Voltage Dropping

[MossyOak07]

I'm having some intermittent voltage drops. I notice it mostly at idle. The headlights will dim a bit, the heater fan slows down and battery gauge drops close to L. Its a 98 4.0 SOHC.

I don't believe its the battery, its only a couple years old. I have not completely ruled it out though. It does start fine also.

I also noticed while messing around today that while sitting in park, if I hit the gas really quick it will idle up like normal, but as the RPM's come back down it will stutter out and die almost every time. Not sure if this is related or a separate problem.

 

[pbnj22]

sounds like the alt is not putting out enough juice at idle..

 

[MossyOak07]

That was my first thought. If that is the case could that cause it to die at idle, but start back up perfectly fine? Or am I looking at 2 different issues?

 

[pbnj22]

well that second issue sounds like a vacuum leak

 

[my98nnj]

Of your idle speed is to low or drops slightly, your voltage will drop too.

Get the truck to idle steadily at the correct speed and your voltage should be stable.

 

[deweyville65]

Have you checked the voltage out put with a volt meter? Preferably a digital VOM. Check at idle and at around 2000-2500 RPM. That will tell you more than any of us here can. Also, clean the battery terminals and battery cables.

 

[SoNic67]

Voltmeter on battery while engine running should be 14.2-14.4V. Test with lights on and off.
Advance Auto Parts also tests the alternator/battery - for free.

 

[MossyOak07]

I'll be hooking the volt meter up hopefully this afternoon.

 

[deweyville65]

The main thing to check is the voltage with engine off, at idle, and at running RPM. Lights on and off is a good idea too, but I have never done that. If you have at least 13.6 v. with the engine running, then the alternator is working.

 

[SoNic67]

13.6V running is a bad alternator (one diode is fried).

 

[bill06447]

13.6V running is a bad alternator (one diode is fried).

Awesome. I've been running a bad alternator then for the past year...

Bill

 

[MossyOak07]

I am running 14.07 volts at idle and 14.41 at 2000 RPM

 

[DjDom]

Doesn't sound too bad ^

It may actually be the battery.
My alternator is new, but the battery is quite old, and at idle, when I have a lot of accessories on, I can get quite a low voltage..
That's with Blower motor on high, rear defrost, wipers, full headlights, aftermarket fogs, radio and stuff plugged into 12V power outlet..
I need to get a bigger alternator.

 

[deweyville65]

Well, I am not an electrical engineer, but I have had vehicles running 13.6 volts for years. Damn lucky I have not had to be towed home then.

 

[SoNic67]

Then either your meter is uncalibrated or the alternator is bad.
See here the voltage levels:
http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery#Voltages_for_common_usages

Typical (daily) charging: 2.37–2.4 V (depending on temperature and manufacturer's recommendation)

You have 6 cells in a '12V' battery.

 

[bill06447]

Wikipedia is nothing more than what someone else wrote and posted, right or wrong. That said, charging rate will vary depending on how discharged the battery is, temperature, the age of the battery (i.e., internal resistance). 13.6v is perfectly normal at certain times. So is 14.2. Anything below 13 will most likely start causing you problems down the road, although I had a Windstar for 6 years that rarely charged over 13.2, battery would read 11.8 in the morning, and it never failed to turn over and start.

The important thing is that the alternator is able to vary the voltage at different engine rpms, increasing after start and leveling off once the battery is recharged.

Bill

 

[blakshukvw]

My alt runs at 13.6 volts as well.

 

[SoNic67]

Wikipedia is nothing more than what someone else wrote and posted, right or wrong.

You can google your own sources.
However, it won't change the facts that a lead-acid cell needs 2.35-2.40V to proper charge. That makes a range of 14.1-14.4V.
A 2.30-2.35V per cell leads to: slow charge time; capacity readings may be inconsistent and declining with each cycle. Sulfation may occur without equalizing charge.

 

[blakshukvw]

What alternator are you running sonic?

 

[deweyville65]

What I have found over the years amounts to this. Reading a book and real life situations are not always the same. Its damn odd that over the years, the standard for alternator output was 13.6 volts. Now we are being told we have been wrong all this time. End of discussion.