mikes84vette

on my car, when i crank it for the first time of the day, or the 20th time of the day, it will not start charging till i rev it up to about 3k rpms. when it is idling @800 the voltage is about 12.7 with fan running. @ about 1k rpms, voltage reads about 13.7-14.0. is this a normal thing? at what rpm is the alternator putting ot its max? when i shift the car into car gear, and my foot on the brake, the rpm will fall to about 500 and volts to about 11.7, until i let off the brake and give it some gas it will stay there. that 11.7 is about where it is when i crank it. it will stay there till i tach the motor out @ 3k rpms. does it sound like i got a problem some where? or is everything doing like it should? once it is cranked and starts charging everything will work fine till i kill the car, and crank it again. then the above takes place again.

Guest

Definately does not sound normal, when my 84 was stock it kept the charge pretty much even at all times. Your battery may be going bad. You can always take the car to a Shucks or Autozone and have them test the alternator output for free. It has and internal regulator so all thats involved in the circuit is Battery, Alternator, wiring. If you don't have the schematic, let me know and I'll see what I can scan.

HTH

jfb

The regulator in the alt is designed to maintain 14.3-14.7 volts and the current is limited to the alt rating by decreasing the output voltage. Max output is possible when the alt is turning about 1200 rpm (by my car voltmeter). At 500 rpm, the alternator output cannot maintain 14.3 volts because it isn't turning fast enough and the battery is pulling a lot of current right after cranking the starter. Normal idle speed is 750 or so. When the alternator is cold , it can maintain 14.3 , but the voltage regulator isn't well temperature compensated and the alt voltage drops to about 13.5 when the alt gets to operating temp (all with driving speed rpm's, not idle rpm). Check your dash voltmeter against one on the battery posts, the dash is usually about 0.3 V low. When the battery falls below 12.0 volts, it isn't being charged and with high amp load and sitting at a light, you might observe slowly falling battery voltage, but don't become concerned unless it doesn't correct when you get under way. Hope this helps.

mikes84vette

i took it to autozone output was almost 15.0volts. when it drops the most is when i shift to reverse. i know the backup lites come on to put more of a load on it, but it will drop to around 10.5volts. you have to keep your foot on the gas pedal to keep the car from stalling. once you shift to drive and get rolling everything goes to normal. @13.5-14.0volts. the battery will hold a 'good' charge.

scorp508

Optimal voltage ratings is 13.8-14.3. You need to maintain at least 13.8 to properly charge your 12 volt system.

mikes84vette

just one more thing to add, then i will shut-up. the alt will only maintain charging voltage, 13.8 or above between the rpm's of 1k-3k. @ lower or higher rpm it will not carge at all. when this is occuring volts at alt or only equal to volts at battery, usually 11.9. does it sound like a bad alt?

Daniel DK

Mine charges even when ideling at 500 rpm but only if the battery needs it bad!! I don't know if there is a relay that hooks up when needed but when ideling my dashboard lights up a little more when it kicks in.... also when i light up the rear window defroster when ideling it always kicks in...

BTW: mine is a 92 so it proberly works i a totaly different way :smash: :rolleyes:

jfb

It sounds to me that you have a defective alternator. Not only that, but I have never heard of such an unusual symptom where the alternator only charges between 1k and 3k rpm. The only thing I can think of that would cause an alternator to do this is an out of round slip ring which keeps the brushes off them above 3k rpm. The test at the parts store didn't vary the rpm and therefore wasn't complete. I'd replace it if it were mine.

Hawaii89vette

I think it sound ok to me. Mine does the same thing at idle if my A/C is on (11.9 volts measured across battery. Car Dash volt meter is 1/2 volts low)
The A/C seems to take alot of juice. If I turn off the A/C , the voltage at idle goes up to 12.5 and upwards depending on the engine temp (the cooler, the higher volts). When the RPMs go above 1000-1200rpms the volts go pass 13 volts.

-Tim

SunCr

I'll get a 11.9 at the gauge; 12.1 at the battery with a hot motor and high a/c. Your blower can suck about 20 amps. Both fans, 15 amps and up. Add the brake lights, radio and what have you, you've let the air out of alternator which is putting out (if you're lucky) 40 amps; hence, you're running off the battery. Now mine, like Mikes84, started dropping a volt in reverse some time ago. Resistance is good to the bulbs and ground, so the only thing I can figure is that the connectors are at fault and I either need new ones or the old ones should be soldered. Also, I've found excessive resistance in the ignition switch was dropping my gauge reading .5 volt. Cleaning it up with electrical cleaner got it to a .2 volt drop.