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.
Mustang temperature, oil pressure,...
Mustang temperature, oil pressure, and fuel gauges are all the same inside. Only the faces and senders differ.
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).
This is a typical voltage...
This is a typical voltage regulator, which can be found on the back of Mustang instrument panels. Twelve volts enter and approximately five volts exit to provide regulated instrument voltage. When instruments stop working, most of the time it's a faulty voltage regulator. Word to the wise is to use a genuine Motorcraft voltage regulator because some aftermarket replacements have proven troublesome. Trouble-free, solid-state voltage regulators are also available.
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.
 Each instrument has two posts-one...  Each instrument has two posts-one with five volts of power (positive) and one to sender (negative ground). |  Here's what the inside of...  Here's what the inside of a temperature, oil pressure, and fuel gauge looks like. Two elements contribute to gauge function-a bimetallic strip and heating element (resistor). As current flows through the resistor (heating element), it gets warm. As it warms, it acts on the bimetallic strip attached to the needle (by expanding), causing the needle to move to the right. The geared elements are the instrument's calibration points. |  As you face the back of the...  As you face the back of the gauge, there are two calibration points. Lower left is adjusted with the ignition off to find true "zero"-"E" on a fuel gauge or "L" on a coolant temperature or oil pressure gauge. Upper right is adjusted with the ignition on with five volts of power to the gauge. This is where you adjust the needle for actual conditions. For example, "F" for a full tank or normal range with engine operating temperature at approximately 180-200 degrees F. Same is true for oil pressure. |
 Here are two voltage regulators....  Here are two voltage regulators. On the right is '65-'68 Mustang. On the left is '69-up as used on printed circuit instrument panels. |  Here, a coolant temperature...  Here, a coolant temperature sender is checked for proper operation by submerging in hot water. At 180 degrees, there should be approximately 40 ohms of resistance, which would put the needle in the normal range. |  Oil pressure senders should...  Oil pressure senders should read 24-36 ohms of resistance at 8 psi or less and 8.0 to 17.5 ohms at 90 psi (gauged maxed out). Here, at 39.8 ohms of resistance, we're just starting to make pressure. This test is performed using air pressure. You can also conduct this test on the engine with the engine running. |