Alternator Problems

Flghtmstr1 · Aug 13, 2012

I've been trying to get the Mustang ready for the dyno, and I've been having persistent alternator / charging system problems. I've gone through three alternators now, and they all had the same problem: they don't seem to be charging the battery even though the idiot light seems to be working properly (key off indicator off, key on indicator on, engine on indicator off). Key off battery voltage is 12.6, but it drops to 12.2 when the engine is turned on, and revving the engine up to 2000 rpm doesn't change the battery voltage at all.

I've verified that the black and orange positive wire has continuity with the B+ terminal, and the alternator case has continuity with the B- terminal. I've checked the fuse in the junction box and the fusible links (both visually and with a multimeter) and they all seem fine. I've also checked the yellow and green wires going into the alternator by back-probing the connector. They read as follows:

Key off:
Yellow/white: 12.6v
Green/red: 0v

Key on:
Yellow/white: 12.2v
Green/red: 1.6v

Engine on:
Yellow/white: 12.2v
Green/red: 11.2v

Needless to say, I'm stumped. The only thing that looks sketchy is the engine on green/red measurement, but I'm not sure what to make of it. Any ideas?

n0v8or · Aug 14, 2012

I may be missing something obvious, but I can only come up with 2 possibilities. Well, perhaps 3.

That the lamp turns off but the battery does not charge is an indication that the alternator is developing voltage, but charge current is not flowing. My first choice is a defect (perhaps not a complete break) in the black-orange wire that runs between alternator and battery. Second choice is a poor connection between alternator frame and battery negative.

The second possibility is easiest to eliminate. Connect a jumper cable between the alternator frame and battery negative post. Be careful to lay the wire where no moving parts can contact it when starting the engine. If nothing changes, that's not the problem.

To check the black-orange wire resistance, you need to pull back the rubber boot enough to get your positive voltmeter probe on the alternator output terminal. If it reads more than 1-2 volts higher than the positive battery post, (engine running) there is either excess resistance in the connection or an unusually high current drain. I know you checked continuity, but even 0.5 ohm would prevent the battery from charging with the engine running. The ignition alone draws ~10A, which would mean a ~5V drop across 0.5 ohm.

wmburns · Aug 14, 2012

I want to go out on a limb and go a different direction. Are you positive that the CORRECT alternator for your application was used?

Some Ford alternators have different wiring and have a separate "I" and "S" circuit. These alternators will not work in a Mustang without jumping the "I" and "S" together.

Flghtmstr1 · Aug 15, 2012

wmburns said:
I want to go out on a limb and go a different direction. Are you positive that the CORRECT alternator for your application was used?

Some Ford alternators have different wiring and have a separate "I" and "S" circuit. These alternators will not work in a Mustang without jumping the "I" and "S" together.

How can I tell if the alternator is correct? I got it from Auto Parts International (I assume it's reman).

Flghtmstr1 · Aug 15, 2012

n0v8or said:
I may be missing something obvious, but I can only come up with 2 possibilities. Well, perhaps 3.

That the lamp turns off but the battery does not charge is an indication that the alternator is developing voltage, but charge current is not flowing. My first choice is a defect (perhaps not a complete break) in the black-orange wire that runs between alternator and battery. Second choice is a poor connection between alternator frame and battery negative.

The second possibility is easiest to eliminate. Connect a jumper cable between the alternator frame and battery negative post. Be careful to lay the wire where no moving parts can contact it when starting the engine. If nothing changes, that's not the problem.

To check the black-orange wire resistance, you need to pull back the rubber boot enough to get your positive voltmeter probe on the alternator output terminal. If it reads more than 1-2 volts higher than the positive battery post, (engine running) there is either excess resistance in the connection or an unusually high current drain. I know you checked continuity, but even 0.5 ohm would prevent the battery from charging with the engine running. The ignition alone draws ~10A, which would mean a ~5V drop across 0.5 ohm.

I checked the black-orange wire's voltage with the engine running, and it was 12.2, same as battery voltage. I'll try the jumper cable trick when I get home from work today.

n0v8or · Aug 15, 2012

Burns has won me over (once again). Given the history of multiple alternator replacements with no change in symptoms, his idea is more plausible than mine.

