Jim Smart
May 10, 2010

If you've driven classic Mustangs for any time at all, you know that their instrument panels go haywire from time to time. Thankfully, Mustang instruments are easy to diagnose and fix once you learn how they work. Aside from the speedometer, which is mechanical and cable-driven, Mustang instruments are all electrical in function. And that's what we're going to talk about here.

Mustang instrument function tends to be about resistance to the flow of electricity across a circuit to achieve a given function. This resistance, also called ohms, creates heat. It also controls how fast something operates or how brightly. Ohms are an important element to understand when you're checking resistance values in a circuit.

Temperature, Oil, and Fuel
Temperature, oil pressure, and fuel quantity gauges operate on five-volts of regulated current thanks to a small voltage regulator on the back of the instrument panel, also known as a voltage limiter or voltage reducer. Inside, the voltage regulator has a bimetallic arm and heating element (resistor). It is this balance of heating element and bimetallic arm with contact points that regulates current flow to your instruments in half-second pulses of electricity. Voltage regulators fail when contact points burn and pit or the resistor burns out and doesn't get warm anymore. "Bimetallic" means two metals - two dissimilar metals that expand and contract at different rates to open and close contact points inside the voltage regulator. It is a micro-fine, constant pulsing process of contact points that keeps voltage close to five volts. Because vintage Mustangs were born in the age of AM radio, this pulsing causes radio interference. A radio noise suppression choke wired in series with the constant voltage supply lead reduces the noise.

These five-volts of regulated current from the voltage limiter flow through your instruments, which have their own unique function inside. Temperature, oil pressure, and fuel quantity gauges are all identical instruments behind the face, although they have different sending units and faces. All sending units, which are variable resistors (also called rheostats or potentiometers), do the same basic thing. They vary current flow to negative ground to provide readings on the instruments. When current flow is high to ground, instruments read high. And when current flow to ground is low, instruments read low. Each gauge's sending unit works like the volume control on your stereo or dimmer switch. Raise the volume or brighten the light and you are reducing resistance to current flow (low ohmage). Lower volume or dim lights and you are increasing resistance to current flow (high ohmage).

Each instrument (except ammeter) works similar to the voltage regulator - heat and its effect on dissimilar metals cause an immediate expansion along with needle movement. Each gauge has a bimetallic band tied to the needle. And each has a heating element (resistor) designed to act on the bimetallic spring and needle. When current flow across the heating element increases along with corresponding temperature, the bimetallic spring expands, moving the needle higher on the gauge face. When current flow is low, the heating element cools, causing the bimetallic spring to contract, moving the needle back to the left. When you turn the ignition off, current flow stops altogether, causing instrument heating elements to cool and returning needles to rest. These bimetallic springs are very sensitive so they respond quickly to temperature change.

Each sending unit varies resistance to current flow to negative ground. The coolant temperature sender in your engine's water jacket has an electrical conducting spring, temperature sensing element, and heat conducting disc inside. Like other sending units, it operates on variable resistance. When you start a cold engine, current flow across the temperature gauge is nil and the needle remains on "L." As the coolant warms, resistance across the sender decreases, allowing more current flow to ground from the gauge. As resistance decreases with heat, current flow across the instrument increases to move the needle higher.

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Oil pressure senders work the same way, only with spring-loaded variable resistors instead of a bimetallic contact. Oil pressure acts on the piston and resistor. When there's low oil pressure, there's high resistance to ground and current flow to ground is low, which makes the instrument read low. When pressure is higher, there's less resistance across the sender and oil pressure reads higher.

Fuel quantity works the same way, only with a float in the tank with a variable resistor. When fuel is low, resistance across the sender to ground is high and the gauge reads low. Fill up the tank and the float rises, reducing resistance across the sender and increasing current flow to ground, which makes the gauge register "Full."

Idiot Lights
Warning lights, also affectionately known as "idiot" lights, are simple in scope and easy to troubleshoot. A coolant or oil pressure warning light works off a grounding "on/off" sender, which is either open or closed. When it is closed, the warning light illuminates. When open, the light is off. A coolant temperature light (TEMP) closes at a specific temperature, typically around 210 degrees F. Like a coolant temperature sender for a gauge, there's a bimetallic switch inside. However, instead of a variable resistor, it's a simple on/off function with contact points. An oil pressure light illuminates at 10 psi when contacts close to illuminate the OIL light. When you start the engine, oil pressure is low and the switch is closed. The oil pressure builds right away, contacts open, and the light goes out.

GEN/ALT lights work an entirely different way. When the ignition is on with the engine off, power flows through the ALT light and a parallel 15-ohm resistor (shunt) through the voltage limiter contacts to the alternator field. The voltage limiter is inside the voltage regulator. Battery voltage through the light is enough to allow the alternator to make electricity once the engine is started. The ALT light is shunted via the 15-ohm resistor just mentioned. When the engine is started, enough voltage is made to close the field relay contacts in the regulator, causing the ALT light to go out.

With generator charging systems, the GEN light is connected between the armature terminal on the voltage regulator and the coil terminal on the ignition switch. This makes the GEN light an integral part of the circuit where it is in parallel with the voltage regulator contact points. If the ignition is turned on with the engine off, the voltage regulator contacts will be open and the GEN light will illuminate. When the engine is started, the regulator contacts close. Power then bypasses the GEN light and the light goes out.

ALT and GEN warning lights are simple to understand and troubleshoot based on the previous description. These warning lights illuminate only when the battery isn't being charged. When they become inoperative, it's typically a burned out bulb or a break in the wiring. Keep the black/yellow wire (37) squarely in mind and you can't miss.

Ammeter, as its name implies, monitors or measures amperage flow in and out of the battery via the alternator. It does not measure battery voltage like a voltmeter. As the alternator charges the battery, an ammeter reflects this charging process as the needle swings right. As the battery discharges, the ammeter also shows this process as its needle swings left. It all happens via a magnetic field caused by the flow of current across the ammeter. Two basic types of ammeters have been employed in classic Mustangs. In '65, Mustangs had an induction-style ammeter with no connections, only an induction loop (also called a magnetic loop) on the back of the ammeter that picks up current flow (magnetic field) through the main power lead.

For '66 and beyond, Ford thought it had a better idea in its shunt-style ammeter where two leads - red and yellow in color - bring current through the ammeter. The red lead goes to the battery and yellow to the alternator. The problem with these post-'65 ammeters is that they weren't durable enough for the demand. Most burned up and failed early in their service life, which is why your ammeter needle probably never moves. We looked at many '66-'73 Mustang ammeters in the used parts inventory at Mustangs Etc. and were amazed at how many of them had burned up and failed. Most do not work.

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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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