Alternator tested good, but seems bad.

[Twista]

Sorry, i thought this was in the car section. Can a mod please move.
My apologizes.


How possible is it for a bad alternator to be tested good? I already had people telling me to replace it.

I got both the battery and alt tested. Both passed.

Reinstalled on the car and it ran good surprisingly after the battery was charged @ the autostore till after about 10 minutes, then the battery meter dropped back down.

Soon as i turned on the lights/ac the battery meter dripped down real low. SOon as i turned them off the meter jumped back up.


Soon as i revved the engine the headlight would brighten then dim back down after the rev was over.

I can jump the car, but it will turn off by itself if you leave it idling to long then it needs another jump.
Driving it doesn't charge the battery up.. its actually drops the battery meter down more and more and more till its dead.

-Car doesn't drain the battery while its off.. so its nothing left on.


:hmm::hmm: Now people are telling me there alternator was tested good, but the problems went away after buying a new alt.

Its a 15 year old car with a 15 year old alternator.
1995 mustang GT
Car just hit 100K miles 2 weeks ago.

 

[Paladin3]

Loose belt not turning the alternator fast enough to supply power to lights/ac and charge battery? Bad wires?

 

[DrPizza]

voltage regulator? Moving the thread to the garage.

 

[Twista]

Thanks for the move.

FYI:
Its a 15 year old car with a 15 year old alternator.
1995 mustang GT
Car just hit 100K miles 2 weeks ago.

 

[Vette73]

What is the voltage at the battery when the car runs? Then check the voltage at the Alt when running?

Could be a bad cable/connection. So if the voltage at the battery is low but the Alt is fine then its in the line/fuse somewhere.
Also take the Alt to Advance since you tried a AZ and see if their machine agress the Alt is good. If it is then check the line coming from the Ignation switch. The Alt also needs a signal to charge. So it could be agian in the line or switch.

 

[JeffreyLebowski]

Could also be battery.
I just had a Optima red top go bad on me and had the same issues you described.

It would charge fine from a battery charger, but would not charge from the alternator. Had alternator tested and it tested good 2x. Put new battery in and problem soved.

 

[LordMorpheus]

is the 95 the first year of the 4.6 mod V8 or was it 96?

Anyway, I've got a mod V8 in my lincoln and I had my alt go bad - bought a Duralast remanufactured part and it seems I'm replacing them every year or so (hooray for lifetime replacement, that transfers to the replacement part!).

Last time it went it was an intermittent problem - it would be 100% fine for a while and then it would no supply any voltage at all for a while. When I drove to the local autozone to get it tested / get a new one, I was running on the battery. accessories started switching off and the engine started stumbling as I got near my destination - and then once I was there, the alt started working again. The autozone people gave me a replacement, and refunded me when I brought the old one back even though it checked out OK. Nice group at this one (Ithaca, NY).

It was really strange driving down - I have an all digital dash, and it just switched off about halfway there, so I had no instrumentation on the way down.

 

[Twista]

No, 96 was the first year of the 4.6L engine for the stangs


Well i took it to Murrays and had them run it 3 times. It passed 3 times.

I said okay.. thanks. Left and went to Autozone.

FAILED with flying colors Exactly what i wanted to hear. The thing works when it wants to and cuts out when it wants to.

LordMorpheus.. mine is doing the exact thing. It works at one point and doesn't at another.

 

[LTC8K6]

Yeah, must have been an intermittent problem. They can drive you nuts.

 

[bruceb]

Could be a loose belt. But alternators have brushes that wear out over time. It can also be bad internal voltage regulator, bad diode (they have 3), check both your battery cables for good connections (both ends of them) be sure they are clean and tight.

 

[kornphlake]

I don't believe all auto parts stores testers are equal. Likewise the counter monkey running the tester may or may not know what he's doing. Alternators have wearable parts that are known to fail, it's the most likely part of a charging system to have problems if the battery is taking and holding a charge. The problem may be a bad ground or corrosion on a terminal, but the odds strongly favor the alternator with 100k miles.

 

[franksta]

No, 96 was the first year of the 4.6L engine for the stangs


Well i took it to Murrays and had them run it 3 times. It passed 3 times.

I said okay.. thanks. Left and went to Autozone.

FAILED with flying colors Exactly what i wanted to hear. The thing works when it wants to and cuts out when it wants to.

LordMorpheus.. mine is doing the exact thing. It works at one point and doesn't at another.

It could be the idiots at Murrays didn't connect it properly. My wife had the same problem with the 35 year old alternator in her '68 Mustang. Advance Auto parts said it was good, Autozone said it was hosed. Autozone had the correct diagnosis.

 

[JeffreyLebowski]

Well if it is bad, don't buy the part store brand crap. Like someone mentioned, the Duralasts, don't.
Spend the extra $10-$20 and get the OEM one if you can.

 

[Twista]

Well i took the ALT to another autozone today. It passed 5 times with flying colors. I brought a alternator anyway..

Car runs perfect now. My underdrive performance pulley sucks the voltage down @ idle/red lights, but thats normal for underdrive pulleys. I might get rid of them soon.


And anyone out there.. take autozone/murray/advanced auto/pepboys alternator test with a grain of salt. I guess if your ALT is not 100% dead then the test will say you pass

 

[dawp]

ya, I've seen that before, it can pass but have a bad internal connection so when there is a load, it fails to charge the battery. as long as they maintained pressure one the plugs it works but when you reinstall, it will lose the connection in a short time.