I have a 1989 f350 dually with a 460 7.5ltr engine. My problem is the battery seems to be holding a charge but when i turn the key on the voltage drops to almost 7 vdc, and therefore not enough to turn over the engine. And i was wondering what is the round thing the battery pos cabble hooked to? it seems to be connected with the alternator as well. I found a frayed and badly decayed wire on this. Maybe this is the problem?

google ... <diagram for drum brakes on a 1989 ford f350 dually> . choose desired diagrams to enlarge.

Someone would need to check the circuits to find the short to ground or a component that is using power.
We usually start with the alternator and check the major components first.

you should have 12.5 volts with engine off at battery and 14.4v with engine running and charging system working. Is alternator light on? please rate, thx.

There are different versions and 1989 is too old to get on line. here is 1993, so take with caution. Also check the fusible link.

Battery Positive Voltage (B+) Output
The generator (alternator) output is supplied through the Battery Positive Voltage (B+) output connection to the battery and electrical system. I Circuit
The I circuit, or ignition circuit is used to turn on the voltage regulator. This circuit is powered up with the ignition key in the RUN position. This circuit is also used to turn the indicator on if there is a fault in the charging system operation or associated wiring circuits A Circuit
The A circuit, or battery sense circuit, is used to sense the battery voltage. This voltage is used by the regulator to determine the output. This circuit is also used to supply power to the generator (alternator) field coil. This circuit is connected back to the load distribution point and is a protected circuit. S Circuit
The S circuit, or stator circuit, is used to feedback a voltage signal from the generator (alternator) to the regulator. This voltage, typically 1/2 battery voltage, is used by the regulator to turn off the indicator.

Did you put a battery of equal or larger cranking amps, if you have dual alts it would be very noticable as you would see 2 alternators. You may also need to replace the other battery also, as this is not a primary and secondary batteries but equal batteries.

1989 f250 4x4 lighting problems. i replaced the floor mounted dimmer switch ,the lights worked fine for an hour untill i turned them off.Now the lights will not turn back on but i have park lights.

not hard at all, remove the driveshaft, undo the holding nut(impact tool helps a lot) you may need a peuuer to remove the yoke, pry the old seal out and gently tap the new one in, the refill the diff

You say that it has power, yet it acts as if the battery discharged due to a faulty alternator or voltage regulator. Turn the headlights on and try to start the engine. If the lights go out when you try, suspect the battery has lost charge, or a poor connection to the battery. Manually recharge the battery overnight to see if the engine will start. If it does, take the car to an auto parts supply who will test the charging system for free. If the alternator has a separate V-belt, check it's tension and condition. Hope this helps!

Check the fluid level in the reservoir for the slave cylinder, or look underneath to find a leak. If the reservoir is empty then you need to replace the clutch slave cylinder. A quick fix is also try to bleed the clutch slave like you would bleed brakes, open the bleeeder push the pedal down and hold it until the bleeder is closed, repeat until solid fluid comes out...will take 2 people.

Sounds like a bad battery connection, either on the battery, or the starter or engine ground, or just a poor battery.

sounds like you need to have your alenator checked just to make sure its charging.