Hi guys. Excuse me but I have to stick my nose in here and tell you to NEVER EVER disconnect the battery while the engine is running. That was a trick done many years ago by mechanics who didn't understand how these simple systems work. The battery is the key component in maintaining system voltage to a safe level. The voltage regulator can't do it by itself. Without the battery in the circuit, if the alternator is working and you raise engine speed, system voltage can easily exceed 30 volts and destroy every computer on your van and the diodes inside the alternator. I did that every year, on an alternator test bench, to prove to my students how dangerous doing that is. My '95 Grand Caravan has an Engine Computer, Body Computer, anti-lock brake computer, and automatic Transmission Computer, along with the radio and instrument cluster which could also be damaged. That's a huge potential repair bill and not worth the risk.
As for the fusible link Jacobandnickolas mentioned, it's plugged into the smaller positive battery cable. If it's bad, you'll find a higher voltage, (possibly as high as 30 volts) on the alternator's output terminal, but closer to 12 volts on the battery, while the engine is running. You should also measure full battery voltage on the alternator's output terminal with the engine off, but that's only a valuable test for a hard defect that's always there. For an intermittent problem like yours, that test is of no value and it won't be accurate because every strand of wire could be corroded apart but as long as there's a hint of carbon-tracking or corrosion in there, the voltmeter will pick that up and incorrectly show a good circuit. There's no way the alternator's 90 amps is going to get through that corrosion though.
A common intermittent problem is a corroded connector pin between the alternator and the voltage regulator which lives in the Engine Computer. The voltage regulator circuit is extremely reliable and trouble-free so the computer should be the very last thing to suspect.
Along with that voltage on the alternator's output terminal, all diagnostics can be done by measuring the two voltages on the two smaller wires. They're held on with two small nuts holding tabs coming out of a small black plastic block. Those two voltages must be measured with the engine running. One will have full battery voltage. The other one must have less but not 0 volts. Be aware a lot of incorrect "0 volts" are read due to those nuts being rusty so be sure your meter probes are making good contact. 0 volts on the second terminal means the brushes are worn. That always starts out as an intermittent problem and is most likely what happened to yours. I just changed them on my alternator for nine bucks a few weeks ago. If you read exactly the same voltage on both small wires, there's a break in the wire going to the Engine Computer / voltage regulator. If you find somewhere between 4 - 11 volts on the second wire, that entire circuit is working. If it still isn't charging, the problem is in the output side of the alternator or that fat wire going back to the battery.
I broke a lot of brushes to create "bugs" that I could switch in and out for my students to diagnose, and after the first '91 - 95 Caravan, I proved those brushes could be replaced without removing the alternator from the engine, at least with the 3.3L engine. Should you ever run into this again, I can post photos of the procedure.
Oh, one more observation. Your fault code 41 is only set when no current is flowing through the brushes and voltage regulator. If the defect is in the diodes or any other part of the alternator's output circuit or wiring, no code will be set even though the charging system isn't working. There's the low-current input side that creates the magnetic field, and the high-current output side that the charging current is "induced" in. The computer only monitors the low-current side.
Saturday, December 10th, 2011 AT 12:28 AM