This should cover the basics for both high and low windshield cars. The stock instrument gauges can be trying at times and they all have their own unique characterisitics. Think of them as seperate entities except for the temperature and the fuel gauges as they have the little voltage regulator under the steering wheel in common.
A word of advice from Eric "In your link for electrical problems, you talk about the voltage regultor for the gauges (under steer. column). People should know that the regultor metal case IS A GROUND!, and is needed for the regulator to work properly. I got a brand new one, and it was working intermittently until I realized the case was a ground, cleaned the area where it mounts to, and fixed the problem. Probably better to attach a wire to the case where it mounts and run a seperate ground wire. Hope this may help someone w/ a gauge issue!"
Let us troubleshoot the voltage regulator first. It is the small rectangular shaped metal box mounted to the right, and above the steering column, right behind the dash. A word of note-it is usual for this box to get a little warm-so do not be alarmed. There are two wires on it. Using a volt meter check the voltage on the two wires connected to the regulator. With the key on you should have 12 Volts to it. The other wire should pulse on and off, going from 12 Volts to zero, about one to two times a second. If you have voltage on one wire, and zero on the other, the regulator is probably bad. The regulator is still available and relatively cheap Early PN 25020-14600-Late PN 25020-25900
The temperature and fuel gauges have a common power source. The temperature sender has a single wire connection and is located on the drivers side of the head close to the thermostat housing. Again, with the key on, placing this wire to ground should swing the gauge to full hot. If it does not then there is either a problem with the gauge or the voltage regulator under the dash. The fuel gauge works the same as the temperature gauge. The wires to the fuel sender are located under a plate located on the trunk floor. Remove the fours bolts securing the plate and you will find the two wires. One is a ground and the other has a clip on style that slips over the fuel sender pin. Ground this wire to see if the gauge goes to full. Again, if it does not swing to full the problem is either with the gauge, regulator or the sending unit.
The ammeter or shall we call it a fusible link handles the main power from the battery/ alernator to the rest of the electrical system. If it is bad nothing works. You are dead in the water. They have been known to short out and burn up. To check the ammeter gauge turn the headlights on with the engine off. The ammeter needle should move to the left towards the negative (-) side. The ammeter in a normal state indicates zero. The exception to this is when power is flowing in or out of the battery. Under normal conditions with the engine is running the battery will be fully charged with the car running off the alternator.
The oil pressure gauges are very unreliable in these cars. Your best bet is to put an aftermarket mechanical gauge under the dash so you can really see what is going on. You can use a tee fitting at the block to keep the old gauge in place. On the later cars they went to a mechanical gauge with an oil line to the gauge. I am not sure when this occurred but it will be very obvious to you once you look. Early oil pressure senders were electric and can be tested by grounding out the single wire to see if the gauge swings on the high side with the ignition key in the on position. Five to ten PSI at idle in not uncommon. A freshly rebuilt motor will show between 40 to 50 PSI at highway speeds.