Jim Smart
May 10, 2010

So what's the fix for burned out ammeters? There really isn't one short of having yours converted to a voltmeter by Auto Instruments. Auto Instruments can convert your ammeter to a voltmeter so the needle is centered when everything is normal (around 12-14 volts). We've learned from Ray Sanchez at Mustangs Etc. that classic Mustang shunt-style ammeters are problematic and can be an unsafe fire risk because they're live all the time and not protected with a fuse or fusible link. This is why it's a good idea to have Auto Instruments do a voltmeter conversion, which makes things reliable and safe.

Your Mustang's tachometer gives an rpm readout based on frequency of ignition point closures. As engine rpm increases, the greater the frequency of these closures per minute along with a higher tachometer reading. The tachometer will have two leads - one red and one black. Red is a male plug that gets its power from the ignition switch, known as "switched" power. The black lead with a female plug goes to the positive side of the ignition coil. When tachometers fail, it's either a bad connection or a problem in the tachometer itself. Tachometers rarely fail. They're as reliable as oil, fuel, and coolant temperature gauges.

When a tachometer fails, the engine typically won't start either because the ignition coil gets its power via the tachometer. In fact, if you disconnect the factory tachometer, your engine will not start. Mustang factory tachometers get their signal via ignition points cycling (closing). With electronic ignition, the basic principle is the same - the cycling of coil collapses and spark plug firings.

Tachometer service and calibration isn't something you should tackle yourself. When your tachometer becomes inoperative and all troubleshooting efforts are exhausted, we suggest sending it to Auto Instruments for service and calibration.

Questions and Answers
My gauges won't register with the ignition turned on. What's wrong?
Probable Solution: Replace the instrument voltage regulator.

Question: Can my instruments be calibrated?
Answer: Yes, you can calibrate them yourself or send them out for calibration. Ideally, you will calibrate them yourself on the vehicle.

Question: How do I know if the problem is the sender or gauge?
Answer: Go to the sender, disconnect the plug, and ground it directly to the body or engine block. If the gauge moves to maximum, replace the sender. If the gauge is unresponsive, suspect the gauge, wiring, or voltage regulator.

Tapping on the Gas Gauge
It is surely an old cliché, but knowing true fuel quantity has never been an exact science. Just because the gauge is on "E" doesn't always mean the tank is empty. Or, it can be empty but the fuel gauge shows a quarter tank. Here's how you can get it spot on.

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When it comes to fuel quantity, there are three forms of calibration designed to get you close to what's in the tank. First, don't begin with handicaps such as a bashed in fuel tank, damaged sender, or trash in the fuel tank. Begin your calibration process with good parts and a clean tank. We're going to show you how variable resistance translates to needle position on a fuel gauge using a multimeter (ohmmeter) and a simple 12-volt light bulb.

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