I put in a new battery and am sitting at 12.2. When starting the truck the alternator is putting out roughly 16.5, and the battery drops as low as 11.7 opposed to running at 13.* - 14.*

I have changed:
battery
alternator
starter
serpentine belt
starter solenoid
cleaned/changed terminals, ground contacts exc

Ive opened the fuse link on the alternator and it was fine.


Im at a loss and any help would be greatly appreciated. Ive been dealing with this for over a week and need to get back on the road.

 

Can you clarify? You say you have 16v while starting?

 

The new alternator is putting out a consistent 16.5v at all times (so was the old)
The battery when starting will drop to 12v<, with lights on itll go to 11.7
amp light has been on


I started out having the issue on the way home when radio and lights went out, and started felling sluggish. I did some work on the truck and took it out the next day only to make it a mile or two down the road. When i got out the battery was down to 6v. I jumped it and got home, it died again. I was thinking possibly the voltage regulator and changed out the alternator, i also changed out the new battery that apparently was dead. I put the new stuff in, did further testing and back to square one. I still have 16.5+v at the alternator, and lower then expected volts at the battery.

 

So you have different voltages at the alt and battery? Sounds like the wire between the two is bad or a bad connection.

 

Sounds like you may have gotten a bad alternator at the store. 16.5 is too high and I have seen alternators do that before. On these alternators when voltage increases current drops when the regulators go bad. I would take it to the parts store and have it bench tested.

 

Ding, ding, ding! we have a winner

 

Its not the alternator, the one i just took out had the exact same reading, and hell i had that one tested 4 times :-p


I can try replacing the wire between the two and see. As of now ive only looked them over, cleaned harness, and all contacts. I was trying to find one at the junk yard but didnt have any luck, the one good one i found someone had already cut in half, the dealership no longer sells the wire/harnesses that go from the alternator to the starter solenoid.

 

Id be looking closer at the fusible links. What year truck are we talking about? Is it internally regulted? The one with that rectangular plug that you are supposed to replace whenever its disconnected?

 

Exactly and here is why.

The voltage at the battery and the voltage at the alternator should be exactly the same, well at least much closer than 12.2V and 16.5V. Question now is where exactly are you measuring this? Between battery posts? Chassis ground or engine block? Alternator output stud?

Here is a further test you may want to do. Measure voltage between the battery negative post and chassis ground. Do the same for the battery negative to the engine block. Any voltage reading means you have a problem with grounds. Now check voltage between the battery positive post and the alternator output post. If there is any significant voltage, you have a wiring problem between the alternator and the battery. You may see a small reading like 0.1V, but this will likely be within the margin of error in your meter. If your numbers are correct, you should see 4.1V somewhere in this circuit and that will pinpoint the fault.

 

Likely a 2G alternator and the infamous 'Fire Plug' has gone bad.
There is no output stud on these...
(he mentions changing the alternator because he felt the -internal- regulator had gone bad)
If the alternator's regulator does not 'sense' output voltage, the alternator will go full field trying to charge the system.

To the OP,
Disconnect the rectangular three wire plug with two orange/black and a white/black wire from the alternator.
Check for continuity between the plug and the starter solenoid stud.
Look to see if the plastic plug has melted internally.

Ford's TSB for 2G charging problems outlines that any time the charging system is worked on this plug pigtail needs to be replaced.
In fact, most alternator manufacturers/rebuilders won't warranty unless it has, and include it in the box.
Almost every aftermarket wiring company makes this pigtail.

 

Exactly. That right there is your clue. Agreed about the "fire plug" being faulty... the OP will probably find it all nice and crispy inside when he pulls it apart.

 

Refer to TSB #96214...
They don't sell it because it has been superseded by a repair kit.

Any of these brands should interchange, and be available locally for around $10.

Borg-Warner PT-780
Standard Motor Products S-542
Power Plug 4628011
KEM Parts 350-220
Pico 5711
Dorman 85123
Motormite 08622
NAPA VRA400

 

If this is the notorious 2G alt, I would suggest doing the 3G alternator conversion.

I just did this conversion on my '92, 302 and pleased at the results. 2nd one now and my other '92, 4.9, I did 6 years ago.

 

We are ALL in the 3G club!

 

Yep, love my 130A 3G. Direct bolt-in replacement for the 95A 3G that was originally there.