Jim Smart
December 1, 2000

All of the technical questions we receive from our readers, instrument function is one of the most oft asked. We’re asked why something doesn’t work or why it has gone completely haywire. But instrument and idiot-light function isn’t as baffling as it may seem. It’s just simple automotive electricity.

Your Ford's electrical system differs from your home system in how the electricity is managed. Household electricity is 110-115 volts of buzzing alternating current (AC). Automotive electricity is 12-15 volts of direct current (DC). In household wiring, we have two wires for function and one for safety. One is hot (black), one is neutral (white), and one is ground (green). Grounding a household appliance keeps alternating current from passing through humans to ground (electrocution). The ground wire carries short-circuited electricity to ground through the green wire, which keeps humans safe from electrocution.

With automobiles, we have a single hot wire (positive) that provides power to the lamp or accessory. The second wire (neutral) is the car's body, also known as the "ground," which carries electricity back to the battery to complete the circuit. This is what we mean by "negative ground." Most automotive electrical systems are negative ground, which means the negative battery post is connected to the body or "ground."

In an automotive electrical circuit, current flows from the battery’s positive post to the lamp or accessory to ground and back to the negative post via the car’s body and negative battery cable. On Ford vehicles, the negative battery cable is connected to the engine block. Because the engine block is insulated from the body by rubber engine mounts, it must be grounded to the car body via a ground strap between the cylinder head and firewall. Without the ground strap, electrical gremlins that cannot be explained sneak up on us quickly—dim lights, erratic accessory operation, engines that don’t start or stall without notice, a brownish glow in lamps that should not be lit, and other electrical oddities. These oddities are simply electricity following the path of least resistance to ground to complete the circuit. This explains the oddities just mentioned. The engine’s ground strap is very important to proper electrical system operation.

Instruments—How They Work
Instruments and warning lights function by the same principles of automotive electricity we’ve just described. Think of the flow of electricity like you would water through a pipe.

Switches are like water faucets. Turn off the switch (open circuit) and the flow of current stops. Switch it on (closed circuit) and you get current flow. Volume controls or dimmer switches are like water-flow regulators—they control the flow of electricity (water) through the circuit (pipes). When we think of electricity, there are two basic ways it’s measured. Current (amps) is electrical pressure, or how much “push” we have behind the electricity. Voltage is volume, or how much electricity we have.

Resistance to current flow is called ohms or impedance. The higher the ohm reading, the more resistance we have to current flow. The more resistance we have, the less power we have to or from an accessory or lamp. With resistance comes heat as a result of the electrical traffic jam. Increase the resistance and things get warm.

Fuel, oil-pressure, and coolant- temperature gauges function based on how much power is flowing through each gauge at a given moment. The more power we flow through the gauge, the higher the needle reading. How does it do that? By regulating the flow of electricity through the gauge two ways. On the back of your Ford’s instrument panel is the voltage limiter (also called an instrument voltage regulator). When you turn the ignition on, roughly 12-14 volts of electricity flow to the voltage limiter from the ignition switch.

The voltage limiter reduces 12-14 volts to approximately 5 volts which is where gauges are happiest. If we ran a full 12 volts through the gauges, the damage would be swift and permanent.

With a regulated 5 volts going to the gauges, it’s then a matter of completing the circuit from the gauge to the ground (negative), which is how we determine how full the fuel tank is, how hot the engine is, or what we have for oil pressure. Fuel, coolant temperature, and oil pressure all get their message to the gauge the same way: variable resistance to ground through a sending unit. Think of a sending unit like you would a volume control or a dimmer switch. Where the volume control and dimmer switch control resistance from the hot wire, sending units control resistance to ground. Remember—high resistance equals low current flow. Low resistance equals high current flow.

How do we control resistance in a sending unit? With a variable resistor that’s affected by coolant temperature, fuel level, or oil pressure. Each type of sending unit works differently, but they all do the same thing: vary resistance to ground. When resistance to the ground is low, current flow through the gauge is high and the needle moves toward the maximum reading. When resistance to ground is high, current flow is low and the needle remains on the low side.

Fuel Gauge
In the fuel tank, the sending unit gets its cue from the float, which is tied to a variable resistor (looks like a spring) via a float arm. When the tank is full, resistance through the sending unit’s variable resistor is low, and we have full power across the gauge. The needle moves to the full mark. As we drain the tank, the float drops and resistance across the variable resistor increases, like turning down the volume or dimming the lights. The needle begins to move toward empty.

When you’re getting an erroneous reading on a fuel gauge or no reading at all, check power to the gauge first. There should be roughly 5 volts. If there is power, the sending unit is most likely at fault. If the gauge doesn’t work at all, the sending unit probably isn’t completing the circuit to ground. Disconnect the sending unit and ground the plug directly to ground (the body). Watch the gauge for operation. If the needle goes to full, the sending unit is faulty. If the gauge does not respond, there is likely a break in the wiring between the fuel gauge and the sending unit. Most of the time, the sending unit is the culprit.

