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Wiring problems Tachometer Mustang -73
#21
if you're referring to the resistance through the tachometer, between the two connection points it will be very low, probably less than 1 ohm. The circuit that runs through the tachometer just runs through a set of windings in a transformer that actually works similar to the ignition coil. As the ignition coil is grounded (points close) to load the primary windings the same thing happens to the primary winding in the transformer in the tachometer. When the ground is released (points open) the field that has been generated collapses in both the ignition coil and the tachometer transformer, the ignition coil generating a high voltage pulse and the tachometer transformer generating a low voltage pulse. This low voltage pulse is what the tachometer circuitry converts into an analog signal (voltage) that causes the tachometer needle to move.

If checking the continuity across the two tachometer connections yields a reading of around 1 ohm the primary winding in the coil is OK, if it shows no reading the primary winding or connections to it have failed.

If the primary windings in the transformer have failed the there will also be no voltage passing through the tachometer to the ignition coil and the engine may start when the ignition key is turned to start, but as soon as it returns to the Run position the engine will die.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#22
An update of the tach. and of course some ?????

I took apart the tach and mesured the resistors and the electrical stuff in it.

What happened was when i measured the resistor with my multimeter, which i did set at ohm, the tach needle went to the top. I also put 3 1,5 volt batterys  together and took some wires and put it in to the contact to feed the tach with volts and the orange pin moved about 1-2 mm.

My conclusion here is that the tach works. There must be some other problem than in the tach, i.e the wiring or the ignition coil?, right ?

So for now in waiting for some parts including the tach circuit board from the USA so there will take some weeks before I can install the tach and try my new resistor which is at 100 watts and 1,5 ohm to se if that one will get the tach to work properly.[Image: 20200106-192836.jpg]
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#23
All that you verified is that the windings in the tachometer needle are intact and moves the needle. You did not verify whether or not the circuitry between the input transformer and the needle are functioning. They have to take the pulses generated by the coil (actually the points) and convert them into a steady voltage that then moves the needle. The faster the pulse, the more voltage generated and the higher the needle moves. There are likely a couple of capacitors, an inductor (choke) or two and some other components, like resistor in the circuitry. I haven't seen the inside of one of the original tachometers, so just trying to visualize what components would be required. Old capacitors are the most likely component to fail, although a voltage spike could also take out the inductors. It's also possible that the secondary side of the transformer is bad.

Did you ever measure the resistance between the two wires that connect to the tachometer? If the primary side of the transformer is still good that should measure around 1 ohm.

I'm still not sure that you need to install another resistor. Did you ever measure the resistance (ohms) between the tachometer and the ignition coil? It should measure around 1.5 ohms if there is a resistor someplace in the circuit.

Edit: if you do install another resistor it will get hot, so make sure it is at a location with ventilation and not likely to come into contact with other wires. The normal place for a ballast resistor is at the ignition coil.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#24
Ok ill try to get a better picture to show exactly what i did, but i take it, his does not indicate that the tach is working properly as i thought!
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#25
Today i measured every single component on the picture i dont know the name of them but got some resistance in all of them (Ohm).
So that should mean that they are properly working?!
Got the new voltage regulator and the new circuitboard today, ill be back when i got it all together and we will see if the tach will work after that if not Angry ....

[Image: 20200115-095401.jpg]
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#26
To accurately measure each component individually requires removing each component from the circuit and test it in accordance with what type of device it is. In the picture I see the a transformer, 2 diodes, a resistor and 2 capacitors, each of which are tested differently. The way to test them in the circuit is to input a signal and then follow the signal through the circuits from beginning to end, and know what the expected readings are after each device. In this case the signal would have to be a pulsing signal, 8 volts loaded through a resistor and unloaded, just like the points load and unload the ignition coil.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#27
Thanx oh, and by the way, broke the tachoneedle today when my 300 USD upgrade for the tach came. Fun. Project now, glue the tachoneedle back in place or glue the whole tachsh**t together....
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#28
(01-15-2020, 04:17 PM)nursieee Wrote: Thanx oh, and by the way, broke the tachoneedle today when my 300 USD upgrade for the tach came. Fun. Project now, glue the tachoneedle back in place or glue the whole tachsh**t together....

Yikes! $300 for a used Faria tach board? I hate to be the bearer of bad news, but you could have bought an entire working tach for less. I'll bet it came from SC....

Gluing the tach pointer on is most likely going to upset the calibration depending on just where the break/repair is. Use as little as possible and use something light - like Super Glue. No epoxy.

The easiest way to test the tach is to fins a running car (preferably a Ford product) using points or pertronix ignitor:
1. Remove the wire from the coil + post, connect it to the tach MALE connector (usually a RED wire but Faria ran out of red wire and started using black for both).
2. Run a wire from the tach's FEMALE connector (usually black) to the coil + post.

Start the car.
If it starts, check the tach function.
If it doesn't start the tach board is bad.
If it starts but the tach doesn't work then either the tach board or movement (or both) has failed.

This is for a 67-68 but electrically it is the same:

[Image: Faria-Tach-wiring.jpg]

Bob

Tachs, Voltmeters, Headlight kits, Wiper delays and more at
[Image: smlogo.jpg]
Rocketman's Classic Cougar (and Mustang) Innovations, LLC
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#29
Just curious about what kind of upgrade you are bought, what it is supposed to do?



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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