Been working this problems for some time now and can't figure it out. Got a 1986 F-150 that use to have a V8 302. Previous owner replaced it with a carburated 4.9L 300 I6. Measuring the alt output at the +/- battery terms I see 13.2V then slowly moving down to 12.2V. Turn the lights on and drops below 11.7V leading to dead battery. I have replaced the alternator with internal regulator 2 times but same symptom (remanu. alternators bought at Advanced Auto Parts. Wiring harness buzzes out good. It is wired for a system w/o amp gauge even thought the truck has a D/C meter on the dash.

Wiring: 2 Bl/Rd wires from Alt (Batt) to Battery + term
1 Bl/Wh wire from Alt not connected to anything.
This is possibly the "STA" wire.
--------
1 Green Wire from Regulator "S" to ignition switch.
1 Yellow wire from Regulator "I" to Battery + term.
Nothing connected to "A".

Tried swapping yellow & green on Regulator plug but no
output from alternator.

What are the odds I got two bad alternator/regulator from AAP. Next step is to bench test a new alt/reg from AAP before attempting to install
this 3rd alternator. HELP!!!

 

New/reman parts aren't always good parts, so it can happen that 2 in a row are bad.
Could also be loose/broke connections inside the plug(s) that go to the alt.
The voltages listed sure point to a non-charging alt, so you're on the right track.

 

The last guy had to replace the VR plug and wire it with all 3 inputs, not really sure why as mine only has 2 wire on the VR plug.
Maybe the new VR on O/H ALT are different than the old ones??
Be sure to read the whole thread.

https://www.ford-trucks.com/forums/1...0-302-efi.html

 

Replace the output plug.
They're cheap (a LOT cheaper than a battery or a truck fire)

 

Thanks for the tip. The VR output is a two wire plug like you describe. The new alternators mention that all 3 must be connected. I've already
replaced the Alternator harness head but not the integrated 2W-VR harness head. Guess it wont hurt to get a 3W and give that a shot. I'll let you know.

 

One last sanity check. In the last post to that thread said "I moved the white
wire to the "S" terminal and it charged up ok". Do you think he looped the
white/black wire from the output of the ALT to the "S" on the VR and moved the yellow from the 2W plug to "A" on a new 3W. This would match the schematic provided.

 

Something sounds goofy in your wiring harness.
The S terminal in both plugs should be looped together, Your I terminal should be running to key switched, and your A terminal should go to B+ on the alternator side of your fuse protection on the 2 big wires going to the output plug.

 

 

Yes I believe we are thinking the same here
<O</O
On the VR plug
Fat yellow wire on “A” is the same on both 2 wire and 3 wire plugs
The green wire on the 2 wire plug “S” connection moves to the “I” connection on the new 3 wire plug.
The small white wire from the power output plug STA will connect to and provide a power input to the “S” connection on the 3 wire VR plug.

<O</O
You will also new to tap into the STA power output wire to power the electric choke, if you have one.
<O</O

Hope this works out for you, please post your results.
Jim

 

Great..thanks!

Picked up the f3W plug last night. Will install it this weekend. Hope for the best!

 

CRAP! No change. I wired the thing with the new 3W regulator plug but it didn't make a difference in the output with headlights and fan on. Still drops to 11.8V. I suspect I may have fried the Sense circuit when trying to troubleshoot it with the 2W plug. So...back to AAP for another alternator.

 

Being that the engine has been swapped what are the conditions of your ground cable?
The alternator is part of a circuit, not a river of electricity.

 

Thanks for asking. The ground cable is probably original but the alternator
is making good contact. I cleaned the mountings, used new hardware and
measured 0-Ohms between battery ground and alt. case. I also replaced
the battery and battery cable lugs.

One problem I do see is the charging-meter on the dash doesn't work but
I measure 12V on the green "I" wire when running so the gauge is not causing an open circuit.

I suspect I may have fried the "S" rectifiers in the Reg. when I had it wired with the 2W plug. The next step is to take the alt back to AAP and
get a new one. Have them bench test the new one and verify 13.5-15.0V
and make sure it stays constant under load. If it works on the bench, it should work in the truck with the new wiring and 3W reg. plug.

Right now it's wired as shown in the 2G schematic shown in these posts.
I measure 12.8V on both yellow and green wires to the reg and 6.5V on
the white wire going to the alternator. The alternator is putting out 12.8V
on both battery (B&R) wires. Turn the lights and heater motor on and
the charge drops to 11.8V. I should take a V measurement on the white wire at this point.

 

As your truck has an AMP meter it just has 12v switched power on the green wire on the VR. You don’t have the stuff inside the box with the dotted line.
To test the AMP meter just turn on the lights, the amp meter should move to discharge.
Here is a good read on the amp meter with a good picture toward the end of the post.
<O</O
https://www.ford-trucks.com/forums/1010607-where-is-the-ammeter-shunt-located.html
<O</O
When you go back to the auto parts store I would have them test the old ALT to see if it bench checks OK, if it bench checks bad it helps verify that your current wiring might be OK.
<O</O
If your battery voltage is say 12.3 volts before you start the engine and does come up to 12.8v at idle… then the ALT has to be coming online and producing some output, just not enough amps, as it should be a little higher voltage at idle and of course not go lower with lights/heater turned on.
<O</O
Jim

 

This is why I love my 3G swap....