Vintage Mustang oil pressure...
Vintage Mustang oil pressure senders look like this. On the left is the larger sending unit for gauge use. The smaller unit on the right is for warning lights. The larger sender is a variable resistor, which varies gauge reading depending on oil pressure. The smaller unit is a simple on/off switch, which closes (oil light comes on) at 3 to 7.5 psi oil pressure.
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."
Coolant temperature sender...
Coolant temperature sender resistance test shows 277.7 ohms cold, which is a lot of resistance. The gauge reading would be "C," for Cold, because there's virtually no power flowing across the gauge. Recommended resistance is 125 ohms cold for '65-'69 and 175 ohms for '70-'73. In actual practice, it doesn't make much difference.
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.
Checking a coolant temperature...
Checking a coolant temperature sender on the vehicle shows 385.3 ohms or high resistance. Again there's no instrument response-or needle on "C."
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.
Here's an oil pressure sender...
Here's an oil pressure sender for a warning light. With no pressure, there's virtually no resistance at 4.8 ohms, which means we'd get an "OIL" light. The oil pressure light should illuminate at 3 to 7.5 psi.
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.
Coolant temperature senders...
Coolant temperature senders vary from generation to generation. On the left is '71-'73. On the right is '65-'70, which is smaller with a single, larger pin.
Here are two basic types of...
Here are two basic types of Mustang ammeters-induction-style on the left with an induction loop ('65 only) and shunt-style on the right with two posts for '66-up. Although the induction-style isn't as sensitive, it is safer and more reliable.
Here's the '65 induction-style...
Here's the '65 induction-style ammeter installed with the main power lead (37, black/yellow) passing through. This is how an induction-style ammeter "senses" current flow. There are no connections, only the main power lead passing through the induction loop.
This is a '66-up shunt-style...
This is a '66-up shunt-style ammeter where alternator/battery power flows right through the instrument. When these ammeters work, they're quite sensitive to charge and discharge.....
....Problem is, very few of...
....Problem is, very few of them have continued to function through the years because they didn't stand up to electrical load very well. This one has burned up and no longer functions. Fragile wiring became so hot it failed.
Here, we're checking a shunt-style...
Here, we're checking a shunt-style ammeter for continuity. Because there's a break in the wiring, the multimeter shows no continuity.