Truck would not start after being sitting for 2 weeks. I had to jump it to get it started. I checked the alternator voltage and it is charging at 13.7v. Is that enough? Or is my alternator bad?


Thanks for the help!

 

You should be running around 14.1 volts. Try putting the headlights on high and a few other electrical loads and see what your reading then.

 

Mine last time I read it was between 14.1 to 14.5. I hear its bad to be over that. but it should still sit for two weeks and start. maybe you have a draw

 

Also possible that you have battery-terminal corrosion.

 

Thats not too bad, driving with my lights on, radion on, heater/ac blowing full blast, and a trailer hooked up. I think I dip down into those numbers at idle.

 

Is that right after you get it started from dead batteries or after 30 minutes of running or so? If right after I would say normal and look for the drain. If it is at that while running for a while, then it is possible you are not getting the amperage out of it. But most time you won't see any charge when the alt goes south.

 

Here's the specs from Ford on the alternator:

Ford Specs
Generator Output (Minimum) 87 A at 2,000 rpm
Generator Output 13.0 - 15.0 V at 2,000 rpm

Do as megawatt suggested. Before putting the extra load on it, hook up a multi meter to your batt and see how far it drops when you first load it with lights, fan etc.

You're okay at 13.7V but recheck it every day for a week or so to see if that is characteristic of yours. My first one read 13.6V but three days later I noticed my batteries seemed a little weak and later that night I was on the side of the road in the rain.

I got a jumpstart and pulled out the multimeter. The alt was only putting out 10.8. It was apparently in decline, and I didn't know. If I had checked it every day I probably would have caught the decline and not been stranded.

Austin

 

mine is always 14- 14.5 , 12 when starting and running the glow plugs

 

It probably also still has a weak battery, let it charge and check it again after you've driven it for a while.

 

Well the batteries are less than a year old, and so is the alternator. Advance autoparts tested the alternator and it was running at 11.1v but that was right after the jump start.

 

I recently changed my dying alt and it was putting out about 11.5v when I swapped it. Is your battery light on in the dash?

 

I found that my Alternator was testing normal on the truck, or I should say not turning up bad, but when I took it off and went inside to test it, it showed up bad. I was putting out 13.6v

Edit: It was also an advance auto parts less than one year old alternator

 

Even at idle you should be putting out more than 14V. The voltage is the same output regardless of RPM. The load is what drops you down. If you run your headlights, fog lights, stereo, dome/map/cargo lamps and neon at idle then your draw will be more and as the amps go up to supply all these loads your voltage will go down (your batteries are also pretty good draw too if they are low). This can be demonstrated with Kirkhoff's Voltage Law below:

Voltage(E) = Current(I) x Resistance(R) (E=IR)

If resistance stays the same (can't change design) and you up your demand (I) then your voltage (E) drops when your alternator can't supply the current needed (the equation must maintain balance).

As you increase RPMs and your alternator can reach rated output (more amps) then it can meet the demand (I) of the load and your voltage (E) will remain at rated output too.

To check your alternator I would drive it around for a while (if you suspect your batteries are low) and then park the truck, turn off all the accessories (including headlights), pop the hood and take a voltage measurement at idle. With very little load you should be well within the capability of the alternator (I) at idle and should have (or be close to) rated voltage (E) which is 14.4VDC for an automotive alternator. If you are not reading at least 14.0VDC I would be concerned about the alternator (or the voltage regulator unit inside). That's my personal comfort level.

Just my .02

 

normally peak amps and voltage come in about 1200 RPM

 

I agree that the alternator reaches peak output above idle, but the voltage should still be at or near rated with almost no load (discounting any draw from major electical aftermarket equipment). The last time I checked my alternator (after I had to replace my dying batteries) I was reading 14.3VDC at idle.