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Can an alternator test good and still be bad?

My 1998 Lincoln Town Car has decided to turn on its little battery warning light on the dash, which I assume would mean that the battery is not charging. The alternator is about 3 years old (purchased at an auto parts store by the previous owner). I removed it from the vehicle and took it to Napa and they said it tests good. I called my usual mechanic and he said it could still test good and have an internal diode burned out.

I measured the battery voltage when the car was off and it read about 13v. I started it and it actually went down to about 12v with just a little bit of meter shaking.

I also removed the thicker (single) wire from the top of the alternator, started the car, and read the voltage at this point on the alternator. I used the alternator metal case as ground. I read about 8 volts and, again the meter was shaking (a bit more), almost like there was some AC in there.

So before I buy a non-refundable alternator and install it, I'm just wondering if it can test good off the car and still be bad?



  • edited December 2010
    From what you said it sounds like there is a problem with wiring on the back side of the alternator. On most cars the battery warning light supplies power to the exciter field of the alternator and that lead should have close to 12 volts on it under normal operation. If the lead voltage goes low for some reason the light will turn on and the alternator will not be able to generate any power to charge the battery.

    You might try removing the rear connector to the alternator and then start the engine to see if the battery warning light still lights up. If it does then there is a problem on the field lead. It may have a short to ground on it.
  • edited December 2010
    If you have a bad diode you will get an a/c ripple. I replaced an alternator with a bad diode on a 90 vw golf. I had the auto parts store test it and they said no its good putting out 14 just like it should. I asked them to do it again and see if it had an ac ripple. Busted... Time for a new one.

    Here is a good vid on how alternators work if you wanted to know.
  • edited December 2010
    Alternator diode checking can be done on the car with a DMM. The spec is no more than .5VAC with the engine running and no more than .5 milliamps on the output with engine off.

    Bad diodes happen (more likely to go open than short) but they are something I have never actually seen in my 35+ years in thee shop. For me it was always full no output that caused me to replace alternators.

    My point here is you are looking for the wrong value with engine off on the output, you should be looking for milliamps with the test you describe and looking for how much AC you have with engine on. Really you should be looking for voltage and amperage output at 1800 rpm or so, and if you see 80% of rated amperage output you are doing fine. What voltage are you seeing at the battery at 1800 rpm? if this is good go ahead and take a look at the AC component (remember less than .5VAC.)
  • edited December 2010
    Thanks for the input. I just did a few more tests after your message. First, the alternator is currently out of the vehicle. I turned the key to the Run position and the dash battery indicator is off. While still in Run, I read battery voltage at two of the three pins at the connector that plugs into the alternator. The schematic shows two pins as "Hot at all times". I'm assuming these are the two pins that should be hot.

    I also notice on the schematic that there are 2 15A and 1 40A fuse in play in the various lines going to the alternator. If the larger one were out, more than this one problem would probably exist but I'll take a look at them since it would be quick to rule them out.

    I'd take it in to my mechanic but he's 48 miles from me and I doubt the car would make it that long driving on battery only.

    Thanks again.
  • edited December 2010
    The last time I had a charging problem was on my 97 Explorer.

    A quick look at the FACTORY charge circuit wiring diagram told me there ws a fuse to check.

    BINGO, the fuse was blown. Replaced it & no more problems.

    Make sure this isnt as simple as a blown fuse.

    The FAcTORY wiring diagrams for your Lincoln are probably about 30 bucks at For this and your next electrical problem the factory diagrams are the only way to go.

  • edited December 2010
    My issue with helm is that $10.00 processing fee, per item. Helm caters to the dealerships (I will spend someones elses money and say they can afford it, but I can't) I go to
  • edited December 2010
    Thanks. You know, now that you mention it, when I saw the large wire output meter reading shaking like there was AC, I put the meter on an AC scale with the car running and I swear it was something like 22vac. I don't remember if this was before or after I disconnected it from the battery.

    I also had turned on the headlights while they were shining on an object at night to see how bright the lights were. I then turned off the car and did not see any change in the brightness. I realize this is not a scientific check and reading voltages and current are for real, but just throwing out other things I can remember doing.

    I may wind up popping the alternator back in the car to do just a few more checks. Like I say, electrical parts are not generally returnable so I'd like to be pretty sure (well, as sure as I can be), before I pop $200 for a replacement.

    Thanks again.
  • edited December 2010
    Yes, thanks. I'm going to check the fuses in the morning when I'm not working in 4 degree weather.
  • edited December 2010
    Should have mentioned in my first post that the fuse I replaced is labeled strangely enough "Generator/voltage regulator." Why Ford still uses the term "generator" is a mystery to me. But i did'nt write the manual.

    On the Explorer the fuse is under the hood in the power distribution box.
  • edited December 2010
    Thanks oldschool. i'll have to check out books for cars when I need my next manual.
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