Originally Posted by veganxxx
I have an 88 f350 that I'm working with. The problem is that the batteries die even after a drive and even if they don't die all the way the truck has a real hard time starting.
"hard starting" is one problem, "batteries drain" is another, both may be related to the starter itself of course. If the starter doesn't turn "too fast to count revolutions" then something is out of spec. But first, do a battery test. Disconnect the cables and let the batteries sit for half an hour. Check the battery volts, which should be over 12; if they are not, charge the batteries, let them sit half an hour, and repeat until they are over 12 (12.6 is a "fully charged" battery) or you give up and decide the batteries are bad. A good charging voltage is 1.5 to 2 volts over battery voltage. Also test for AC volts across the battery terminals, if you get more than 20mV then your diode pack is bad. If your battery is not charging then you probably have a bad alternator or regulator.
Try disconnecting the voltage regulator overnight to see if your batteries are still drained. Also the starter relay. If that doesn't help you need to play investigator with a trouble light or ammeter between one battery (the right-hand one) and the clamp, and pull fuses/relays until it goes out. Fun times.