I’ve got a ‘66 f100 with a 240. I put a 75 amp alternator in there and it’s just barely adequate. Its fine while driving but with everything on, you can watch the voltage drop at idle.

I daily this truck so I need something that will be adequate in the winter when I will potentially have more things on in the truck. It was suggested that I find a 3g motorcraft alternator. I noticed on some one wire 10si units there there are provisions to keep using the dummy light. It would be cool to be able to keep it working, but a reliable vehicle is more important to me. Does anyone have suggestions on something that would be a good fit?

 

If you want more current from an alternator at idle you are going to have to change pulleys and spin it faster. Alternators need to spin pretty fast to produce full output.
I don't know what the ratio is on an I6 but on a stock 351C it is 3 to 1. I reduced the pulley size on my engine because I couldn't stand the thought of the alternator turning 19,000+ rpm when I hit the rev limiter.

 

This could be a cheaper way for me to go! It would hurt a lot less to be able to keep this alternator since it’s brand new, but not brand new enough to return.

 

What all are you running that it draws from the battery at idle?
What gauge, AMP or Volt, is it factory or after market and is the wiring still factory?
What was the ALT AMP stock for that truck as I am sure it was not 75 AMP maybe close to 55?
How far are you driving the truck after you start it and how much of that is idle time?

If you dont drive the truck enough after starting or restarting, like stopped for gas or store each time out, then you will never recharge the battery.
Even spinning it faster may not be the answer if not driven enough.
Rule was for every time you started the motor you needed to drive at speed (spin the ALT) for 15 min. to recharge the battery.

I have a 81 F100 with a 300 six and factory ALT, dont know the AMP?
I drive it a lot both winter (heat & lights) and summer (AC & lights as I work crazy hr) and I never had an issue with the battery going dead.
Now I do drive it 40 miles to work (high way) and 40 miles home on back roads (with some idle for lights).

Weekends I use the truck for weekly trash run, maybe to the store a mile or so down the road and to / from local car shows.
Again heat or AC on as needed, fan on low for vent if no heat or AC, and factory radio going 100% of the time all the time.

The only time I may see any kind of issue is when I first start the truck to go to work, between 12am & 4am, and with the head lights on pointing to the garage door I can see they dim at idle. But once the truck has run to recharge the battery from starting I dont see this.
I also have relays on my head lights so they are getting power right from the battery and this could be why I see the dimming just after start up?
Just my .02
Dave ----

 

I agree with the smaller pulley on the alternator. I put one on my '65 240. I will still get down to 12 volts when idling in gear with all accessories on. You will never turn that engine much up above 3K, so I have no concerns. The other thing that helped was putting LED lighting on the truck. The lights are much brighter and draw less current.

 

I very simply put a 100, 1 wire alternator, very simple to install and it eliminates a lot of unnecessary wiring.
I at first was unsure of the whole 1 wire connection, but there are a lot of videos on YouTube explaining it and showing how to install

 

when I’m driving it’s charging at 14, it’s just when I’m sitting at idle and it’s every damn accessory on. Right now during the summer it’s a non issue, I’m thinking worse case scenario during the winter and sitting in construction traffic.

I dont have the original wiring, I did a painless rewire.

 

I really like this idea. It saves me a couple 100 bucks and it takes care of the real issue of the alternator not spinning fast enough at low rpm. And you’re right, my tractor of an engine will never see high rpms.

 

I did the maths and I wasn’t at a 3:1 ratio. I perused powermasters site and all of their alternators are recommended at 3:1 for street use. I found a pulley that got me much closer to it. I’ll see how it goes when it arrives.

 

Figure your least expensive option would be to increase your engine idle speed.

 

So heater, head lights and maybe wipers on all at once?
Stuck in traffic and have the truck die from a dead battery would not be cool.

So what size pulley was on yours now and what size did you get and from where?
Dave ----

 

66 240 manual here. Stock replacement alternator 55a. LED lights all around. No issues holding 14v with all loads on. Idling 700rpm. I replaced my voltage regulator with a new solid state one. Make sure your belt is tight and not slipping.

 

I just got my pulley in and tested it and it solved my issue! I got it from Summit. It’s promaster v belt pulley 111. It’s outer diameter is 2.342”. The one that came on it was about 2.625”.

I had everything on. Brights, wipers, blower in high, and when the electric fan hit, it dropped to a stable 12.6v at idle.

Hopefully when I get to adding a stereo, it will still work out for me. If not, that’s fine. It’s not on the near future and I know to expect that I’ll need a different alternator.
I haven’t ditched the old headlights for leds yet, so that is an option as well.

 

I responded to a later reply with an update. I just tossed a 2.342 and it changed it enough to make a pretty big difference for me.

I just tested it out with blower on high, brights on, wipers on, electric fan on and it went to a stable 12.6v while idling.

That left me feeling a bit more confident.

 

thanks so much for the changing pulley tip! That got me ina. Much more desirable place! With everything on, voltage dropped to a stable 12.6 v at idle.