Alternator question

charltons · Post by **charltons** » Wed May 10, 2006 7:46 am

I recently switched out my alternator & installed a new battery actually, a new almost dead battery- I realized the alt. was bad after swapping batteries)

. My ammeter reads around + 30- 40 when I amd driving, then drops just within the negative when I'm idling. Is this normal? And if not, how do I go about testing it? I drive the truck about once a week, so is it just charging the battery? I know this may be a simple question, but I have usually driven newer cars, and the old Beetle I had didn't have an ammeter.

Jeffc · Post by **Jeffc** » Wed May 10, 2006 10:04 am

swings in charging like that happen more often if
the battery is not fully charged. How long will it
take to fully charge a batter from dead? depends on
the battery and its storage ability, but just a few hours should fully charge the battery in daytime use.
It has been known that charging a commpleatly dead
battery can "kill" a alt, always better to put them on
a charger before use (overheats the rectifiers, capacitors, ect. in the alt and can kill one or more).
You might want to have the output on the
alt checked just in case. Anything less than 13.5 volts
at around 1000 engine rpm is too low to charge the battery (and run everything else).
Since your alt puts out less a low engine rpm you may see the needle swing too the D side if under load of lights ect. If it goes past the frist mark on D side
you got a power drain somewere in the system if the truck is running.

Most auto parts stores will test the system for free
in hopes of selling something, of course if you need it your already there.......

charltons · Post by **charltons** » Wed May 10, 2006 4:12 pm

Thanks. I did put an almost dead battery on- I was in a hurry to get a rental back to Home Depot; I hope I didn't kill an alt just to keep the rental @ 4 hours! How would I go about finding if there is a power drain?

Jeffc · Post by **Jeffc** » Wed May 10, 2006 7:42 pm

Frist thing to check for is a voltage drop (everything off including the engine) between the battery and the alt. (should be around 12v if battery is fully charged at both alt and battery), then the same with engine running (should be around 13.5 to 14.5 both at the alt and battery)..... if battery is near full charge then the readings may be closer to 12, turn on headlights and retest, should be in the higher range.

A drop in voltage between ether side says there is
a problem, ether poor grounding, poor wires, or any combo of ether.
Low voltage output at the alt is ether
low engine rpm, or dio problem in alt, or bad (or) out of adjustment on voltage regulator (sometime this last one is because the regulator is not getting the right voltage, see below).
Note: if pully is too large on alt, your alt will have a hard time keeping up the right charge at low rpm. If the parts store has 2 alt's on the shelf, choose the one with the smaller pully if you can>always...

next is to check at every fuse clip with the key in both run (to the right of off) and accessories (to the left of off), both sides of fuse (fuse in place),
for the same. Sometimes you will find a drop across the clips even though the fuse is just fine, most often because the fuse holders are rusted.

Next, engine off, key in accessories, hook tester at battery + and ground other side (use a helper if needed to read meter) take reading, pull each fuse and look
for a voltage incress. If you find one then you have a drain on that set of wires.

It helps to have a wiring sheet to be able to trace were everything goes. Testing things one at a time
and find out what does and does not work......
If I recall, Scott has some posted on site to down load.