1971 Factory tach - how to check - Chevelle Tech
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post #1 of 12 (permalink) Old Feb 25th, 16, 8:50 PM Thread Starter
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1971 Factory tach - how to check

I found a 71-2 factory tach to replace my 71 factory tach that doesn't work.How can I check the part that I'm buying is a working tach ? is there any way to bench test a factory tach ?

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post #2 of 12 (permalink) Old Feb 25th, 16, 9:06 PM
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Re: 1971 Factory tach - how to check

Can't you just hook up a volt-ohm meter and measure the resistance? Measure the non-functional one and note resistance, then the new one. What's the difference in ohms?

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post #3 of 12 (permalink) Old Feb 25th, 16, 9:48 PM
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Re: 1971 Factory tach - how to check

On a tach I have for my 74 Nova, I identified the connections then hooked it up to a power supply and a signal generator. The first part of the video's pictures are of the guts of the tach but later it shows connections to the power supply and signal generator.


I did this years ago and don't know what the settings were on the signal generator but it seems to me I used a square wave output and there was a relationship between the signal frequency and the reading of the tach as to RPM's being displayed.

Another option is to find a car with a points ignition and use some test wires off of the tach to the vehicles battery for power and ground and another wire for a signal from the coil. I would just do it carefully so nothing get's shorted and maybe put in a 5A inline fuse on the power line.

Jim

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post #4 of 12 (permalink) Old Feb 26th, 16, 12:01 AM Thread Starter
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Re: 1971 Factory tach - how to check

Quote:
Originally Posted by 70SSTHUMPER View Post
Can't you just hook up a volt-ohm meter and measure the resistance? Measure the non-functional one and note resistance, then the new one. What's the difference in ohms?
The non-functional one is still in the dashboard of my ElCamino and the one I am considering is still in its owners possession.
I should have made it clear I was looking for a quick way to check it before I bought it.How would I even do that or is it a crap shoot ? and hope for the best.

John
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post #5 of 12 (permalink) Old Feb 26th, 16, 2:40 AM
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Re: 1971 Factory tach - how to check

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Originally Posted by 71350SS View Post
The non-functional one is still in the dashboard of my ElCamino and the one I am considering is still in its owners possession.
I should have made it clear I was looking for a quick way to check it before I bought it.How would I even do that or is it a crap shoot ? and hope for the best.
Would the seller be comfortable if both of you knew exactly how to wire it and then temporarily wired it up safely to a car under the hood using the cars battery for power and ground and then a connection to the ignition coils negative terminal ?.

You asked if there was a way to bench check it and this is the only way I can thing of without using any high tech tools like I did with a power supply and a signal generator. A meter is not going to tell you how well or poorly the needle will move as the only way to really test it is in a real life condition.

If all else fails, give it a sniff test and visual look. If it smells like burnt wiring or there is more than just a little surface rust you might pass on the deal BUT if we are only talking 5 bucks or less, then take a chance. If it's way above market value, and it's being sold as-is you might wait for something else.

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post #6 of 12 (permalink) Old Feb 27th, 16, 12:52 AM Thread Starter
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Re: 1971 Factory tach - how to check

Quote:
Originally Posted by Jim Streib View Post
Would the seller be comfortable if both of you knew exactly how to wire it and then temporarily wired it up safely to a car under the hood using the cars battery for power and ground and then a connection to the ignition coils negative terminal ?.

You asked if there was a way to bench check it and this is the only way I can thing of without using any high tech tools like I did with a power supply and a signal generator. A meter is not going to tell you how well or poorly the needle will move as the only way to really test it is in a real life condition.

If all else fails, give it a sniff test and visual look. If it smells like burnt wiring or there is more than just a little surface rust you might pass on the deal BUT if we are only talking 5 bucks or less, then take a chance. If it's way above market value, and it's being sold as-is you might wait for something else.

Jim
Thank you Jim.I'm well aware of how to hook up a tachometer but I doubt the seller would go for that unless he understands what my risks are.
What's a good price for a used factory tach ? Or would I just be better off removing my tach and sending it out for repair ?

John
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post #7 of 12 (permalink) Old Feb 27th, 16, 8:59 AM
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Re: 1971 Factory tach - how to check

Quote:
Originally Posted by 70SSTHUMPER View Post
Can't you just hook up a volt-ohm meter and measure the resistance? Measure the non-functional one and note resistance, then the new one. What's the difference in ohms?

No, that won't tell you much of anything. Tachometers are not as simple as a fuel or temperature gage and even then resistance measurements are not reliable.

Quote:
Originally Posted by Jim Streib View Post
On a tach I have for my 74 Nova, I identified the connections then hooked it up to a power supply and a signal generator. . . . . . . .
I did this years ago and don't know what the settings were on the signal generator but it seems to me I used a square wave output and there was a relationship between the signal frequency and the reading of the tach as to RPM's being displayed.

Another option is to find a car with a points ignition and use some test wires off of the tach to the vehicles battery for power and ground and another wire for a signal from the coil. I would just do it carefully so nothing get's shorted and maybe put in a 5A inline fuse on the power line.
Jim

Jim is correct. I use a Signal/Function Generator and a 12V DC source also. I use a 30% duty cycle ground referenced 12V peak square wave for these checks. This is a snapshot of the functional test worksheet I use:
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post #8 of 12 (permalink) Old Feb 27th, 16, 11:15 PM
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Re: 1971 Factory tach - how to check

You could hook the tach power wiring to a 12 volt source and then use a simple pushbutton switch to simulate the signal with 12 volts through the pushbutton to the siganl wire.
Just push the button rapidly and watch for movement of the needle.
Quick and dirty.

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post #9 of 12 (permalink) Old Feb 28th, 16, 8:50 AM
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Re: 1971 Factory tach - how to check

Quote:
Originally Posted by wihaltom View Post
You could hook the tach power wiring to a 12 volt source and then use a simple pushbutton switch to simulate the signal with 12 volts through the pushbutton to the siganl wire.
Just push the button rapidly and watch for movement of the needle.
Quick and dirty.
Although this will yield some "twitching" of a functioning tachometer's needle, keep in mind you would have to press the button 33.33 times per second for a V8 tachometer to indicate 500 RPM which I don't think you would be able to do. Consequently you would still have no way of knowing whether it is linear or accurate.

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post #10 of 12 (permalink) Old Feb 28th, 16, 9:11 AM
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Re: 1971 Factory tach - how to check

I just pop the hood of the car it is supposed to go on and connect it temporarily with clip leads to power, ground, and the coil. If it works, it works. If not, you aren't guessing anymore.
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post #11 of 12 (permalink) Old Feb 28th, 16, 10:36 AM
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Re: 1971 Factory tach - how to check

Reproduction tachs are $159 on ebay.


Give him a "tested" offer, and an "untested" offer.
or ask him for a guarantee...


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post #12 of 12 (permalink) Old Feb 28th, 16, 11:32 PM
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Re: 1971 Factory tach - how to check

Quote:
Originally Posted by David Bates View Post
Although this will yield some "twitching" of a functioning tachometer's needle, keep in mind you would have to press the button 33.33 times per second for a V8 tachometer to indicate 500 RPM which I don't think you would be able to do. Consequently you would still have no way of knowing whether it is linear or accurate.
That is completely correct.
I just read his post as asking how to tell if the tach worked.
This quick little test would at least let him know if the device did function, with no accuracy or linearity implied.
You would need a very quick finger to get more than about 5 hertz with that little pushbutton!!

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