replace alternator or is there something else wrong 1990 f350 5.8 l thanks guys

 

Connect the negative cable?

Motorhead26

 

I disconnected the negative while engine was running battery is new cables new my neighboor said the alternator is dead and I have to replace it is he right

 

Most engines will run two to five minutes and then die. When you take the cable off with it running you are very lucky if you did not fry something as the voltage can go over 100 Volts when you do that.
You should never do this, you may get away with it a few times but it will get you in the end.

 

I would have to agree with subfrod. Why on earth would you do that? Are you trying to get shocked? If you can start it temperarily then take it and get it checked. If not you might want to check your solenoid. You might have fried it like subford said. Thats the little box your red cable runs too. Could be a number of things.

Motorhead26

 

Pulling cables from a running engine was fine in the old days, but I'd never do it with a computer controlled vehicle.

 

It might have been ok in the old days, but computer controled or not, you touch the wrong thing even by accident and zzzzz, lights out. I've had friends get shocked many a times just messin' with battery terminals, and not just on new vehicles either. You still never said why you pulled the negative cable! What was the accomplishment, and I am not trying to be rude at all.

Motorhead26

 

I've seen it done to hot-swap batteries on a dead vehicle where jumper cables were not available... but not in the last 20 years.

Things have a tendency to go BOOM! or ZAPPPP!

 

As Fbama73 stated, it was a way to find out if the alternator was dead on the older pre 80's models. But yes, if the motor died as soon as you pulled the cable, that normally meant the alt. wasn't putting out the juice required to operate the electrical demand.

 

As stated, the alternatorwould be one of the components to check first before taking any major steps toward any other electrical component. Although there was never anything that you stated to be in great detail so I was working with what the subject was when written. Just be a little more detailed so we can help you out and give you some insite on what might be going on with your truck. I like to give funny/ sarcastic comments back when only one sentence is written to start off a topic. At first I had no idea what you might be looking for as far as answers but these guy will tell ya, and yes there are right the alternator should have help the charge enough for the engine to run for a little bit before cutting off. Are you going to check it out or just replace it? I love new stuff!

Motorhead26

 

Not for sure on the fords, but on the chevys, if you just start them and dont touch the gas, they're usually not charging. On them it takes some engine rpms to get the alternator to energize and begin to charge. my k10 wont energize till i bump it up to about 1100, but after its going it stays going.

 

just noticed this, and am I getting this wrong? disconnecting the negative cable to the battery shouldn't affect anything since the computer, alt and entire elect system are all connected and grounded to the chasis. Not saying it's a good idea, but aside from a big spark, I don't see how it will ahrm anything.

However, it's not a test for the alternator. Usually disconnecting the positive side of the battery will let you know if the alternator is working. If the vehicle can't run without a battery, the alternator isn't putting out.

If I'm wrong, let me know.

 

I would say you are wrong.
The body (not chassis) ground return is just a large wire going back to the battery, it does not do anything electrically other than conduct the power back to the battery.
Yes I know that the flow is out the negative side of the battery, through the load and back into the battery on the positive side.

To the trucks electrical system it does not know the difference if you take off the positive or the negative battery cable. It is an open circuit either way.
But it is always safer to take the negative cable off first if you are working around the battery so you do not have any accidents.

The A/C Generator with a built in regulator has an information wire ("A" wire) going back to very close to the battery to see what the charge state of the battery is. When the battery is disconnected with the engine running it tells the A/C generator that the battery is very low and will go into a full charge state and this can be over 100 volts and fry anything that is turned on including its own regulator.

The battery is also the filter capacitor for the electrical (charging) system to smooth out the A/C ripple.

There is not any residual magnetism in an A/C generator and it can not charge without DC voltage being feed to its Field windings to start things going. This DC exciter voltage also comes in on the “A” wire. So if you are not getting any voltage to the “A” terminal on the A/C Generator it will never charge at all. You can test it and it will test good at an auto parts store and you can change it out with a new one and it will still not charge without this exciter voltage.

It also needs power on the "I" terminal to start charging, as this is how you control it off and on with the key.

 

It shouldnet be done but if doing it, put the headlights, and blower on to absorb some of the voltage spike. Hot swaping batterys is a great trick, but should only be used as a last resort. Also you shouldent relly get shocked unless something shorts or grounds out. Just use common sense.