A new headlight switch can...
A new headlight switch can cure a lot of ills, including nonworking instrument panel lights.
When you turn on a turn signal-let's say the left one-two things happen. First, the circuit for the left brake light is interrupted, and second, the circuit of that brake light is connected to the turn signal flasher and the flashing sequence begins. This is why when you're stopped with the left turn signal on, the left brake light flashes but the right brake light stays on. The nighttime dimmer taillights are unaffected.
Sometimes a brake light won't work because the switch has failed internally or has become weak. The weight of the turn signal lever can take it just past center, thus interrupting the brake light circuit but not quite starting the flashing sequence. It can be a real puzzler. In this case, the switch must be replaced.
Dash Lights Don't Work
The dash lights get their current from the headlight switch. Current flows from the battery to a headlight-switch contact; through a rheostat on a white, ceramic turn wheel; then to the bulbs. The rheostat is a small circular spring installed in the ceramic wheel that dims or brightens the dash lights as you turn the headlight-switch knob. The spring can get rusty or faulty and cause dead spots or not light the bulbs at all. In this case, the switch needs to be replaced.
In '65-'68 Mustangs, the instrument-cluster housing grounds to the car when it's installed, so keep that in mind when testing. Sometimes you need to attach a ground wire from the housing to the dash frame to perform a good test of the bulbs. The '69-'73 Mustang dash lights ground through the printed circuit board.
A bad constant voltage regulator,...
A bad constant voltage regulator, located on the back of the instrument cluster, will prevent the gauges from reading correctly or at all. It can be inspected with a test light.
Fuel, Temp, or Oil Gauges Don't Work
Your gauges are really very simple. Current flows from the ignition switch to a device called a constant voltage regulator, to the gauge, and then to a sending unit. The voltage regulator changes the 12-volt current to a pulsing lower-voltage signal and then sends it to the gauge. The sending unit of that gauge acts as the ground to complete the circuit. It increases or decreases the quality of the ground, and the gauge reads the average voltage of the circuit through the pulses. If a gauge isn't working and it's definitely a good gauge, it either isn't getting the pulse from the voltage regulator or the sending unit is faulty.
Check the voltage regulator with a test light by checking for 12 volts or a pulsing signal. If all is well, examine the sending unit by removing its boot terminal and testing it for the pulsing signal. If you have a pulsing signal, then your sending unit is bad. If you have no signal at all, then the gauge is at fault. You can test your gauge by grounding the sending unit's boot terminal directly to ground to quickly see if the gauge reads all the way. If it does, it's okay. Don't let it stay grounded for long or you could damage it.
Another problem is that gauges may not be perfectly centered in the cluster housing, allowing one or both of the posts to touch the edge of the opening. If the hot post touches, it can short the gauge and it won't read properly. If the sending-unit post touches, the gauge will jump to full when you turn the key. The gauges must be perfectly centered with the posts not touching the metal cluster housing.
A test light checks for a...
A test light checks for a pulsing signal from the voltage regulator. If you get the pulse, the sending unit-in this case, for oil pressure-is probably bad.
Won't Start
It's frustrating to hop into your Mustang, turn the key, and nothing happens. There are a number of things that could be the problem, but nothing that can't be diagnosed and fixed.
If nothing is happening or there is a clicking sound from underhood, the first thing to do is get a test light, remove the boot terminal from the starter solenoid terminal marked "S"- usually a red-with-blue-stripe wire-and have someone turn the key while you test the terminal to see if the light turns on. If it doesn't, the start circuit from the ignition switch is at fault. In that case, either the ignition switch is bad or the circuit in that red and blue wire needs to be checked. If your Mustang has an automatic transmission, the neutral safety switch could also be bad.