Jim Smart
May 10, 2010

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
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.

Photo Gallery

View Photo Gallery