Electrical Short

[Ben90]

Hello everybody, I have a 87 Fiero GT that seems to have developed a short somewhere. The battery gets charged and holds it fine as long as I disconnect the terminals after I finished driving it. It seems to lose its charge in a few hours if the terminals are left attached.

I believe the alternator is shorting out internally. The voltage is perfectly fine until taken above 3000RPMs, then it slowly starts to drop as I rev higher. I noticed both of these problems happening around the same time.

I whipped out my trusty multi-meter and disconnected all the alternator terminals I could find. Taking the reading between the battery and battery-post-on-the-alternator reads 11.5V. I'm guessing this is indicative of a short? I'm still a car noob, but I really want to do most of the work/diagnosing myself so I can start learning. Thanks.

 

[PottedMeat]

try driving it, then disconnect the alternator only. measure the voltage. the next day check the voltage and if it's the same, you've probably found the culprit.

looking on rockauto it says you've got the really common DELCO 12S or some variant alternator, there are lots and lots of troubleshooting documents online for those and you can get the internal parts very cheaply.

 

[phucheneh]

Your post is confusing.

What does the voltage read at idle? What does it read above 3000rpm?

If you're getting 11.5v all the time, all you're reading is a mostly-discharged battery. The alternator is not charging. Check the voltage supply and/or sensing wires in the small connector on the alt.

 

[Ben90]

12.0V when the car is off and everything disconnected
14.5V when the car is running and under 3000RPM
Less than 14.5V when the car is above 3000RPM, probably around 12 judging from internal guage.
11.5V between the battery/alternator connection when car is off.

In any case, it does seem to be the alternator as the battery doesn't drain when it is disconnected. Do you recommend trying to diagnose the alternator or just replace it? How difficult is it messing around with the internals on those things.

 

[PottedMeat]

12.0V when the car is off and everything disconnected
14.5V when the car is running and under 3000RPM
Less than 14.5V when the car is above 3000RPM, probably around 12 judging from internal guage.
11.5V between the battery/alternator connection when car is off.

In any case, it does seem to be the alternator as the battery doesn't drain when it is disconnected. Do you recommend trying to diagnose the alternator or just replace it? How difficult is it messing around with the internals on those things.

looks like a remanufactured one is ~$40. i've fixed a 10si before and they're really easy to work on (assuming it isn't crusted over or anything). if you're going to do that i'd replace the brushes too. i believe there are lots of websites that will walk you through a diagnosis.

 

[phucheneh]

12.0V when the car is off and everything disconnected
14.5V when the car is running and under 3000RPM
Less than 14.5V when the car is above 3000RPM, probably around 12 judging from internal guage.
11.5V between the battery/alternator connection when car is off.

In any case, it does seem to be the alternator as the battery doesn't drain when it is disconnected. Do you recommend trying to diagnose the alternator or just replace it? How difficult is it messing around with the internals on those things.

I still don't seem to follow. I think you're not quite grasping the function of your charging system. Not trying to be insulting; just to help.

The battery not losing its charge while disconnected means absolutely nothing. If nothing is connected to it, it's not going to discharge. Yeah, it will go dead after sitting for a long period of time, as all car batteries do. But it's not any kind of test.

Now, some voltage basics: Voltage is 'potential difference.' You are simply measuring one point in comparison to another.

There are multiple ways to check a charging system with a voltmeter:

1) Probe the alternator output terminal (fat red wire) and the other on the alternator casing (ground). This should give you the actual output of the alternator.

2) Probe the alternator output terminal and the negative battery terminal. This tells you approximately the same thing...however, if the number is lower than above, you have an issue with a ground connection.

3) Probe both battery terminals. This tells you the actual voltage that is reaching the battery. There will always be some drop across the cables, but it should be minimal. E.g. 14.4v at the alt and 14.2v at the battery is not likely to be an issue.

You can also check across positive and negative cables, e.g. alt terminal to B+, or B- to chassis or engine ground. These numbers are 'voltage drop'...voltage lost across a cable. Normally, voltage should only be consumed by a load (lights, electric motors, ect; this is how 'positive' becomes 'ground').

Are you using a voltmeter or trying to diagnose with the a dash gauge? The latter is simply not really possible. You need to be measuring at the main connection points mentioned above.

If charging voltage is indeed normal at idle, but drops at high RPM, it is possible that it is the alternator. But a bad cable is also a likely cause.

As a first step, I would recommend taking the battery to a parts store and having it fully charged and tested (free). Then pick up a cheap multimeter if you don't have one.