here's the entire recent history:
1: drove to work. System voltage was about 11.9v the entire way.
2: truck would not start. acted like battery was dead
3: charged/jumped batteries. truck started, but voltage started counting down like the bomb in an action movie: about 0.5V/minute, drove about 3 blocks
4: got home via tow truck
5: replaced alternator twice, most recently with new NAPA alternator

the "draining voltage" problem went away with the original alternator, but neither of the replacements are generating any power. System voltage matches battery voltage.

I've seen some very strange voltage readings with all the wires connected, like -0.7V relative to battery negative at the output of the alternator... which sounds to me like a protection diode is going off?

Resistance from the case of the alternator to the negative driver's side terminal is 20 Ohms

any advice? debugging checklist?

 

it sounds to me, like the circuit from the Cluster Panel to the " I " terminal on the Alternator is no good....
there is a high resistance, or corrosion involved.

that circuit requires Battery Voltage to excite the alternator, and tell it to charge.

Fuse F2-45 is the source, trace straight down, and verify that all is good.

I would put a voltmeter on the " I " terminal first, and see what is there??

it better be close to battery voltage, or it will never charge.

 

at 11.9 those batteries were worse than dead,
they were submerged below the surface.

 

If you don't find the Parasitic Draw,
you best install a Knife Blade switch in the negative lead of the Batteries....

when you get out of the truck, open the knife blades so the batteries don't get discharged.

invest in a good Clamp On Ammeter, and trace where the current is going?


I bought two of these, a small one at first, then the 1,000 amp model later.
this is the cheapest, high quality clamp on ammeter that I have run across.
both of them are very good, and accurate.

https://kaiweets.com/collections/clamp-meter

if you can wait awhile, you can get them even cheaper from the source

https://www.aliexpress.com/i/3256801...apt=4itemAdapt


both of those links are cheaper than Amazon..

 

Found an interesting document. Since my alternator only has two wires in the "data" connector, I assume I have a "type 1". https://www.electricalrebuilders.org...regulators.pdf

Based on this, and the diagram above, there's quite a few fusible links and fuses that could be bad... but at least I know where to start looking.

Many thanks, all.

 

No parasitic draw, beyond something in the original alternator (or something that was "fixed" by popping a fuse?)

I'll fill in the info as i find it.

it started beautifully... I literally said to myself "I'll be fine til the weekend..."

 

Disconnect both batteries and have them tested at the auto parts store in the morning.

Post back with the results

 

With the batteries installed, and the new NAPA alternator in place:

• The batteries can be charged (by a decent benchtop charger) to 12.6V (maybe 12.5? depending on where I take the measurement)
• They hold that charge steady for at least a day
• The truck can be started and driven, with no unusual current draw

My suspicion is, when "the incident" happened, there was a short in the field coils of the alternator, and the batteries were discharging through the field "sense" line and through that fuse shown in the diagram above.

Given the history with this truck, I'm pretty sure I'll find the P.O. put a penny in the fuse slot or something.

 

What is the battery voltage while driving?

use a DVM in the Power Port

 

John, you’re the best!
For everyone else, he knows his stuff! I had alternator issues with voltage in the 13.6 range and barely marginal charging. With a little coaching and personal research, I ended up with a 240 amp Mechman alternator. It bolted right on with no mods required. Also connected the alternator power and body directly to the driver side battery with a direct connection between the batteries negative side. The positive is already connected. Now I have a constant 14.8 - 14.9 voltage. And, the engine runs like never before. Even the windows roll up & down faster. These trucks are WAY under served with that dinky 110 amp piece of crap. I’m more than happy now!
Thanks John!!

 

ah shucks, glad I helped you though.

 

I think I have that exact DVM.

Before the disaster, it was about 11.9.

Now, it's battery voltage (low/mid 12's).

I'm about to go dig into the harness and see what I can ohm out.

With reference to the post just below yours, do you suggest wiring the alternator differently than Ford did? I was thinking about changing things up a bit, myself.

 

My testing results:

1: belt and pulley seem to work correctly. No squealing or slipping that I can see
2: proper connection (close to 0 ohms) between B(charging output) and SENSE ("A")
3: I pin sees battery voltage when engine is running
4: no connection (OL on meter) from B charging end of cable (removed from alternator during test) to + lead of battery

This tells me, based on the diagrams, that I cooked off the 2 or 3 fusible links somewhere in the wiring harness, and that my best option is to build my own charging cable to go from + to B, with a fuse on it.

I have some 1/0 wire long enough to get from B to driver's side + via a route that should minimize bouncing and belt/heat contact. I have wire ends and some smaller wire that I could use to make a new sense line. Even got a hydraulic crimp.

So, I'm thinking, my best bet is to make a new charge cable. Get a 150A fuse, plus a 10A fuse holder for the sense line. put the 150A fuse very close to the battery, since the alternator will turn off when I kill the ignition if something goes wonky.

Does this sound like a decent plan, or am I missing something stupidly simple?

 

to me,
it sounds like you need to start from scratch with new cables.

the ones on the truck now, are highly suspect.

Put a Voltmeter from the positive post of each battery,
a DVM will work better here, as it will just show a ( - ) symbol if you get the leads backwards.... an Analog meter will try to go below Zero and you can't tell what is what.


do this before you start the truck, so that the glow plugs and starter motor will drain the batteries quite a bit.....

have that Voltmeter where you can see it from the driver's seat.

as soon as the engine starts, what is the Voltage Drop between the two batteries???? this is when the MOST charging current will occur.

if it is more than about 0.1 or 0.2 volts, replace the cables.

I suspect, you are going to see 1 volt plus or more...

 

That's a good idea.