Ford truck electrical problem

Ford diesel pickup would not charge so I put a new alternator on it.Old one was bad.Started truck with new alternator hooked meter up and still no charge. I messed around some more and could not figure out what was going on so while truck was running I unhooked grounds on both batteries and checked alt again and it charges 14 volts with out the batteries hooked up but as soon as you hook grounds back up the alternator wont charge. My load tester says batteries are good. cables all look good . if anyone has some ideas I would love to hear them. Thank you

For gosh sakes, don't unhook the batteries while it is running. A GREAT way to fry an alternator!

What year is the truck... internal or external regulator?

It is a1990 regulator is on alternator. I have never fried an alternator by doing that,and I had to try something anyway. it wasnt going to fix itself even though I wish it would. lol. It had an external regulator but i switched it over to a delco alternator because motorcraft alternator was about 300 bucks. so I am not using the external regulato now, just big wire to hot post on solenoid and a hot with key wire to excite and one other wire from big post to other spade terminal.

WOW, who would have guessed... a Delco alt. conversion!

The #1 "switched excite" wire needs to have an idiot light, a diode, or a resistor in it, or you will eventually fry the alternator's internal regulator or diode trio.

We need some voltage readings here....

Battery (-) terminals to engine block/alternator?

Large output stud on alternator/#2 terminal to ground?

Alternator's #1 "switched excite" terminal to ground?

<img src = "http://ndtc3500.stellarnet.com/~blweltin/Bob/AlternatorHarness3.jpg">

<img src = "http://ndtc3500.stellarnet.com/~blweltin/Bob/DelcoSIA1A.gif">

<img src = "http://ndtc3500.stellarnet.com/~blweltin/Bob/DelcoSIB1A.gif">

... Bob ... alternator repair guy ...

That is exactly how it is hooked up with diode also .I have put plenty of alternators on and converted alot to delcos and never had this problem before. The volts read around 12.5 if you check at batteries and if batterise are hooked up and you check at big post on alternator it is the same 12.5 and the alternator seems to be running hot. if you unhook grounds from batteries the alternator puts out 14 volts just like it should and the alternator runs alot cooler. although I only ran it enough to see if it was charging. So I guess I am thinking the problem is after the alternator somewere. Even though my batteries load test fine could there be a problem with one or both of them still?

NOT a Delco convertion. Most vehical are internal regulated nowadays. One exception is my 89 Dakots. The regulator is in the engine controller.

Kent

How about a shorted cell in one of the batteries?

Try charging the batteries separately at low charge overnight, then load/voltage test them.

If OK, you've got a bad stator or rectifier in the alternator.

(Belt isn't slipping, right?)

Get the truck started and warmed up.
Shut the truck off.

Disconnect both truck batteries.

Get a spare battery from some other vehicle, a battery you know to be good and fully charged. Remove one of the truck batteries and install the spare battery in its place. The truck has kind of a "master" and a "slave" battery tied in parallel, the "master" position would be probably work out better for this test. Leave the slave battery cables disconnected.

Now start the truck and evaluate the alternator. If you get 14 volts now, one of the existing truck batteries is bad (shorted cell). If you don't get 14 volts, the problem lies in the truck's alternator or wiring.

I concur with the other poster. Disconnecting the battery grounds with the truck running is a truly terrible idea. You risk destroying the alternator, and damaging any touchy electronics in the truck (like the ABS computer, for example). The charging system relies on the battery to filter out the 3-phase alternator's ripple current, and to damp any electrical spikes that arise as the regulator turns the field current on and off. It would be even worse if you tried this stunt on a gasser or a later model Powerstroke. You risk wiping out the fuel injection computer.

Belt is not slipping but with batteries charged up they load test fine. Could one still have a shorted cell and load test ok? Also the alternator seems to be trying to charge if you hold throttle at around 1800 or more it puts out 13.2 volts but seems to be running a little warm. I am assuming with the batteries unhooked there is no load on alternator and that is why it puts out 14 volts. Could the alternator be to small? It is only 90 amps.

Are you monitoring the battery voltage while you load test them? (Load tester vary from brand to brand.)

Otherwise, it sure sounds like an internal problem in the alternator.

I'm thinking I'd be testing the amperage output from the altenator when the engine is running..........if it's on the higher end of the rating of the altenator that would definitely answer for your lower than normal voltage readings and as the others suggest possibly a bad battery, a warm/hot altenator tells me there's some amperage flowing somewhere..............if the amperage is low then one would have to question the condition of the altenator or it's control wiring................

Hello dieseldoc
If you have 2 batteries and they are not fully charged, it may take an our or two for the alternator to bring the voltage up to 14V + or -.2 volts.A fully charged battery load test is 1/2 the battery capacity in amperes, hold the load for 15 seconds. Voltage should not drop below 9.6 volts at 80degrees F. If they pass the load test, then give the alternator time to recharge them!
a couple of hrs. at least, and then check the voltage at the batteries.
You can also load the alternator and see what its output is. A load placed on the alternator will make it put out close to the max or at least 95%
of its rated output.