Early 944 Charging Problem... Alternator is good! Need Help!

[earlr85944]

yeah, hit me up when you get a chance
why do you think its broke? maybe the battery is weak?

[x2nervisx]

ok I started the car and the battery light was on. I took the blue thin gauge wire off the alternator while it was running and I looked at my dash and the battery light was still on. So now I guess there's a ground somewhere in the line? Or could it still be the alarm?

[x2nervisx]

Originally Posted by Rasta Monsta

To add to the above, you can also disconnect the exciter wire, jump directly from the big lead to the exciter post, and then check your voltage.

So I can just take a wire and jump it from the positive post on the alternator and hook it up to where the blue wire would go? This would make the alternator charge my battery?

[79MALIBU]

I spent a great deal of time this weekend trying to figure out why the new alternator on the Porsche would not charge.

So, alternators use EMF to create current. A magnetic field is created in the rotor, the part that spins, by running current through it. That field is then spun through wire that surrounds the rotor or stator. Magnetic fields passing through wire causes current to flow. This current is fed into the battery.

Notice you must have some current to create the field in the rotor. The amount of current passed through to rotor is proportional to the field the rotor creates and thus proportional to the amount of current fed to the battery.

A voltage regulator decides how much current to pass through the rotor and prevents over charging the battery.

Well, then there is the Charge Lamp. Basically the lamp is powered by the battery, but seeks grounds through an auxiliary charge output circuit. The light will come on you turn on the key because the engine is not running, thus rotor not turning and therefore no charge current is being generated. When the engine starts, the current is created and the Charge lamps ground circuit becomes a 14v charge circuit. The lamp then goes off. (It is not perfectly that simple, but basically that’s how is works.)

So count your wires,

1. Charge to battery
2. Ground to battery
3. Lamp Ground/Auxiliary Charge output
4. Power to Regulator which then passes current through rotor.


So what did the crazy Germans do? They decided to combine wires 3 and 4. Which means the Regulator is expecting to a Voltage of 12v – (the voltage drop through the lamp). The regulator is very sensitive, and must receive the exact voltage it was designed for.

So what does this mean? First of all, if you charge bulb blows, then you will have no indication of an alternator failure and the alternator will not work at all.

So I went on line to find out how much resistance the bulb was supposed to have. I found numbers close to 50ohms. I measured 28ohms to ground, which I later personally determined to be OK for a 1987 924s. So I hot-wired it with a 50ohm resistor to see if it would charge. It did charge, kicked up the 14v on the battery, but then promptly burned the resistor into two pieces.

I thought this was strange, so I decided to take the dash apart and take a look at the alternator lamp to see if there was a resistor burned out of any thing abnormal looking. Did not find anything, but did confirm my understanding of the circuit when I notice the alternator lamp was deliberately sized differently than any other lamps. Apparently using a larger lamp with less resistance.

At this point I am confident there is no problem with the charge circuit. So I took the new/reman alternator to my local parts store to have it checked. It was bad. . .Grrrr….I installed the second new alternator. . . It worked . . . the end

[79MALIBU]

Originally Posted by x2nervisx

So I can just take a wire and jump it from the positive post on the alternator and hook it up to where the blue wire would go? This would make the alternator charge my battery?

Be Careful doing this. I tried this experiment. Work great for certain GM Alternators. This is how I always run the alternator on my 79 Malibu. But is the alternator is bad, it may start to charge, but the charge current may also flow through you jumper wire. I could melt that wire and start a fire. When I made the jumper with a 14awg wire is caused lots of arcing within the alternator. I you want to try this I would use a fuse-able link or circuit breaker. Read my other post above.

[Hellraiser]

Had this one myself once,
resulted in this: