Poobs

The alternator light has been going on for a couple of weeks now while at the same time the amp meter goes towards the minus side. It usually does it right off idle at anywhere from 1 to 2 thousand RPM. Sometimes the light will just stay on even if the motor returns to idle or even if I shut it off and re start.

Here's the weird part - I have taken to 2 garages and one AutoZone store where the battery and alternator have checked out fine with their equipment.
One garage did say that the alternator had a bit of ripple on the aoutput but that was considered normal.

All connections are clean as are the fuses and fusible links.

Initially one 25 amp fuse was found blown in the engine compartment fuse box but replacing it has not affected the problem. There's no way to tell how long that fuse has been blown.

A little more background about the problem...

This is a 98 GT with about 66K miles.

The car was purchased 2 years ago with under drive pulleys already installed but the light was never noticed going on until now.

The alternator belt has recently been replaced but appears to be ok. I shimmed the belt tensioner to temporarily apply more tension to the belt in case the belt was slipping - the light still came on.

A new TB and plenum was installed recently. The idle is slightly above 500 RPM mark. The needle rest approximately 1 needle thickness above the 500 RPM mark.

I think the problem may be caused by either the a low idle setting off the light or some other electrical false alarm.

Oddly the both the charging light are on and the amp meter are reading low while the alternator has been under test and passed.

Any ideas will be greatly appreciated?

Derf00

could be the main fuse block/harness is shorting out. What 25amp fuse was blown? Is the right amperage? Since you never noticed it before it could be a higher rating that what it's suppsed to be.

Poobs

I don't know what the fuse protects. I looked under the fuse block cover but it makes no sense. The car did not have an owner's manual so I replaced the fuse with the same rating that was there.
However before I installed the new fuse I conected an amp meter across where the fuse connected to see if it was drawing more current than the fuse rating. I don't remember getting any reading.

The pic shows the under hood fuse block. There is a tape arow right on the fuse

Thanks

Poobs

Sorry I can't get the freaking pic to load

Derf00

I just went back and reread your post. Have you kicked up the idle to see if that fixes the problem? What is the voltage at the battery with the car idling?

Poobs

Thanks, I just kiked the idle up with a small nut threaded into the throtle stop stud. After that I started the car but the light came on soon as I reved it up.

I was trying to follow the ground wire to see where it goes so I can clean it. It snakes under the motor someplace........

Poobs

Well, I followed the ground wire to its bolt near the oil filter. It did not look dirty or corroded but I sprayed it, loosened it and re tightenend it. I did the same to a nearby ground strap that goes to the chasis.
The light still came on [&o]
It goes on exactly on the same point as I watch the tach and goes off as the motor returns to idle.

Derf00

Try having a friend help you with the following.

Start the car let it warm up and idle. Find out exactly where the light kicks on (what RPM) and then take your volt meter (when your friend can duplicate it) and see if there is a drop off at the battery when the light is on. If there is a drop off then it's the alternator. Autozone only checks things at one RPM on their bench test. If they checked it on the car then it probably wasn't at the right RPM or the light was off indiating there was no problem when they tested it.

If the voltage is not affected when the light is on, then your indicator is bad. You may also want to see if you can get your hands on an AMP meter to check that along with voltage. Amps and Voltage are somewhat independent.

Poobs

Thanks man !!! I really appreciate the help.

I did as you suggested. It was bit tricky since there was no one around to help and I was eager to find out what the deal is[8D]

Basically the battery voltage starts to drop off when the dash light is on. The voltage is actually higher at idle than at 3000 RPM ( ! )

I think those AutoZone testers are BS. They follow a procedure and if the alternator is putting out at some point during the test it just concludes the test as passed.

This would make it hard for them to detect an intermittent problem. On the other hand if they har a pile resistor and volt amp meter type tester like in the old days they would probably se the needles suddenly drop

Derf00

The auto part shop testers usually look at the average throughout the test. That's why they dont' do a real time display on them.

Then you have your answer. The alternator IS bad. You could try a rebuild kit if you wanted to since it sounds like it's the stator assembly or a faulty diode (kit comes with all of that )

Or get a new/rebuilt alternator and be done with it.