Hi all. I have a "new to me" 79 f250 w/a 351M/C6 4WD. I've run into a bit of a problem. When I bought it, it was not charging correctly. I charged the battery, bought and installed a new "Auto Zone" alternator, and had the same undercharging problem. I took the new alternator off, and had Napa Auto Parts test both. The old alternator was still good, but the new alternator was bad.
I re-installed the old one, and put a new belt on it. I replaced the Batt. to Starter Relay cable, the Battery to engine block ground cable, and the relay to starter cable. I replaced the external voltage regulator as well.

Not long after all that, I started smelling a "rotten egg" smell. When I lifted the hood, the battery was steaming from under the caps. I popped the caps when it cooled down, and topped off the six cells with distilled water. I had the system tested at Advance Auto. Good alternator, good regulator, bad battery. Bought a new one, still overcharging. The old one still works, it's just got 18 volts reading off of it.
I looked at the connector to the regulator. It only had three slots, not four. There was nothing going to the "I" side of the regulator. I bought a new connector with the four slots/six wires, and wired it according to the haynes manual. The old battery was at 18.4, and climbed when I started the truck, so I think it's still overcharging. Any Ideas?
When I looked at the Starter relay from the passengers side fender, from left to right, I had the cable to the starter on the first lug, nothing on the second, a red/blue wire on the third, and the black wire from the alternator, a black yellow, and the red to the battery on the right lug. According to the manual, I should have had a brown wire on the second lug, so I wired it there. It still overcharges.
If the battery was at 18V, would a working alternator still charge it? Thanks in advance.


Zach

 

The alternator is definitely working, since it's overcharging the battery. And you do not need the "I" terminal on the regulator if you have factory gauges.

First check the yellow wire going to the regulator "A" terminal. Make sure it's connected to 12v bat +. Check the "A" terminal with a meter to make sure you have 12v there with the truck off.

If that's good, I would then take the meter and with the truck off, measure the voltage on the "f" terminal of the regulator. You should get zero volts. If you get 12v, then I believe the "F" wire to the alternator has melted against the output wire of the alternator.

 

One thing not to forget is to check the cround side of the system. Do a voltage drop test at each connection point and engine to frame and body. If the grounds are bad you will not have a complete circuit and will result in an overcharge problem similar to what you have

 

I have voltage matching the battery at the "A" side of the regulator with engine off. Nothing on the other three. I'll check the grounds. Where should I check, and how? I'm guessing a cab to either engine or frame ground? Frame to engine block? I know that the engine block to battery ground is good, I just replaced it. Do I just go one side to positive side of battery with engine off, and ground side of meter to cab, frame, and engine and look for the same voltage as the battery pos. to neg. sides?

 

Yes, exactly.

 

Any idea of a decent way to drain the battery? It's up to 18.5. I thought about pulling the alternator belt and driving it with the headlights on for a while, but I don't think 18+ on 30 year old wiring and circuitry will turn out very well over an hour or so....

 

Just pull the headlights on with the engine off. I have a feeling it will drop right away. The 18v is just battery voltage floating up with no load on it, and being overcharged aggravated the voltage.

 

I'll give that a shot. I didn't want to fry anything. Thanks. According to the manual, my wiring is a little different. It shows a white wire going from the alternator straight to the regulator. In reality, it goes into a factory two wire plug with, I believe< a red/green wire. This white wire goes into the wiring harness towards the front of the cab. I haven't traced it out yet, I just tapped into it for the regulator. I hate not being able to drive the truck, and this is starting to drive me nuts....

 

I assumed you had the setup with factory gauges. There is another way they wired it when they used a "gen" light in the dash.

F reg wire still goes to F on the alternator.

Large Bat wire still goes to the battery + in the wiring harness.

A on the reg still goes to the bat+.

S on the reg goes to the S on the alternator like you described.

I on the reg goes to the red/green and that goes into the cab to the dash light, and then to key-on power.

 

From the time I got the truck, none of the four smaller gauges (Oil Pressure, Gas, Charging, and Temperature) work. The previous owner installed a Sunpro three gauge set. The Coolant Temperature and Oil Pressure guages are hooked up and are working properly, though the backlights aren't hooked up. The Voltage gauge is not hooked up. I don't know if the original gauges are just not hooked up, or if they're damaged. I have to guess with the gas gauge, so I fill it up every time I drive it. It has both gas tanks, but just the rear is operable. I have everything I need but voltage, so it hasn't been a top priority, until now.

 

OK, here's what I've got. I pulled the alternator, voltage regulator, and new battery. I also took a voltage reading on the old and new batteries. 24v and climbing, even with the alternator pulled. I smacked myself on the back of the head like Gibbs on NCIS and promptly changed the multimeter battery. 12.5 v. Much better. I had the Alt., Voltage Regulator, and battery tested. All good, with the alternator putting out 15V.

I re-installed everything, and wired the regulator as best I could to the Haynes schematic. The only differences was a red/orange wire and yellow/black wire were hooked to the yellow wire on the old regulator, so I hooked them to a yellow on the new, and I didn't hook the green wire from the reg. to the relay..

Fired it up, and it's not charging. 12.30 volts at the battery. I disconnected the alt. wire from the starter relay, and read .03 volts with the engine running. I had minimal voltage (.003) at the orange and white wires on the alternator. I had the same .03 on the lug of the alternator, so I don't think the wire from the alt. to starter relay is bad.

Also, I took voltage readings from the battery to the engine block, inner fender, frame, and cab with no voltage loss. I also tested the continuity from the ground of the battery to the innerfender/regulator with good continuity.

Any Ideas?

 

From your description, I believe this is the wiring diagram you should be using. Make sure you have voltages in the correct places with the truck off. And then make sure you have voltage on the "I" terminal with the key in run.

 

That looks right. I know a guy who has a parts truck with this harness. I may just buy the harness, regulator and relay and just splice the new harness to the old and get rid of the old chopped up mess. I should be able to make it look pretty decent if I solder the wires and heat shrink the splices. That would give me the original plugs and fusible links. Plus, it's unmolested, so I would think it would take care of my problem. I am going to wire my current mess to match your diagram though, and see if it takes care of my trouble. I certainly appreciate the help. Thanks again!

 

Make sure your old connector to the alternator is good. When I've replaced my alternator, there's always a note in the box to replace that plug as they can get quite junky with all that current flowing through them all the time.

 

Make sure your old connector to the alternator is good. When I've replaced my alternator, there's always a note in the box to replace that plug as they can get quite junky with all that current flowing through them all the time.