Normal voltage drop?

[LUV2DRV]

What is the normal voltage drop with a L98 at idle?
I haven't replaced the alternator in several years, and suspect mine is about to bite the dust. Last night, with the heater and headlights on, it would drop to ~12.5v @ idle, then pick back up to 14 or so upon acceleration.
The battery was replaced about two weeks ago.

Thanks for any insight.

 

[callaway4fun]

That sounds about right you have a load on the system and at idle you are not spinning the alternator fast enough . Then when you are driving it is fine .

 

[vigman]

That is correct

@ 600 ENGINE rpm ( where a vette should idle) the ALT can do VERY little to charge a battery. as you get closer to 1000 engine rpm that's when the voltage should climb..

Remember... the ALT charges using increased CURRENT..... Voltage is a by product of increased current..

Do the following experiment.

turn the key ON ( do not start )

Read the voltage gauge

Turn on the headlights
( make a note of the voltage)

Turn on the blower for heat or AC on HI.
( make a note of the voltage)

The current drain has increased to the battery by OVER 25 amps

How much has the voltage dropped ?
leave all this on for a minute..
UNLESS YOU HAVE A WEAK BATTERY

Turn em all off
( make a note of the voltage)


Now start the car and let it idle

read the voltage gauge

turn all the STUFF back on
( make a note of the voltage)

and SLOWLY raise the rpm's where you achieve 13.4 volts..

Note here... the red light in the dash voltage gauge will come on @10.4 volts

The batt light will come on when there is no current flow from the ALT..

HOWEVER

If a Diode is blown in the ALT.. you will still put out current.. and get no idiot lights.. but the battery will not charge correctly due to AC voltage being passed down the power buss in the car.

and the Alt pulley vs crank pulley is about
( rough guess here) a 1 to 4 ratio (crank/alt).. so at idle the alt shaft speed is ROUGHLY 2000 to 2400 rpm.

Head hurt yet?


Vig!

 

[LUV2DRV]

Thanks for the replies. You've eased my worries :cool