volt meter drops at idle

[90rangersuper]

I think i know this is a bad alternator just wanna check, when i have the lights on, come to a stop all the lights dim and the volt gauge drops to about 12, idle is at 700 rpm, as soon as i apply gas it perks back up to about 15 volts, i think my alternater is going, i have had the truck for 2 years now, it has always done this, just wondering if anybody else has noticed this or the alternater is bad

thanks

 

[Big Jim M]

Sounds like the voltage regulator is a bit defficient.. I'd just run it till it stops. If it charges with everything in the truck running it is really Ok.
In the old days...which I well remember.. vehicles had generators and all of them did the same as you describe.. In fact the AMP meter on the dash had a MINUS side as well as a charge side.. Every time the engine went to IDLE the amps had to come from the battery and the amp meter would read on the minus side.
I wouldn't worry about it.
Big JIm

 

[MAKG]

Stick an AC voltmeter on it and see what it comes up with. That can detect a blown rectifier diode (it will work OK with one blown diode, but not with two).

As long as it isn't discharging at idle, I wouldn't worry about it (yet).

 

[ak96]

i used to have this prob, not as bad when i had all my lights, heater, defroster on and came to a stop...then i threw a new 130amp alt and a overdrive pully on it...no more issues even with my extra lights on also..made me happy

 

[90rangersuper]

thanks guys, it doesnt discharge at idle so im just gonna let it go til it dies,

 

[Bob Ayers]

thanks guys, it doesnt discharge at idle so im just gonna let it go til it dies,

If the voltage is dropping below about 12.6V, then the battery is discharging.

 

[shadetree]

What makes an alternator charge at idle as opposed to a generator is the small pulley size. It spins the alternator faster producing more voltage. 700 rpm isn't very much engine speed, and with a load on the system, such as your lights being on, the alternator isn't spinning fast enough to produce the voltage necessary to maintain a higher charge rate. With the voltage coming back up with engine speed, I wouldn't worry about it.shady

 

[90rangersuper]

It hase been going strong for 2 years now, even had a 900w sub going up until 6 months ago, gonna pass on getting a alternator swapped this weekend and gonna get that damn front u joint changed. Thanks guys