power in the US is 60 Hz not 120. How can your charger be putting out 120 Hz?
Dave W. located in Gilbert, AZ
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Re: Bench Test and Calibration of your Tachometer

Unread post by BobWitt » Sat Feb 28, 2015 8:10 am

OK - now I understand the issue - having a grey moment...

Brad - I have the dash on the bench now! don't want to reassemble to learn it needs calibration...

Did i need to do that so publicly? Thanks guys..
BobWitt - South East Michigan
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Re: Bench Test and Calibration of your Tachometer

Unread post by bbodie52 » Sat Feb 28, 2015 8:50 am

BobWitt wrote:...I have the dash on the bench now! don't want to reassemble to learn it needs calibration...
The dash on the bench can be positioned near the engine compartment, and the tachometer can be temporarily grounded, wired to the battery, and the sensing lead can be temporarily connected to the coil negative terminal to perform a "bench check" for function and calibration.
Brad Bodie
Lake Chatuge, North Carolina
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