I have a 1991 f250 4x4, short or ground problem. If I pull the negative cable off batttery,truck dies..does the same when I do the positive cable....It has a new alternator and battery. any Ideas on where to start to look for problem,any help will be appreciated. Thank you

 

Ho, Ho, Ho Wylee,
The ole days of pulling off a battery cable to check a charging system with an alternator don't work too good since the output of the alternator is no longer controlled and the little elves in there may cook the alternator. That worked with a DC generator, but should not be considered with an alternator because there is no place for the output to go and the alt can go crazy.
Would you care to tell us why you changed the alt and what was the original problem? Just in case the vehicle has no other problem, Merry Christmas!

 

I got in it to go to work and the lights went dim started chugging and wouldn't start again. checked voltage on battery 5 or 6 volts charged battery started it and the voltage kept dropping,so I replaced alternator still kept doing it got to the auto parts checked battery.drove it home next day same thing.so I got on line and did some research and started checking on things,came across a test where you you hook up a test light between the + post and the + cable it light up so I started pulling fuses, it went out on #8 interior,dome,courtesy,warning buzzer and radio. so I took out the radio and checked all bulbs.still don't know whats going on FRUSTRATED!!! Any help will be great.....Merry Christmas. and Thank You.

 

Here is something you can try,
You have a voltmeter, now just connect it to the battery terminals and it should read somewhere around 12 volts if there is sufficient charge in it. If the vehicle will start, with the meter still connected to the battery, and engine running the voltage reading should go up to around 13-14 volts. If it does the alt is putting out something. If it doesn't ,then the alt is not putting out anything. If you aren't getting any output it could be the alt is no good, poor connections with the system or high resistance in the wiring from the alt to battery where the system thinks the battery is fully charged and cuts the output . A thorough check of the ground connections never hurts.

 

could it possibly be a weak fusable link? or do they just go all the way bad.

 

It's possible , but not likely.
What results did you get with the last suggestions I offered?

 

voltage stays the same,I had the alternator tested by the auto parts it tested good.

 

What do the plug connections to the alt look like? A thorough check of the connections for corrosion and looseness is always called for.
I believe your vehicle has an external voltage regulator to control alt output.Have you checked that?

 

3 small wires on a plug and larger plug below it also has 2 large wires and 1 small wire. I'm wondering what if any voltage should be on the top plug 1 green wire 1 yellow wire and 1 white wire.

 

Not sure which system you have on your vehicle.
Did the place that tested your alt say anything about the voltage regulator whether it is integral with the alternator or is separately mounted?
The wiring is different for each set up.
If you choose you might confirm that there is power from the starter relay to the battery and also to the heavy (larger) wire going to the alt.(could be black & yellow)