this is an '85 f150 4.9 auto. I'm stumped. The alt is not charging the battery. When I first start the truck the alt will charge for about 1 1/2 to 2 minutes then quit charging completely. here's what I've done so far.....Checked voltage before cranking 12.28. immediately after start voltage climbs to 14.40 holds steady for a while then drops continuously until well below 11 volts. Harness was removed and checked (new plug installed just to be safe). Alt was tested at two different places (both good). I have 12v at the ign wire (green w/red stripe) in run. Grounds are good.
Heres something weird....I pulled the wifes expedition up and hooked the jumper cables to the now discharged battery. With the cables on and the expy running I still only got 12.02v on the battery in the '85 (expy showed 14.04v)
Ideas?

 

What's the history of the F150's battery?

Based on your description, this sounds like perhaps one of 3 things might be going on. (my guess it's the first on the list)

1) battery is toast... internal shorting and thus overloading the alt's VR. following sufficient warmup (12.28V initial reading is a clue)

2) Alternator's internal windings or rectifiers might be breaking down following sufficient warmup.

3) Something else (electrical fault) in the truck is potentially kicking in... again, following brief initial warmup

Here's an easy & relatively quick test:
Run through your previous start & run-for-awhile routine. After the truck is good and warmed up - then disconnect the battery altogether and turn everything in the truck OFF. Then use the meter and do a continuity test between the +battery cable-end and a good frame ground point. Do you see any continuity? -- If so then switch the meter to read ohms and see what kind of ohms reading you see. The results *might* indicate a heat-related ground fault.

If you don't have any evidence of a ground fault (from above procedure) then try the following:

Now allow the truck to completely cool down (I'd give at least a couple hours) -- Install a known-good battery with a good full charge (maybe from the Expy?..maybe not ~ cuz terminals might be different)

After installing a good battery, test the battery's voltage with a your meter and make a note of that. Then start the truck and conduct a live monitoring session as you've done initially (meter connected). Do you see any similar decline in voltage reading over the test period? -- If so, then is the decline much more gradual? Or is it about the same order of voltage-drop (over a similar time frame) as the first scenario.

Report back your findings.

 

With the engine running, check the field voltage on the "f" terminal on the regulator. See if it's high, then slowly drops when the output voltage drops.

 

'85= ext VR.... duhhh!

Do what Franklin2 suggests first... I've had a long hard day.. not thinking too clearly.

 

Well don't be so hasty to discount yourself...'85 was a big changeover year for ford. This truck has the internally regulated two plug alt..
I just installed a new battery and ran through the same tests.
Got 12.60 before start, 12.41 running at idle and above, 12.19 with headlights on high beam. Checked voltage to the trigger wire (same as battery voltage). All fusible links test good, if I'm thinking right, this only leaves the alt itself as suspect. what do ya think?
Greg

 

Well Greg... here's what I think.
I think you're a graceful fellow... and perhaps considerably more adept than I hastily presumed. My bad.

If it were me... I think I'd spend the few minutes needed to yank the alternator and put it on my 'errands' list. I'd then take it to my most trustworthy auto parts guy.. probably would do it while on my next trip up to the grocery store.

I'd have him spin it up on his relatively modern computerized test rig. -- I would want more than a pass/fail indication however... I'd want to see the actual output-parameter readings. And whatever the outcome, I'd threaten to rescind his beer allocation if he refused to re-run the test-spin for me again several times... just to be sure.

~ Then I'm guessing I'd go ahead and just spend the extra dollars necessary to buy an *all-new* alternator...one with a *lifetime* warranty on it....
........... just as I did early this morning in fact.

cheers
dave

 

I missed that, you have the alternator with the famous plug that likes to melt. I have a ranger that had that alternator, and it went bad. I was going to turn it in and replace it, but found out they are pretty expensive, even rebuilt ones. And knowing all the problems they had with these things, I modified the brackets and installed a GM alternator off a 86 monte carlo I had laying around.

I don't know if you are up to doing something like that or not, but these alternators like you have(I believe they are called 2G?) do not have a very good reputation. I saw at some of the parts houses, they will not even warranty the alternator unless you buy a new plug. I don't know how you prove that, unless you have to show them the receipt where you bought the plug.

 

Thanks for all the input and help. I've never been a big fan of the 2g alternators, never had any serious issues, I just don't like the design. So I have decided to just install a 3g and be done with it. Thanks again for all the help.
Regards Greg

 

If you are up to a little fabrication, I believe that's the best route, even if it turns out not to be the problem.

 

Heres a picture of the regulator plug on my truck. If you stare at it for about a week you'll notice that the ign wire (green/red) is in the s position as it plugs into the alt. I finally saw this and made a temp harness with the correct connections and it charges perfectly....I have no idea how this happened, maybe a previous owner, maybe a mid year change, who knows. Have you ever seen anything like it before?
Greg

 

Hmm.. well that's curious...and kinda' begs a couple questions then.

Was this thing previously working OK and then just recently began behaving differently as you first described?

If so...then what's different now; why would it change all of a sudden?

 

The older alternators with the external regulator did have two ways of wiring, one with the green/red going to the "i" terminal on the regulator, and another with the green/red going to the "s" terminal. One had the dash light, and the other did not, only the ammeter gauge.

I am not sure if they had two ways of wiring these internally regulated alternators or not.

 

Well heres the history. This truck sat in a field for the last 15 years. I got it as a winter hobby/resurrection project. After getting it started (new fuel tank/pump/carb/battery...fired right up) the alt was making noise (dry bearings i assumed) and was replaced with a new one. thats when the trouble started. I never knew there were two different ones for light vs. ammeter, Mine has ammeter. Very curious indeed. But oreilleys has the correct reg. plug as a pigtail. I plan to splice that in and all should be good.
Thanks for all the help guys. This one had me scratching my head for a week.
Regards
Greg

 

The wiring on the Voltage Regulator (VR) plug is stock. My truck had the same setup.

The Newer VR require all 3 wires to charge. The original VR would work with 2 wires, the turn ON power to get the ALT to charge could go in either the S or I. New ALT with new VR need BOTH S and I inputs to get it to charge.

When it was bench checked they used all 3 wires. You are not the first one to get caught on this newer style VR issue.

I think the 2G ALT is a very poor design and would recommend the 3G conversion. I did my 3G wiring so I could still install a 2G ALT and thus carry my old 2G behind the seat with my ride along spare parts.
Jim