First off the car runs pretty good, no real serious problems while driving... aside from when I'm sitting still at a light or parked while running... the volts drop from 13-14 where they're at when in motion to about 7-9??? Whats the deal?

I replaced the cap & rotor, new spark plugs, even a new alternator.... it is a 140 amp one though if that means anything... got it brand new for $50... couldn't pass that up... anyway what could be causing my low volt problem?

 

hmmm, i've heard that underdrive pulley's can cause that since you do have them on your car. or maybe dying battery?

 

Yup. I'd be very suspicious of that last item in your Mods list. If the voltage goes normal with a little more engine RPM, you've already answered you own question.

 

V man is right on one point. Other point is that a 140 amp alt will generate LESS output at low rpm than a 105 or lower. Many times with a high amp alt the manufacturer recommends an under size pulley to bring up alt rpm. i know it sounds crazy, but more is not always better. High amp alternators will very quickly bring the batt up to a "surface charge", but they do NOT deep charge the battery. What you are experiencing is probably normal and really not an issue. From a practical point of view a 60 amp unit is better than any high output unit. One of these days I would love to write an article about charging systems and dispell all the myths that seem to prevail about them. Guess I'll have to get invited for that, but trust me it is my area of expertise. Once a month trickle charge the batt and keep the charging system in good order and you should be OK.

 

I understand that it is within your area of expertise. Consider yourself invited.

 

OK, Danno. Please explain to me when I swapped to a caprice 140 amp alternator (junkyard) my bolts at idle NEVER drop below 13 volts even fully loaded down with everything on. My stock 105 amp could never do that, it would quickly drop to 10 volts during the same test. That's a main reason why I swapped to the caprice alternator.
Also I beieve my performance has been improved since the electrical system always has full battery voltage now.

 

Quick question for you then: what would be the ideal alternator to use in a car with a lot of current draw from the audio system? I was thinking about upgrading my alternator help the system along and keep the voltage up when it's on. Would a higher output alternator compliment this?

 

mine did this anyways....without under drive pullies...i could turn on all my accessories and could get my gauge all the way down...just a crappy guage i guess...

 

John, was the drive pulley the exact same size. The engineers usually design the charging system to give a fairly linear charge rate over the normal operating rpm range. High output alternators are usually installed in vehicles where electrical demand in normal use is higher than normal. Police cars(light bars, high power 2-way radios, sirens, etc). The disadvantage of a high out unit is that the battery tends to get "rapid charged". It's whats common with Ni-Cad batteries that are quick charged. What happens is that the terminal voltage comes quickly up but the alt because it only really looks at voltage never gets a chance to slow charge the batt. I'll be brief, here's a case in point. My 89 sits a lot. Popped the hood and noticed "green eye" out indicating a discharged state. It started well enough, but no amount of normal driving brought it up. New batt and alt so no prob there. had to trickle it 16hrs or so and it's fine. Nothing wrong with a 140 unit, but no matter how big the alt it's not a battery charger and will in many cases reduce the life of the batt. Lot of night driving, extra accessories bumping up from a 105 is probably a good idea. A normal TG with a well maintained charging system the stock is fine.

 

My volt guage is not accurate. It drops to 8-9 volts while idling sometimes, and sometimes while cruising. We've hooked a voltmeter up to the battery and alt. while this happens and the 8 volts on dash isn't corresponding to the 13.8 at the battery. The alternator shows 14.8. You may want to check and see if yours is accurate.