My truck with new alternator on the 390 is only charging at like 12.1 volts and dont reach 14 till 3000rpm

 

Any heavy electrical draws such as electric fan or fuel pump? Maybe the alt. pulley is too large, letting the alt. turn too slow. Mine is about 13.8 to 14.1v @ idle, but I have the large 100amp alt. on mine.

 

unplug the regulator and jump the "+" and "F" wire , if the voltage goes up the regulator may be weak.

 

The voltage measured also depends on the state of charge of the battery.

 

also your idle speed. the alt needs 900 rpm to do its thing.

 

I've had several vehicles that wouldn't show a charge unless you revved the engine once to get them started.

 

There are so many variables to take into account that it is difficult to answer your question. The state of the battery, ambient temps, battery terminal connections, condition of the alternator and regulator, idle RPM's, etc. It sounds pretty much like a normal system although 3000 seems a little high to reach 14 volts. I would think that 2000 RPM would be more ideal. If it is reaching 14.1 volts I would suspect that the regulator is doing its job. I would see if I could borrow another battery from someone to see if the idle charge would come up a little. It sounds more like a weak battery to me. I have seen alternators lose one of their diodes and have symptoms like yours. You could also put your battery into another vehicle and see if there is any change in the charge rate. That would be a lot less expensive trouble shooting procedure than just shotgunning by changing out expensive components until the problem is found. (If you have a problem at all!)

 

That is running an Optima Deep cycle, full charge, no accessories on at the time, all new cables, completely newly built wiring harness, brand new NOS Ford regulator, and the factory size pulley. And i am wanting to stick plenty of lights on this thing later on too.

 

You should have posted all this info on your first post. Your alternative are,

1. Change the pully to make the alternator turn faster, but if you plan on high revving the engine this could cause the alternator to spin the solder off of the rotor at high rev's.

2. Put on a heavy duty alternator designed for fire trucks, school buses, etc. Only disadvantage high cost/size/weight.

3. Convert the lighting to LED's. This is simple and easy to do. I changed all of mine out and decreased my current flow from over 20 amps, with the brakes on, to less than one amp. The only lamps that are not direct replacement for your truck yet are the headlamps.

If you want info on LED's let me know and I will get you the web page to check out.

Good luck, Gary

 

<i>completely newly built wiring harness</i>

I think maybe something is screwed up in the new harness, or the new regulator or the new alternator is fuxored.

You might have either inadvertantly grounded something, or the wiring to the regulator may be wrong. Simplest check would to remove all fuses except those absolutely needed and see if the voltage goes up. Then retrace the wiring between the regulator and the altenator. Then swap the battery, then get an el cheapo regulator and swap that in. (El cheapo regulator may not work correctly, but it shouldn't be broken in the same way as the current one.)

ash
['Fast, not neccessarily good.']

 

What you describe is Normal

The alternator is only the dynamo.

The voltage regulator doesn't kick-in until 850-900 rpm.

The VR is what does the extra output

Deep cycle batteries are designed to be charged SLOWLY, not rapidly.

I assume this optima is an AUX battery hooked to a battery isolator ?

 

is that place with the LED's reasonably priced? I have got LED taillights and 194s for all around the truck when i get time.

 

Thats funny, my truck idles in gear at 650 rpms (factory spec) and my volt guage reads 14.5 volts all day.....running at higher rpms it goes up around 15-16...and thats a stock replacement alt. too.....ive seen new alts (actually remanufactured i guess) that only put out 11 volts cause they werent wrapped correctly...Ford truck guy, take off your alt and have it tested, that way you can at least eliminate it as a possibility and go from there(plus its free)....you can have your voltage regulator tested at the same time, but ive had autozone tell me my VR was fine, but it really wasnt, and it caused me alot of headaches cause i thought it was good to go.....

 

Mil1ion:<i>What you describe is Normal</i>

12.1 volts? Something had been bugging me about that - 12.1 volts has the battery <i>discharging</i>.

My harness is original and charges at 13.8V or so @ 600-650 rpm. (Excluding the shape the taillight assembly section was in, my harness is solid.)

<i>completely newly built wiring harness, brand new NOS Ford regulator</i>

That's it! (It took some beer to get that gear shook loose. Happy NY) Is the new harness wired for guages or the idiot light? And which was was the truck setup?

With guages, the power to engage the VR comes from one direction. Without guages it comes from a different direction.

ash
['I'm close, dammit.']

 

I had a charging test chart from firsthand testing located on one of these threads in here.

Basically the systemes I have tested run around 11.8v -12.8v at idle without any load on the charging system at the time.
Once revved up to 900+ this value goes up to about 15.1 v.

With increased load on the system by running:headlights,heater & wipers this voltage value drops accordingly.

Testing (adjustable) VR's in vehicles from the 60's was a lot more intense than from the 70's/80's with the advent of the newer style VR's.

I think it is so easy for these newer type VR's to be built cheaply and end up failing or having electrical values all over the place ......than it was for the older ones.

Back in the 60's it was mostly un-heard of replacing a Voltage Regulator.
They all seemed to be still intact on a car or truck that was or is in the wrecker's.