bad alternator?

[malohnes]

After a lengthy top end rebuild, my 91 is up and running again. One snag I discovered is that my alternator has crapped out.

According to the Haynes manual, the procedure to check the regulator and alternator has resulted in a need for a new alternator. Back probing the A and I pins show severe undervoltage with key on and engine running.

Does anyone know of other checks I can perform before I bite the bullet and buy another one? I guess now would be the time to upgrade to a 130amp model.

Thanks!

[Anime]

With the engine running, if the red battery light comes on, and the battery voltage doesn't go to 14+ volts, it'sthe alternator.

Sometimes disconnecting/connecting them can fry the voltage regulator that's attached to the back, and simply replacing it along with the brush assembly brings it back to life. Many times though, especially with high mileage, the internal shaft is also well worn and it's better to just have them rebuilt or get a new one.

[malohnes]

I thought as much Anime. Thanks for your input.

The thing that gets me is that the battery was disconnected the whole time I started the top end rebuild and it was working fine just before then. New brushes did nothing and continuity checks from the harness shows the connectors are fine. Sometimes electrical stuff just simply gives up without warning or provocation.

[Anime]

I pulled mine to replace the brushes just because they were getting worn and the dash lights and headlights would pulse a bit. In the process of replacing the brushes the voltage regulator fried and so it no longer worked. Took it in to have it rebuilt professionally and the guy hit the housing with hammer, cracking it, and told me I just needed a new one.

If you want a 130A anyway, use it as an excuse to buy it. You might still consider getting the old one rebuilt as a backup, or even just getting a cheap voltage regulator to try out and see if it works again, but still just keeping it as a backup.

[malohnes]

Good advice...I did the same. Ordered a new 130 amp alternator and ordered a new regulator just in case the old one would work with the new brushes I have and regulator.

[Anime]

The regulator and brushes are the same for the 95A and 130A, but it seems to me the cheap voltage regulators on the china-made aftermarket alternators don't regulate the voltage as well, nor do the units themselves put out the same amperage at the same RPM. They get over this by using a slightly smaller diameter pulley on the unit, which makes it turn faster to get more amps.

I think the original 95A actually put out more than the new 130A WPS brand I replaced it with, but I haven't had any issues with it yet, so the cheap ones do work for what they are.

[malohnes]

I am not surprised that the cheap 130 amp alternators are weaker than original 90 amp units. Happens all the time. I spent the money for a quality, rebuilt alternator, not a cheap one. For some things it is not worth it to go cheap.

[malohnes]

Okay, something interesting and funny: I received my new 130 amp alternator only to find out that it too did not work. Back probing the alternator pins on the connector showed a similar but different under-voltage problem. This time the I pin showed no voltage, yet a continuity check showed no open...WTF. Started wire chasing and found that the bundle went to the fuse block. To my pleasant surprise there was a field alt fuse, 15 amp, that I did not know I had, was blown! Perhaps my original alternator was not bad after all, but I got the excuse and approval from my wife to install an upgraded one!

Anyways, the point is, check the damn fuse block cause the solution is sometimes not an immediately obvious one!