Chonic Alternator Failure

I "thought" Matt had done some testing on this. The main causes of troubles IN NO particular order...........

Voltage drop in the ground circuit

Voltage drop in the ignition circuit

Regulator out of spec

Battery problems

Resistance in the charge wire, USUALLY the age old bulkhead connector / ammeter problem

I think this "blown diode" thing is being overstated. If there's enough resistance in the charge wire to cause diode problems, you are going to have smell and smoke from bad connections at the bulkhead ammeter.
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CHECK

VOLTAGE DROP IN GROUND CIRCUIT

Make these checks with engine running fast, warm, and with battery "as normal" as you can get. Check first with all accessories off, and again with lights, heater, etc on.

Stab one probe of the meter onto the battery NEG post. Stab the other into the regulator mounting flange. There should be very little voltage, zero is perfect. More than a couple of tenths of a volt means the grounding is poor.

Clean and re--mount the regulator with star locks, and consider adding a ground (no10 or larger) from regulator and ECU to block. On a V8, there are holes in the driver side rear head (look at the front of the pass side!!) to add a "starter" (ring to ring) cable to the firewall

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VOLTAGE DROP IN IGNITION CIRCUIT

This ALSO involves poor connections in the regulator connector (70/later)

The path is battery----starter relay--fuse link--bulkhead connector--ignition switch connector--through the switch--out the switch connector--out the bulkhead connector-- to underhood loads (ignition, regulator, alternator field, idle solenoid, choke, etc)

Key on, engine not running. Find switched ignition under the hood, either alternator blue field wire, (70/later) or the "key" side of the ballast. Hook one meter probe to that, hook the other meter probe to battery positive. More than .3V (three tenths) means you need to improve the voltage drop. ANY VOLTAGE DROP in BOTH the ground and ignition circuit ADDS to regulator sense point.

The easy way to repair this is to break the output of the ignition switch coming through the bulkhead, use that to fire a relay, and fuse the relay off the starter relay. Then supply underhood loads off that relay

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REGULATOR OUT OF SPEC

AFTER you check voltage drop, try a different battery, if charge voltage is too high (optimum 13.8--14.2 warm) and not below 13.5 nor above 14.5, replace the regulator

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BATTERY PROBLEMS

BEST way is to swap in a known good battery

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RESISTANCE IN CHARGE LINE

This, in old mopars is a large part of why I always preach the MAD electrical page:

http://www.madelectrical.com/electrical-tech.shtml

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

Now, even Ma knew this was a problem, look up "fleet wiring" sometime, which essentially beefs up the wiring through the firewall and BYPASSES the bulkhead connector

The path from alternator to battery is...........

The eyelet at the alternator stud..........through the bulkhead..........through the ammeter...........back out through the bulkhead..........through the fuse link..........to starter relay...........to battery

This is easy to check. With the engine running fast, and charging "hard"

(to simulate this, crank and kill the engine several times to run the battery down a bit, then test with lights, heater, etc, on)

Hook one probe of your meter to the alternator output stud. Hook the other to battery POS post. Now "THIS OPINION" can vary. This voltage will vary. That's because there can be a LOT of current. On a STOCK harness which might have a 50-60 amp alternator, I'd say as much as 1 volt is OK. This is under HEAVY charging.

Along with this last, you can "wiggle test" the bulkhead connector, and stick your (careful!!) hand up in the dash and feel "for heat" at the back of the ammeter.