ok so after trying just about everything that i thought of including,NEW alternator,regulator, battery is newer but not brand new..volt meter says battery good..anyway truck starts and runs fine but its not charging ..cant run heater and/orheadlights with out sitting on side of road with dead battery(found this problem the hard way).... My truck is 1979 f250 4x4 300 six just bought it a short time ago, runs fine with out heater etc. on. i checked and rechecked everything i know as far as wiring which isnt much..previous owner had his own wires ran from alt. to reg. i went tto junkyard and got factory alt to reg. connector boot deals..and plugged in and still no charge. what are wires that go into the cab and could my problem lie there??... i have a dummy light by the way on dash....didnt think these rather simple systems could give me such issue.PLEASE HELP !!!

 

Welcome to FTE. Since a previous owner has had his hands in the wiring (I don't know why they do that), the first thing to check is the wiring.

With the ALT light setup, you should have one wire coming from the cab (according to my diagram, it's RED with a GREEN stripe). This is switched power from the key, downstream of the ALT light (meaning when you turn the key to RUN, current flows through the ALT light in the dash, to the I terminal).

You should also have one wire connecting the F terminal of the regulator to the FLD terminal of the alternator, and another wire connecting the S terminal of the regulator to the STA terminal of the alternator. The B post of the alternator should go the the battery-side of the starter solenoid to connect the alternator output to the battery. This connection is heavier gauge and should be protected by a fusible link. The A terminal of the regulator needs to connect to this connection as well in order to sense the battery voltage. Finally, the regulator grounds through its mounting bolts - so make sure the fenderwell metal underneath the regulator is clean and rust-free, and that you've got a good ground strap connecting the firewall to the engine block.

When you say the truck runs fine without the heater on - do you mean you can run it for a very long time and the battery voltage won't drop? Also, if you turn the key to RUN, but don't start the engine, does the ALT light come on?

 

ok alt light does not come on when key is turned on. and yes i can leave it idle in garage stays around 12 to 12.4 volts which i know is low and not charging(havent ran very long but had buddy rev motor a little with no voltage gain just a drop from 12.4 to 12 volts.) went for a drive about a week ago when it was almost dark ( lights on and heater) and lights gradually got dim and i barley made her home even after i shut all heater and lights off. this was right after i replaced alt. thinking it would be fine...being this truck wasnt drivin until i got it i figured alt was bad from sitting for 3 plus years. i just in for night from garage,but i need to check to see if there is a ground strap on firewall to block..right? and why wouldnt my alt light work?

 

When the key is turned to RUN, 12 volts is applied to one side of the ALT light. This is done in the cab behind the dash. The other side of the ALT light is connected to the I terminal of the regulator. There is a path to ground at the I terminal of the regulator. When you first turn the key to RUN, but the engine has not started, the ALT light is supposed to come on, because you're applying power to one side, and ground to the other side (it's not exactly ground, but it's close enough to make the light glow). This also generates a field current in the alternator.

Once the engine starts running, the alternator turns. Because there is field current in the alternator, the rotation of the alternator generates voltage at the S terminal of the alternator. This voltage is applied to a relay in the regulator that jumps 12 volts to the I terminal, which is connected to one side of the ALT light. The other side of the ALT light is permanently connected to 12 volts, so since there are 12 volts on both sides of the ALT light, no current flows and the light shuts off. This is how the system works when properly connected. The ALT light is a major player in the system.

If your ALT light doesn't come on at all, then the following could be the case:
1. Power is not getting applied to the ALT light because of a wiring issue or problem with the ignition switch.
2. The other side of the ALT light is not getting grounded because of a fault in the regulator, or the wiring between the dash and the regulator.
3. The ALT light is burnt out.

Since you've already replaced the regulator, that can be saved for last. It's possible that the light isn't coming on because the light is burnt out; but even then, the system should still work because there is a resistor in parallel with the ALT light, so that current can still flow to the I terminal in the event that the bulb burns out.

The next logical step would be to disconnect the regulator connector, and check that you have 12 volts at the I terminal of the connector with the key in the RUN position.

And yes, you should have a ground strap between the firewall and the block - this is how the voltage regulator, and all other loads in the cab and on the inner fenders ground.

 

I will do as you say,you obviously know alot more than i. i really appreciate your responses' and think your on to something.bare with me i check it all out tomorrow and post asap for you.thanks again!

 

so i redid all grounds including firewall to block.. battery to frame.. and block to frame...got it charging as i ran wires as you explained YAY! but...i noticed a smell in cab took dash apart and it seems a red and green wire coming from the ignition is getting to hot to handle after just 15 seconds of igntion on!! dont need a fire...no wire in engine comparment get hot when igntion on..just in cab.... in fact i unpluged regulator all together and it still gets hot...thinking a hooked something up wrong...and yes ALT light does work now as it is charging..but why is that wire getting smoking hot...could ignition be issue..tumbler and all that seems to have alot a play in it. or could there be a ground issue still somewhere in cab?? urrgh thought i had her dialed in because it was finnally running 14.1 volts running...was going to go for a cruise when i noticed the "hot" smell of wires in cab. now what...i thought. i hope that you have an answer for that for me also..you solved my charging issue..just need to get that wire up to snuf.
thanks

 

fixed her..replaced starter relay no more hot wires...and 14.1 charge/running.thanks FMC400

 

Glad to hear it's charging. As for the hot wire, it's not quite clear to me why replacing the starter solenoid solved the problem. RED with a GREEN stripe is hot-in-RUN from the ignition switch. If it was getting hot, it was shorted to ground somewhere. RED/GREEN meets a BROWN wire which join to go out to the coil - RED/GREEN is power to the coil in RUN, BROWN is power to the coil in START. I guess if there's a short to ground at the "I" terminal of the solenoid, excessive current will flow through the RED/GREEN wire. What's concerning to me, though, is that this didn't happen before you fixed it - it's strange that the starter solenoid would happen to go bad in the process of fixing the charging issue. I think it would be good to go through and double check the wiring to be safe, and keep an eye on it. I don't believe that the RED/GREEN circuit is protected in the fuse panel. If something in that circuit shorts, it's a very bad situation. These era Fords have pretty poor circuit protection schemes in my opinion. Best of luck, let me know if there are any more questions.

 

Also, as a follow-up: I went out to my '79 to double check, and the wire going to the "I" terminal of the regulator is actually GREEN with a RED stripe, backwards of what I have told you. It appears that the wiring diagram I was using on Autozone's website is incorrect. Their wiring diagram shows the "I" terminal of the regulator as being powered directly from the key which is incorrect. If that was the case, the ALT light would never fit in the circuit. I apologize for the confusion.

I also verified operation of the ALT light by turning the key to RUN but not starting the engine - the ALT light turns on. By disconnecting the GREEN with RED striped wire going to the "I" terminal of the regulator, the ALT light turns off. This is expected, as this removes the ALT light's path to ground. Under normal conditions, when hooked up correctly, the regulator jumps 12 volts to the "I" terminal of the regulator internally once it senses that the alternator is charging, which shuts off the ALT light to let the driver know the system is working properly.