Admittedly, I am grasping at straws here. I have seen some Mustang alternators with a right angle adapter that bolts to the output termunal. If you had to re-use the same adapter with all of the replacementy alternators, and it was damaged internally, then all alternators would produce this same symptom. Of coourse, if your original alternator did not have this adapter, or the replacement alternators came with a new one already attached, then forget I even mentioned it.

Flghtmstr1 · Aug 15, 2012

n0v8or said:
Burns has won me over (once again). Given the history of multiple alternator replacements with no change in symptoms, his idea is more plausible than mine.

Admittedly, I am grasping at straws here. I have seen some Mustang alternators with a right angle adapter that bolts to the output termunal. If you had to re-use the same adapter with all of the replacementy alternators, and it was damaged internally, then all alternators would produce this same symptom. Of coourse, if your original alternator did not have this adapter, or the replacement alternators came with a new one already attached, then forget I even mentioned it.

I have been throwing away those adapters, as my setup doesn't require them.

Flghtmstr1 · Aug 15, 2012

I tried jumping from the alternator case to the negative battery terminal, and it didn't change a thing. WMBurns, what did you mean by separate I and S circuits? How can I check to see if my alternator is correct?

wmburns · Aug 16, 2012

The alternators used in Mach1 applications and other Ford applications have 3 active connections on the small connector. Some Mustangs use a connector with (3) terminals but there is a jumper going to the middle pin. The wire color is WH/BK.

The alternator will only use pin 1 and 3. So you might try jumping pin 3 and 2 and see what happens. I would use a jumper with a safety fuse just in case.

Also, what if the small connector isn't really making a good connection? This would keep the alternator from powering up. Have you really looked at the connector to see if there are any bent/pushed/dirty pins?

Consider getting yourself a full set of Ford wiring diagrams. I can help. PM.

Flghtmstr1 · Aug 17, 2012

wmburns said:
The alternators used in Mach1 applications and other Ford applications have 3 active connections on the small connector. Some Mustangs use a connector with (3) terminals but there is a jumper going to the middle pin. The wire color is WH/BK.

The alternator will only use pin 1 and 3. So you might try jumping pin 1 and 2 and see what happens. I would use a jumper with a safety fuse just in case.

Also, what if the small connector isn't really making a good connection? This would keep the alternator from powering up. Have you really looked at the connector to see if there are any bent/pushed/dirty pins?

Consider getting yourself a full set of Ford wiring diagrams. I can help. PM.

The alternator has 3 pins and the harness has a 3-pin connector with pins 1 and 3 occupied and pin 2 empty. What, exactly, should I try jumping? I have the Ford wiring diagram, but electrical systems aren't my strong suit.

wmburns · Aug 18, 2012

Look at your wiring diagrams. Jump the +12 volt supply pin #3 (YE/WH) to pin #2. Note, it won't hurt anything to jump pin #1 to pin #2 (especially if a fused jumper is used).

Flghtmstr1 · Aug 23, 2012

Sorry it took so long for me to respond; I wasn't able to work on the Mustang all week.

I don't think there's any way to access the #2 pin when the harness is plugged into the alternator because the harness-side connector does not have a pin in the #2 slot. It has a recess for the alternator-side pin to fit into, and this recess is sealed from the outside by the plastic connector body itself. The only way to access the pin with the connector plugged in would be to drill a hole in either the connector or the alternator itself, which is obviously not something I am willing to do.

I did notice that the alternator pulley gets extremely hot (too hot to touch) after only 1 minute or so of engine run time, while the alternator case itself remains relatively cool to the touch. I'm not sure if that tells us anything, but it seemed strange to me.

wmburns · Aug 24, 2012

The #3 is the dash charge alarm so it's not required to power up the alternator. So try taking the small connector out and power pin #1 and #2 directly from a fused jumper.

Flghtmstr1 · Aug 27, 2012

Turns out it was the wrong alternator. Nice call wmburns!

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