My battery light come on this morning. The volt meter is sitting right where it always has though. I noticed that the battery light is kind of pulsating with rpm and at idle its very very dim.

E99 F-250 PSD

Where should I start?

 

Tests should be performed with Batteries Charged.


WIRING:

Check the condition of the Plug at the Alternator and Wiring. Often, the wire will chafe where it enters the bundle or becomes loose / damaged where the connector is. Clean connection with the KEY OFF,

VOLTAGE OUTPUT TEST:

1. Using a DVOM, at the B(+) Alternator Post and the Battery (-), measure voltage. 13,8 - 14.2 is a normal functioning Alternator at 1k - 1.5k RPM.

2. Perform the same test on the Battery(s) using Battery (+) and (-). 13.6 - 14.2 is acceptable.

RIPPLE VONTAGE:

1. Ripple Voltage Ripple voltage or (AC voltage) can be measured by switching your DMM to AC and connecting the black lead to a good ground and the red lead to the "BAT" terminal on the back of the alternator, (not at the battery).

2. A good alternator should measure less than .5 VAC with the engine running. A higher reading indicates damaged alternator diodes.

ALTERNATOR LEAKAGE TEST:

Alternator Leakage Current to check alternator diode leakage, connect the Multimeter in series with the alternator output terminal when the car is not running. Leakage current should be a couple of milliamps at most; more often, it will be on the order of 0.5 milliamps. Use care when disconnecting the alternator output wire; make sure the battery is disconnected first.

 

Thank you! I'll check it out and report back!

 

Nice post by pirschwagon! I had the same thing happen to me. I replaced alternator with my spare and all was well. I saved this post for the future.

 

Well, I charged up both batteries independently last night. Before charging they were each at 12.6 volts, and after each battery was hooked up to the charger on the 40 amp setting for 90 minutes, they read 12.8 volts and 12.9 volts. That seems a little low to me. Especially for two interstate batteries that don't appear to be very old. I've only owned this truck since June.

I'll do the rest of the testing tonight, but I'll add that if it weren't for this light I wouldn't suspect that anything was wrong.

 

I noticed mine doing the same thing yesterday. I did have my gauge out for radio install and changing the gauge bulbs to led's. Everything seems to be working good with volts reading 14.2 after fired up. So, I stuck a pic of my daughter over it 😉

 

Just because you are reading in the 12VDC range doesn't mean all the cells are working. If you are concerned about the batteries most auto parts stores will test them for free (you just have to take them out of the truck for them to test them).

Of all the electrical issues I've chased over the years only once did my battery light come on (with the engine running) and it was the alternator. Conversely, the only time I've had an alternator problem/issue the battery light came on.

Just my .02

 

12.65 is about the standard reading for a decent (wet lead acid) battery that has not been just freshly charged. When I had the flickering battery light (flickering just like yours), it was my alternator that was bad.

Start the truck, turn on every accessory, and see what kind of voltage reading you get at the battery.

 

My test results above in red. I pulled the alternator because I think the voltage was too high. Once I did that, I spun the shaft by hand and can hear the bearings making noise, so it'll need replaced anyway.

 

Replaced alternator - problem solved.

Rejects at the parts store need to go back to school though. Tested 3 times - belt squealing every time and they think that's an OK way to test.

 

Thanks for the follow up.