Auto mechanic types: Alternator help needed!

My 1991 Ford Tempo is having electrical problems AGAIN!

You may recall that a year ago my alternator was diagnosed as dead by my mechanic. Thanks to the help and encouragement of several Dopers, I replaced it with a used (but NOT rebuilt) alternator myself, thereby saving mucho buckos.

Well, once again, there’s trouble in Tempoville tonight. Yesterday, out of the blue, the dashboard “AMP” (w/ battery icon) warning light began to glow pretty steady. Minutes later, all the elecrtical functions in the car started to dim and die; I just barely made it to my parking spot.

I found the battery terminals horribly salt-encrusted, so I cleaned them bright, thinking that was the trouble. No such luck. The AMP light still kicked on after a few minutes of revving.

Okay, maybe the battery is just shot, I thought. I took it out of the car and had it tested at Pep Boys. The battery tested good. And the AMP light is still on.

So now I’m thinking it’s another dead alternator – but I can’t really be sure, right?

How can I test my alternator, preferably while it’s still in the car?


  1. If I recall correctly, there are 4 wires coming out of the alternator: One big, thick one that is held in place with a nut (covered by a rubber boot), two that snap into the housing via a 2-wire plastic connector, and one that snaps into the housing via a 1-wire plastic connector.

  2. A fast search of the internet tells me that the olde school test method of disconnecting the battery while the car is running (“if car dies, then alternator is bad”) is dangerous and unreliable.

  3. The AMP warning light goes out when I rev the car really fast (in neutral) but it does not stay out long. (One internet source said that may be because of residual magnetism in the stator making an otherwise-dead alternator look like it’s working – but I may have read the details wrong.)

  4. I have no real test equipment to speak of. I can rig a 12V test light with an extra car bulb; I also have an “autoranging pocket multimeter,” but I don’t really know how to use it. (It was a well-intentioned gift.)

Thanks all, in advance.

That multimeter will probably be of no help. You’d need a high-range “amp clamp” to test the amperage output reliably.

If you’re going to remove it yourself, lots of parts stores have test equipment to do a “running” output test on the loose alternator.

When I buy a battery, I ask them to check the alternator & they do for free…Better get
a remanf one.

Ah, it is very easy to diagnose your problem. Start the car and with the engine running, remove the negative battery cable from the battery.If the engine dies, it is the alternator. If the car keeps running, it is the battery. The above test is not recommended for newer vehicles but it will cause no harm to your 91 Tempo. I use to be in the used part business, a year out of a used alternator sounds about right.

Do NOT do this to test your alternator!

It was fine a long time ago, when cars still used generators that rectified their output current via a split ring commutator setup.

An alternator rectifies its output with an electronic device, and this can be damaged by removing the cable while running.

Here is some good advice:

Note the “Myths” section.

My brother had a Tempo about the same year as yours, it blew the alternator on me one day and when I got the replacement (rebuilt) they handed me a sheet that said there was a recall on the pig tale (i.e. the part of the wiring harness that hooks into the alternator), as it could cause a fire. Get the VIN# of your car and log on to you can enter the VIN to see if your car is effected by the recall. If it is, you might be able to get Ford to reimburse you for your alternator.

Racer 72 & Joey G. How do I choose between the two?

Help! Help!

(Oh, and Truckerfan, thanks for the recall tip. But I checked the Ford website – no alternator-related recalls.)

To be honest, knowing how screwy Ford can be about recalls, it might not be listed as an alternator recall, it could be listed under something else. Or it could be one that exists, but Ford doesn’t like to talk about. I bought the alternator at one of the big chain parts stores, you could stop in there and have them pull an alternator off the shelf and you could look in the box for the notice. That’s where the one for my brother’s car was.

For a quick and easy test to check to see if your alternator is charging the battery, all you need is that pocket multimeter. Set it to read DC voltages, and put the read lead on the positive battery terminal, and the black lead on the negative terminal.

With the engine off, note the voltage. This should be around 13 volts.

Start the engine, (BE CAREFUL! Keep your hands and clothes away from all moving parts) and let it run for a few seconds, then recheck the voltage at idle. It should be slightly higher than with the engine off. The standard range is about 13.5 to 15 volts, with most systems running at 14 to 14.5 volts.

Turn on all of the electrical devices in your car, such as headlights, heater fan, rear-window defroster, radio, interior lights, air conditioner, etc. When you turn on a heavy load you may see a momentary drop in the voltage, but it should recover very quickly to the 14-14.5 volt range, in just a second or two at most.

Under heavy load, it may be necessary to increase the engine speed to slightly above idle to get the voltage back up, but it shouldn’t require more than about 2000 RPMs to keep the voltage at 14 to 14.5 volts under all load conditions.

If the voltage stays up in all the above tests, your alternator (and regulator, although these are usually built in to the alternator these days) is most likely good.

If the voltage is up under low load conditions (i.e. engine running but no accessories or lights on) but drops under load, or requires a very high engine speed to get to the 14 volt range, the alternator (or regulator circuit) is working a little, but is failing, and probably needs to be serviced or replaced.

If the voltage never increases beyond the voltage you noted with the engine off, or never gets up to the 14 volt range, the alternator (or regulator, etc) is dead.

Note that this test won’t test for a failing diode, because these can be bad, and in some cases the regulator will compensate. I’ve seen cars with a bad diode charge ok, but drain the battery while the car is shut off. This is pretty unusual, however.

Good luck!


Is that diode in the regulator or the alternator?

RJK is spot on, check the voltage. Sometimes you can also turn the voltmeter to “AC” and it will show a bit of AC voltage, if a rectifier is shot. This can toast a multitester if you’re not careful, so the regular DC voltage test is still best.

Removing a battery terminal from a battery while it’s running is bad, and dangerous as well- a spark at that point will cause the battery to explode, I’ve witnessed this personally and it’s very, very bad. On the other hand, it’s no different than the cable being so corroded that no current passes. In both cases, the battery is effectively isolated. This happens to a lot of cars a lot of the time; fortunately the systems are pretty rugged, and while not designed for this kind of abuse usually survive it.

Check your ground as well. A lot of attention paid to the positive side while ignoring the ground; you’d feel fairly stupid if you spent a lot of time and money and you ended up at an overpriced mechanic, who might tighten the ground strap and make it work OK.

Whatever the case, when you find and fix the problem, clean your battery terminals and grease them.

I use an anti-sieze compound called “cop-graf” to grease my terminals- in twenty-odd years and twenty-five or more vehicles I’ve never had a corroded terminal. You can also use wheel bearing grease but make sure there isn’t any between the contact surfaces. The Cop-Graf is conductive so this doesn’t matter.

Best of luck!


Doesn’t sound like it applies to his alternator. Some Fords of that era had the battery connection IN the pigtail itself, rather than a seperate heavy-gauge connection. These caused problems when the connectors started to corrode. Resistance would go up, voltage would go down, current would go up, and it would overheat and burn.

I know when you buy such an alternator at a NAPA store, they will not warranty it unless you replace the connector too. The connector is part number 13-91L, for those who are interested.

Sounds like the voltage regulator to me.