Coolant Temperature Gauge
The coolant temperature gauge works just like the gas gauge. It operates based on the amount of power passing through. We control the ground resistance from the temperature gauge through a sending unit screwed into the engine’s water jacket. Inside the sending unit is a variable resistor just like we have in the fuel tank, only it’s much smaller. The variable resistor is controlled by water temperature and a bimetallic contact. As the engine warms, the bimetallic contact moves across the resistor. And as the engine warms, there is less resistance across the resistor, which allows more power to flow across the gauge to ground. The needle moves toward maximum. If everything is working properly, the needle will climb into the normal zone. But what if the engine overheats? In this case, there is even less resistance across the sending unit to ground, driving the needle toward the dreaded “H.”

If the sending unit is bad, the temperature gauge will be either inoperative or it will climb to H and stay there. Two things can happen to a sending unit. Corrosion inside creates too much resistance, rendering the gauge inoperative. Or there is no resistance, driving the gauge to maximum. As with the fuel gauge, check for power to the temperature gauge first.

Oil-Pressure Gauge
Like the fuel and coolant-temperature gauges, the oil-pressure gauge reads based on the amount of power flowing across it to ground. The oil-pressure sending unit has a spring-loaded piston inside. The piston moves a contact back and forth across a variable resistor. Power flows through the contact across the resistor to ground where the sender is screwed into the oil galley passage. When there’s no oil pressure, there is high resistance across the sender, which leaves the needle at the far left side of the gauge. Fire the engine and put oil pressure to the sender and watch what happens. The contact moves across the resistor to a lesser resistance value, which increases the flow of electricity to ground from the gauge. The needle moves toward maximum.

When an oil-pressure gauge isn’t working, most of the time it’s a faulty sending unit or severed lead to the sender. When in doubt, ground the lead and watch the gauge. It should peg the needle.

We’re addressing this one separately because it is entirely different in function. Instead of operating on variable resistance like fuel, coolant, and oil-pressure gauges do, the ammeter operates based on current flow in and out of the battery. When there’s current flow out of the battery, the needle will swing left. If there’s current flow from the alternator into the battery, the needle will swing right of center. Ammeters are “hot” all of the time whenever the battery is connected. This means the ammeter is live with the ignition on or off. When there is no current flow in either direction, the needle remains centered. Whenever we have current flow in either direction, the magnetic field generated by current flow through the coil inside the ammeter moves the needle in either direction.

Where ammeters fail most is coil overheat. Believe it or not, the ammeter coil can overheat and start a fire because it has power all the time and is not protected by a fuse, circuit breaker, or fusible link (fusible links weren’t installed on Fords until 1969). This can happen only with current flow in either direction—overcharging or a short to ground. We suggest inspection of the ammeter coil at least once a year to be on the safe side. Discoloration or a burning smell is grounds for ammeter replacement. Should this happen, determine why before buttoning up the dashboard.

You can install circuit protection for the ammeter simply by putting an in-line fuse between the positive wire at the starter solenoid and the main power lead into the harness. Use a 20-30–amp fuse and 12-gauge wire. You can also install a fusible link instead of a fuse. This offers protection should the ammeter develop a short circuit.

What Else Can Go Wrong Now?
As you can see, troubleshooting gauges is easy. Most of the time, it’s not a faulty instrument, but a troubled sender, which has the toughest job of all. Senders are exposed to harsh environments—oil, heat, gasoline, and coolant. As a result, they fail more times than gauges.

If you turn on the ignition and all of the needles peg at max, replace the voltage limiter behind the instrument panel. When voltage limiters fail, they either create no resistance at all, which pegs the needles, or they open the circuit, which produces no reading at all. When the needles peg, this means the voltage-limiter contacts have fused together. And when the gauges sleep, this means there’s no contact whatsoever. In either case, replace the voltage limiter.

Whenever you’re troubleshooting your Ford’s instrumentation and are unable to find answers, get familiar with a voltmeter and an ohmmeter. A voltmeter or a test light will tell you if there’s power and where. An ohmmeter will tell you how much resistance there is across a circuit or a sending unit.

Idiot Lights
Ah, yes, the humble “idiot” light. This is a lamp without respect because it means we’re in big trouble by the time we see it. But this is the simplest warning device your old Ford has. And it’s not always “lights out, Pluto!” when it illuminates, either. Oil-pressure lights typically come on at around 5 pounds of oil pressure, in plenty enough time for you to shut it down and get to the roadside.

Temperature lights illuminate around 220-230 degrees, early enough to get to a place of safety for a cool down. The ALT or GEN light comes on the minute the battery isn’t getting a charge, which gives you time to find a repair facility or the home garage.

So how do warning lights work? The same way gauges do, through a controlled grounding in the sending unit. Oil-pressure and coolant-temperature lights receive power from the ignition switch. They light whenever we complete the circuit to ground. Instead of these guys being variable resistors, they’re simply an on/off switch to ground. Overheat the engine and the sending unit’s bimetallic contact closes the circuit and completes the electricity’s path to ground, “ohmygosh, an overheat!” The oil-pressure sender is a spring-loaded contact to ground. When there’s oil pressure, the sender is open with no current flow to ground. The lamp is dark. Lose oil pressure and the contacts close, yielding a path to ground, which illuminates the oil-pressure light.

