Jim Smart
December 22, 2006
Here's the variable resistor, located in the heater plenum.

Controlling The Flow Of Power
Electrical component operation depends on how much power we route through the component to ground. It is easy to assume we control the flow of power before the component, but we can also control the flow of power to ground. Heater-fan speed is controlled that way. We control the flow of current to ground after the component instead of at the switch by having a two- or three-way switch that routes power to ground via a two- or three-wire resistor in the heater box. The resistor is in the heater box in order to run cooler, keeping heat away from the switch. This is also the case with in-dash air-conditioning systems.

When you flip on the fan switch, power is routed from the fan motor to ground via the switch. In other words, we already have power at the motor. Without a ground, the motor does not function. When we complete the ground, the motor functions. If we create resistance to the flow of electricity to ground, we control fan speed. The more resistance there is, the more slowly the fan operates. The less resistance the faster the blower wheel spins.

The fan switch routes power to ground through this variable resistor.

Heating and air conditioning fan switches are not variable resistors. They are contact switches that route electricity from the motor to a variable resistor to ground.

Air-conditioning systems function via a couple of traffic-cop switches.The compressor clutch is an electromagnet that engages the pulley and compressor shaft when it is energized. We energize and engage the clutch via the fan and thermostatic switches with under-dash air conditioning. With in-dash air conditioning, we have to jump three hurdles to get power to the clutch. This means if your compressor clutch isn't engaging, you need to check all of the switches.

With under-dash air conditioning, bypass switches one at a time to determine fault. If both switches check out good, test the compressor clutch by running a jumper lead from the positive terminal to the compressor-clutch lead. If the clutch doesn't engage, it's faulty. Don't forget to check grounds while you're at it. A faulty ground will stop clutch engagement as quickly as disconnecting the battery.

This is a typical three-speed fan switch. It is not a variable resistor as many believe. Resistance takes place at the variable resistor at the heater plenum.

In-dash air-conditioning compressor clutch engagement requires that three switches work properly-the fan switch, the thermostatic switch, and the vent-door switch. All must make contact in order to get power to the compressor clutch. On newer Fords, you must have a pressurized air-conditioning system (serviced with refrigerant) or the pressure switch won't allow operation either.

Reliability depends on the switch and how often it is used. Windshield-wiper switches are normally reliable. Headlight switches seem to have a lifespan of five to seven years in regular use. Some last five years-others last 40. Much depends on climate, conditions, and use. Headlight switches fail when circuit breakers cycle headlamps off and on. Corrosion gets the best of the contacts inside.

It is easy to improperly install an alternator wiring harness during replacement. Snap photographs of your original alternator before removing the harness. Checking your Ford Shop Manual is also a good idea. This is something you don't want to get wrong.

Charging And Starting System Problems
Likely the largest electrical system struggles for most of us are charging and starting systems. Defective voltage regulators-especially mechanical types, cause dead batteries and no-charge conditions. Mechanical regulators suffer from burned contacts, which hamper charging. Rarely will it be the generator or alternator.

Dead batteries call for a slow trickle charge, known as deep cycling, over an eight-hour period, then a load test. If the battery doesn't survive a load test after deep-cycle charging, it's defective. This can be tested at any reputable auto parts store. Because these folks sell a lot of batteries, alternators, and starters, they're equipped to check your battery. They are also set up to test alternators and generators.


Starter power begins at the battery and solenoid. Solenoids are nothing more than electromagnets-heavy-duty switches (relays) we energize with the ignition switch-that get power from the battery to the starter and ignition coil. When you turn the key to start, the "S" lead fires the solenoid, which makes contact between the battery and starter. At the same time, power passes through the "I" lead to the ignition coil. Solenoids fail for two basic reasons: They can stick in the "on" position and continue cranking our engines, or they just don't work at all due to internal failure. They cannot be serviced, only replaced.

Getting Started
Starters fail primarily because they work in a hot and dirty environment. Brushes become contaminated. Breaks develop in the copper windings. Extreme header heat, especially after shutdown, can cause a starter to seize up short term until things cool off. Starter drives can also fail, making it impossible for the starter to engage the flywheel or flexplate. Sometimes all your starter needs is clean-up and lubrication. White grease and WD-40 applied inside the Bendix starter drive makes all the difference. Clean slippers and brushes can also make an old starter like new again.

Starters get blamed for solenoid woes, too. It isn't always the starter. Check the solenoid for continuity by disconnecting the starter and checking for contact at the terminal with a test light. Listen for the "click" when someone turns the key to start and watch the test light. If the test light doesn't illuminate or you don't hear the click, further investigation is necessary. When solenoids don't respond, it isn't always the solenoid. Make sure the "S" lead is getting power to the solenoid from the ignition switch. Sometimes the ignition switch has burned and pitted contacts. The best solenoid function test is bridging the path between the battery terminal and the "S" terminal with a screwdriver, taking care not to ground the screwdriver (or better yet, a remote starter switch). If the solenoid clicks and your test light illuminates (starter terminal), the solenoid is good.

How Do Senders Work?
Instrument senders complete a circuit to ground. There are two basic types of senders: on/off and variable resistor. On/off types complete the circuit to ground to illuminate the light and get your attention. On/off oil-pressure senders complete the circuit to ground when oil pressure drops below a predetermined point. On/off coolant temperature senders complete the circuit to ground when coolant temperature gets too high.

Variable resistor senders vary the resistance to ground, which controls instrument reading. When there is no resistance to the flow of electrons to ground, the gauge maxes out. When there's a lot of resistance to the flow of electricity to ground, the instrument reads low. Instrument senders control the flow of electricity to ground to either illuminate a warning light or move an instrument needle.

The fuel-sending unit is another example of a variable resistor, which works off a float in the fuel tank. When the tank is empty, resistance is high and the needle rests at "Empty." When the tank is full, resistance to current flow is low and the needle races to "Full." A quick test for gauge function and wiring integrity is to ground the sender's wire lead. With the key on, the light on the dash will illuminate or the mechanical gauge will peg to the high side (full, hot, and so on).

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