!993 F 150 5.0.
Alternator light was on, battery slowly drained when sitting, alternator output was nothing. Voltage on alternator red single wire was 11.5 volts, same as battery voltage after having battery on a charger for a while.

Bought new alternator on the net, put it on and the same thing. Engine running, multimeter red lead to single red wire on terminal black lead to ground. 11.5 volts, alternator light still on.
Thought I must have got a bad alternator.
Sent it back got another one.
Exact same thing as before.
What do I do next to diagnose the problem?
I am assuming I did not get 2 defective alternators in a row.

 

I would check the cabling and connections in the charging circuit. Grounds should be solid and clean. When properly charging, the alternator should be putting 12.75 vDC or more to the battery. Make sure you have a charged battery when you put a new alternator in. Might need a wiring diagram to check things properly.

 

Definitely check for a wiring issue. The few inches of wire that first come off of the starter relay going to the alternator are a fusible link, so check that for failure. And the rest of the wiring is pretty simple. The big wire on the back, goes to the hot side of the starter relay, the small single wire plug on the side of the alternator goes right back to the voltage regulator. The green wire going to the voltage regulator should have 12 volts with the ignition on. And the last wire on the voltage regulator should join with the main power wire right before it turns into a fusible link at the starter relay.

 

Thanks, I'll check it out.

 

You should put ALL the truck's details & history (as much as you know) into your signature so it shows with each post, as this page explains:

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Phone apps don't always show signatures, so you may need to switch to a real browser in desktop mode on your phone, or just use a desktop/laptop computer. Put your location (nearest city) in your profile & upload an avatar of the truck. The more pics you post of the truck, engine, wiring, labels, & undercarriage, the more likely we can help you. Not all its details are relevant to these issues, but you don't necessarily know which ones are relevant, so just put everything in now.For how long (time/miles)? What happened just before it came on? Have you followed the published owner checks & maintenance schedule? Post pics of the engine, battery, battery terminals, alternator, & alternator output stud. Read this page & the NEXT:

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That's normal - all batteries do.There isn't a Red wire going to the alternator. Your truck's wiring must have been modified - post several pics of the alternator harness section.That battery is FAR beyond ruined.Why? What brand, PN, & source? What test did the previous alternator fail? Did the replacement pass the same test? Do you still have the previous one? Post pics of both.Did you clean its mounting surfaces down to shiny metal?

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Exactly where? Did you clean that point down to shiny metal? You should be using a clean spot on the battery POST (NOT the clamp AROUND the post).What you should have done originally: learn how the system works, so you understand how to diagnose it. Read this page & follow the links on it:

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Bad assumption. If you got the same brand from the same source, it's probably the same.