As laroke said, a condensor on #4 is for the radio.
If it was shorted, it would connect the alt "field connection" to ground through the regulator at all times and also the battery to ground when the ign key is on. The condensor is probably good.
You don't have 18 volts. If you did, all your light bulbs would be gone, the alt would be smoking, the bat would be cooked dry, the heater fan could clean out the inside of your truck and the cigarette lighter could cook lunch.
Take your voltmeter and compare it to another one. Or, lift the hood of some other car. One that runs good and has no electrical problems of any kind. A good battery, sitting in a car with nothing on and no drain, save for a clock, should read 12.5 volts. If your voltmeter reads 12.3-12.7 volts, then consider it good.
Now go back to your truck. Start it up, turn on the lights and check for bad grounds using the voltmeter. I said use the VOLTMETER. Do not use the ohmmeter. Do not look for bad grounds with your eyeballs.
That means that if you measure the voltage from the neg side of the battery to the frame, the voltage should be "0" volts. That means that if you measure the voltage from the neg side of the battery to the engine block, you should have "0"(zero) volts. That means that if you measure the voltage from the neg side of the battery to the distributor housing or alternator housing, you should have "0" volts. That means that if you measure the voltage from the neg side of the battery to the firewall and dash frame, you should have "0" volts.
By using the same idea, if you measure the voltage from the battery positive post to the starter solenoid large post, you should have "0" volts. If you measure the voltage from the battery positive post to the alternator "Bat" post, you should have "0" volts. If you measure the voltage from the battery positive post to the #3 post on the regulator, you should have "0" volts.
Voltage drops of a couple of tenths of a volt can be expected. Find the big drops and fix them first.
