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1971-1973 MUSTANG TACH/FUEL/AMP/OIL/TEMP.GAGE TESTING.
#1
Hi Was wondering if anyone could chime in on a question I have concerning 1971-1973 Mustang Instruments? I would like to know if anyone out there knows how to bench test a tachometer/ speedometer, temp, oil, battery, fuel gauge. The tachometer is the most important one at this time. Would like to test my stuff prior to install Thanks Aypsears
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#2
The Tach works by using the points as a source of a PWM wave. As the points open and close they approximate a square wave at a frequency corresponding to the firing of the cylinders. So if you have a signal generator you can create this wave for a known perfect RPM point. Let's say 600 RPM (because it makes the math cleaner) So at 600RPM you're at 100RPS or 100Hz. So a 100Hz Square wave fed through the Tach should create a reading of 600RPM. What I'm not sure of is the voltage, I think it should be 12V but it might be 6V, this point has always slightly confused me.

The Temperature, Oil, and Fuel gauges, all use a scale of 0-5VDC (Based on the voltage regulator on the back of the tach cluster) so you should be able to feed them a straight voltage, sweep it from 0-5VDC (make sure you current limit yourself, shouldn't take much to sweep them) and make sure you get good sweep from the needle.

The Ammeter measures current, and is a little harder to test. I'm not sure what its' "Nominal" or "Centered" value is supposed to be, someone a little more knowledgeable would need to chime in on that.

I'm not sure if you can really test the speedo without having some way to spin the cable at a known "speed" and I'm not sure exactly what speed it spins at since I know there are some gears involved in that.

Hope this helps!

2013 Ford Focus SE Flex Fuel 5spd - Daily Driver
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#3
Sorry, MechEng, but your math is faulty.
Quote:et's say 600 RPM (because it makes the math cleaner) So at 600RPM you're at 100RPS or 100Hz. So a 100Hz Square wave fed through the Tach should create a reading of 600RPM. What I'm not sure of is the voltage, I think it should be 12V but it might be 6V, this point has always slightly confused me.
I believe 600 revs/minute corresponds to 10 revs/second, or 10 Hz.

To bench test the ammeter, briefly attach a C or D battery (with leads/alligator clips) to the two ammeter leads. You can test both sides of the swing. Be very brief, as there's very little resistance in the ammeter gauge, and you can burn up the wiring fairly easily.
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#4
Midlife, you're exactly right on that. My math would be right for 6000 rpm not 600! Thanks for catching that. I should know better than to try and do math early in the morning


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2013 Ford Focus SE Flex Fuel 5spd - Daily Driver
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#5
MechEng;169607 Wrote:Midlife, you're exactly right on that. My math would be right for 6000 rpm not 600! Thanks for catching that. I should know better than to try and do math early in the morning


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The battery thing works fine for testing sweep gauges. Amp gauge it works also, but TAP the gauge with the test leads from the battery, do not hold it. oil and water move slowly, amp gauge snaps to position suddenly and will melt if held more than a couple of seconds.

For tach, I have signal generators...ac or dc will work...or in a pinch, any car with a 12v coil unplug the 12 power lead from the coil...jumper to the male lead of your tack...jumper female lead to the coil terminal that you unplugged, now 12v lead to coil is passing THRU the tack...start car. it will work normally or it will do weird stuff, but that connection is identical to what it does under the dash...just upstream of the resistance wire...but positive side input straight to coil
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