2003 Eclipse Alternator Problem

Immediately above and to the right of the “nut” in the center of the picture, there is about an inch of bare wire, not covered by the black and yellow coating, and it is attached to the chassis there by a “clip” held in place by the bolt there.

If that is not a ground, if it is just holding the wire in place, why would the wire be bare there?
I guess the photo doesn’t show that clearly. Double-click on the photo and it shows up better.

You are correct @TwinTurbo. This would indicate if there is voltage drop.

I was looking in the direction of comparing the voltage at the B+ post on the alternator to the voltage at the battery.

In regards to the ground on that balck/ yellow ground wire; I think that that is a ground terminal for the engine ground.
The Op should check that this terminal is not broken off at that point.


So, try to follow the black/yellow wire coming from the battery negative terminal, to where it attaches to the engine? It is hard to follow past where it disappears in the picture.

I was leaning toward that when everything else failed, but wanted to see if I could rule out everything else.

FWIW, I cleaned and reconnected that connection in the picture, and it didn’t make any difference. The battery was 12.7, but when I started the engine it went to 12.17.

The clip in the middle of the cable is the body ground, the cable continues to the engine/transmission as the engine ground.

OK, thanks.

Like a blind pig looking for an acorn . . . I found the engine ground. It is solid and intact and clean-ish, considering where it is. I cleaned it cleaner and made sure it was tight.

Then . . .

The battery was 12.79 . . . I started the engine and the battery was 12.24. I shut the engine off and the battery was 12.54.

The same.

I am beginning to sense that even as hard as it was to get the alternator off, and that it tested good three times (off), and as hard as it was to put it back on . . . it might be bad.

Are you sure all the wiring to the alternator is solid? If it bench-tests okay, it should be okay when installed. The other possibility is a problem with the ECU.

Are you sure the alternator belt is getting a good bite on the alternator pulley?

You might get on the alternator pulley with a boxed end wrench or socket and try to rotate the pulley against the belt.
The pulley should either bind tight on the belt or at most be very difficult to turn.

Yosemite … when the alternator is charging the battery current flows in that wire from the alternator to the battery, so there’s usually a measureable voltage drop you can measure. On my Corolla it’s around 0.1-0.3 volts as I recall when first starting the engine. Since the OP measures 0 volts, that means there is no current flowing in that wire, confirming the alternator is not charging the battery. It also means the alternator isn’t discharging the battery either, suggesting the alternator diodes probably are not to blame for this symptom and something else is afoot.

@Nevada_545 's idea to check the field voltage is a good one to do next. OP, the field is the magnetic coil that is on the rotating part of the alternator, the current fed from the voltage regulator to the rotating field coil by the slip rings. It’s magnetic field sweeps past the alternator’s stationary coils and is rectified to DC by the diodes to produce the voltage and current necessary to charge the battery. If the field coil isn’t energized by the voltage regulator, the alternator will spin but not produce any voltage or current in its stationary coils, so it would basically appear to be working but not actually doing anything but spinning around in a circle.

How is Yosemite wrong? If probes of a VOM have one attached to the battery + post and the other to the B terminal there will not be any voltage. One of those VOM probes is going to have to touch ground to get a volt reading.

That might be a valid test if the VOM as turned to the Ohm scale and the cable checked for continuity.

Just b/c two points in a circuit are connected with a wire between them doesn’t imply they are at the same voltage. Wires are pretty good at conducting current, but not perfect. All wires have some resistance. To calculate the voltage difference between two points connected by a wire, it’s just ohm’s law,

Voltage = Current * Resistance

So whenever there’s current flowing in a wire, there’s also a corresponding voltage difference. If the battery is being charged by the alternator, there has to be a voltage difference between those two points.

Suggest to try the experiment yourself. First thing in the AM, before your car has been started, connect a volt meter set on a scale that you could clearly measure something in the 0.1 to 0.3 volt DC range. Then start the engine and measure the voltage between those two points. Post here what you measure.

I suspect your alternator does not work, despite that it tests OK. A bad diode would not show up in your bench test, but could actually draw energy away from your battery.

[quote=“kurtwm2010, post:32, topic:95685, full:true”]
I suspect your alternator does not work, despite that it tests OK. A bad diode would not show up in your bench test, but could actually draw energy away from your battery.
[/quote]A test with the proper equipment will detect bad diodes.


yup, a multimeter with a “diode” test function. One lead on the red hot connector and the other on the body of the alternator. Battery cables must be disconnected.

I don’t understand the purpose or logic behind testing the battery to alternator lead with both probes of the VOM and expecting to get a voltage reading.
Just for hoots, I checked 2 cars here this morning using both the 20 V and the 200/2000 Millivolt scale and 0 on both.

kurtwm2010 said:

I suspect your alternator does not work, despite that it tests OK. A bad diode would not show up in your bench test, but could actually draw energy away from your battery.

I’m new here, and, as you can see, I haven’t figured out how to “quote” a post, but I. also, suspect the alternator is bad, since everything else tests OK, appears to be properly connected, grounded, etc.

I had the alternator tested off the car because I did not trust it to be able to take it somewhere and be able to get it back home. That’s the first thing I tested, it being the most likely problem, and I was disappointed when it test(ed) OK.

But it is exactly correct that the battery charges, and holds it’s charge, and tested OK, but as soon as I start the engine, the battery voltage is a lot lower, and drops pretty quickly with the engine running.

I did the battery test you suggested earlier, and it held it’s charge for two days with the battery cables disconnected.

a voltage test will not show anything. You will need a multimeter that has a “diode” test function. Here is a nice link explaining it nicely.

get a new alternator and in case you continue to have that problem, take it back. What you have to lose, but a little time? If you are inclined to replace the thing yourself.


I’ll report back.

Thanks everyone.:slight_smile:

Any resistance anywhere in the circuit will drop voltage. You don’t need a common ground point to measure the voltage drop across any resistance in the circuit, you can just measure directly across it.

For example, if you have an oxidized battery terminal, you could put your voltmeter across the battery post and cable terminal. It will show a voltage across that connection if it has enough resistance to create a problem…of course, good connections show up as, for all intents and purposes, as zero volts on most average quality meters…