Year: 1977
Model: F-150 Custom 1/2 ton
Engine: 4.9L 6 Cylinder
Transmission: 4-speed on floor w/Granny Gear
Other Stuff: 4wd w/4" Lift on 35's

As I've previously posted, I recently had the engine rebuilt, and I've only put about 300 miles on it. Every since the rebuild, I've had some electrical issues, mostly the batter going dead. At first, I thought it was a short or some switch left on (like my turn signals) that was draining it. But after a while I decided something was really off.

I bought a new battery (Super Start Extreme) two weeks and installed it. Life was good for about a week, and then it drained to the point that it wouldn't crank.

I started looking at the alternator, I was getting 13.8 off the back of the alternator (idle speed, not revving up rpms), but it wasn't making it to the battery. I installed a new voltage regulator, and things got interesting.

After installing the new voltage regulator, I cranked the truck no problem. I got out and went to check the voltage, and I was seeing something close enough to 14v at the battery (idle speed) that I believed the system was back in working order. After looking around a bit, I turned off the engine, cleaned up my tools, closed the hood, and tried to crank it again.

I had nothing, no power, no ... nothing. Checking back under the hood, I discovered that a fuse had blown. It is an inline fuse between the battery terminal of the starter relay and the BAT connection on the alternator. This inline fuse looks like it was spliced in, and it doesn't seem to appear on my wiring diagram.





Alternator Charging System - Ammeter

So I went out and purchased a 5 pack of replacement fuses (AGC 30A), replaced the fuse, and the truck cranked again as expected. However, upon turning it off, the fuse had blown again.

Some additional troubleshooting lead me to discover that my alternator was overcharging. I took it in, had it tested, and they confirmed that at top RPM's it was putting out 15.7V.

I bought a new alternator (Ultima - 42 Amp) and installed it last night. Now, I've got a new Battery, a New Alternator, and New Voltage Regulator ... and a 30AMP fuse spliced in that keeps blowing still.

I spent the morning pouring over the wiring diagrams and trying to comprehend why that fuse is there (I believe it was an added, aftermarket safety measure) and why it insists on blowing. There doesn't appear to be a short anywhere, I believe this because...

With the engine running, and the fuse blown, I still have battery + terminal voltage on one side of the fuse and I have alternator output voltage on the other side of the fuse. I think if something was shorted, then one of those sides would be burning.

Which leads me to believe that the combination of the new alternator and the new battery means that there are more than 30Amps running through the line, which would necessarily blow the fuse each time.

1) I'm tempted to just take the fuse out, and run the alternator directly to the battery (thru the ammeter it appears).
2) I'm not confident of how to sort out how many amps should be running there, and properly sizing the fuse.
3) What if it should continue to be less than 30Amps (but the Alternator says it puts our 42Amps, and the battery claims 890 Cranking Amps [725 Cold Crank])?

I'm confused, frustrated, and haven't done electrical math since 1994. Any assistance, insight, or criticism would be greatly appreciated at this stage in my life. Thanks in advance!

 

The AGC 30A fuse has been added by someone, Ford did not use a fuse at that location from the factory. They did use a 16 AWG fusible link for circuit protection. This link is typically smaller than the wire leading to the alternator and is able to protect the circuit from short and sustained overloading, due to the fact that it is a high temperature wire and has typically a short length, so it will carry full output current but will blow open under fault conditions. I would recommend removing the fuse and replacing it with the fusible link with a 5/16" ring terminal on one end, available at most auto parts stores. The ammeter on your truck is what's called a shunt type meter, the alternator output does not flow thru it like a series connected meter.

 

I agree with the fusible link post. It is there for a reason. I also suggest that you revrify your wiring against the diagram and return it to the original setup. The alternator output wire on mine is connected to the starter relay post on the same side as the battery wire is connected to so that there is always a direct connection between the battery and the alternator. The new engine is probably pretty tight and will require better battery voltage for starting than it did with the old engine.

Once you know that you have that fixed, take a multimeter and put it on AMPS and insert it in between the negative terminal of the battery and the negative cable. If you have more than .050 amps with the key off, you have a parasitic drain. Start disconnecting stuff until the drop goes away. If you had a bad regulator, that could cause a drain.

 

Sounds like the voltage regulator helped... but blown fuse now. Agree get the fuse out and replace with 16 gauge fusible link and ring terminals. The fusible link should be available at the AP store. Crimp or better crimp/solder the ring terminals and assemble terminals with a short 3/16" screw and tape it up.

A 42 amp alternator and 30 amp fuse is not a happy combo... .