ALT and GEN lights offer immediate news the battery isn’t getting a charge. Where the charge light differs from the ammeter is how it gets its power. When the battery is being charged (current flowing from the alternator/generator to the battery), there is no flow of electricity from the ignition switch via the voltage regulator to ground (light off). When the battery is discharging, there is flow from the ignition switch to the voltage regulator to ground (light on). With a charge light, there is no doubt about charging-system function. If the light is on, your charging system is in a state of discharge. If it is off, the system is charging, although it’s unknown to what degree. Aside from the possibility of a burned-out light bulb, the charge light is idiot-proof.

We get a few letters on these, too. The tachometer is a sensitive instrument that functions based on input from the engine’s ignition system. The tachometer gets its power from the ignition switch (ignition on) and its input from the negative (distributor) side of the ignition coil. Tachometers are rarely serviceable because they tend to be factory sealed. Absence of function is most often caused by a disrupted connection between the ignition switch and the tachometer or between the ignition coil and tachometer.

This one stumps people more than troubleshooting instruments. But car clocks aren’t any more complex than the Big Ben tick-tock sitting on your night table. Vintage car clocks are actually wind-up units with real balance wheels and mainsprings that electrically rewind themselves. If you listen to a working car clock, it makes perfect sense. They tick for a number of minutes, then rewind themselves with that resounding “click!” we’re so familiar with in older cars. That ticking is the balance-wheel movement at work, just like Grandpa’s old pocket watch. The “click!” is the solenoid that rewinds the mainspring movement.

Whenever you install a car clock, the movement needs to be wound (contacts opened to the maximum travel). As the clock ticks, the contacts slowly close. When the contacts touch, 12-volt power energizes the solenoid, which swiftly opens the contacts with a “click!” which rewinds the mainspring. The process begins all over again—tick-tick-tick until the contacts touch, energizing the solenoid and rewinding the movement again. Old car clocks work this way instead of an electric motor to conserve battery power. The occasional “click!” uses far less power.

Vintage car-clock movements get tossed in the trash when they’re actually quite easy to service. Old car clocks must be serviced periodically, which involves a very light dose of thin grease on the movement and dressing the contacts with an emery board. You can do this once a year and enjoy a working vintage car clock. Car clocks quit when contacts become dirty or burned, preventing the rewind solenoid from doing its job. Sometimes mainsprings break, rendering the movement deceased. Most of the time, it’s burned and dirty contacts that send most car clocks to their graves.

Unlike the rest of the instruments we’ve addressed, vintage Ford speedometers are not electrical. They’re mechanical, driven by a gear package and cable tied to the transmission’s tailshaft. Like the instruments we’ve already addressed, speedometers are simple in scope. Splines on the transmission’s tailshaft drive a small nylon drive gear tied to the speedometer cable that drives the speedometer head. The speedometer head is a magnet that spins around inside a hollow shell linked to the speedometer needle. The spinning magnet’s magnetic field attracts the shell that moves the needle. The faster the magnet spins, the higher the needle moves.

Speedometers get into trouble when cables bind or magnetic heads cease due to the absence of lubrication. Like the humble car clock, speedometers need periodic maintenance, too. The spinning speedometer head needs occasional lubrication (speedometer-head lubricant). Pull the cluster out, disconnect the cable, and feed modest doses of lubricant into the head once a year. Don’t overdo it. While you’re at it, pull the speedometer cable out and bathe it in white grease and a low-viscosity engine oil. This combination will keep it happy for thousands of miles.

Speedometer calibration is mostly a matter of using the correct drive gear at the transmission. We suggest finding a pace vehicle with a known accurate speedometer. Begin by taking the pace vehicle and heading for an open highway where there are mile markers. Get the vehicle speed to a steady 60 mph and look at your watch. If the speedometer is accurate, it will take exactly 60 seconds (one minute) to travel one mile.

If the speedometer proves accurate, saddle up your Ford and pace the test vehicle. If your speedometer reads high, you need a drive gear with more teeth. If your speedometer reads low, you need a drive gear with fewer teeth. A drive gear with 18 teeth is going to turn faster (and read faster) than a drive gear with 21 teeth. If your speedometer is reading 10 mph too high and you have an 18-tooth drive gear, you need a 20-tooth drive gear. The highest number of teeth available on a Ford speedometer drive gear is 21. After that, you must run a reducer that was common to Fords with 3.89:1, 3.91:1, 4.11:1, and 4.30:1 rearend gears. Then it’s back to the drawing board with drive-gear sizes. Be prepared to try several different sizes before lighting on the right one.

Tire size also affects speedometer readout. Taller tires will make the speedometer read lower. Shorter tires will make it read higher. Even tire inflation can affect speedometer readout.

Speedometer shops can rebuild and calibrate your speedometer. A rebuild involves new bushings and the replacement of any other worn or damaged parts. If you’ve got an unusual speedometer, such as a Shelby 0-140–mph piece, rebuilding the speedo may be your only choice. A speedometer shop like United Speedometer can help.

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