Are you saying a failing/failed diode will not drain a battery when engine is shut off? By the way, I completely disagree if you say it will not drain a battery. I have seen it in the shop. Had vehicles in the shop that had a problem of drained battery overnight and found that if unplugged the alternator overnight that the battery did not drain. Failed diode causing current both ways thru it instead of one way like it is designed.
More information from an independent customer with a like problem:
Problem
1. The Charging System would sometimes cut out, and even when it was "working" I noticed that the Alternator Gauge would gradually read lower, and lower, over a period of a few months, and turning on Lights would drop it even lower.
2. I took the Truck to the Shop, several times, as things got worse, and each time their Tester said the Electrical System was "OK".
3. Finally, the Charging System stopped working altogether.
4. I took the Truck to the Shop again, and this time they said the Alternator was bad, and installed a Rebuilt Alternator.
5. I parked the Truck, while I went on a 2 day trip, and when I got back the Battery was dead.
6. I measured the Current, with a Multimeter, with the Engine and all Electrical Devices off, and found a 2.5 Amp continuous drain on the Battery.
7. I traced the source of the current drain, by pulling Connectors, Fuses, etc., and found that if I pulled one of the two Connectors on the Alternator, the current drain went away.
8. I took the Truck back to the Shop, they checked it again, and said their Tester said the Electrical System was "OK", it's not a problem with the Alternator they had installed, and that it's an Electrical System problem, and I need to take the Truck to an Automotive Electrical Shop.
9. I went on the Net, found Bob Hewitt's very excellent Automotive Web Site, and sent him an Email explaining the problem, and asking him if the Alternator could be bad, but the Shop's "Tester" say it's good.
10: Bob sent me a very detailed reply, explaining that it certainly could, if the Connector that I had pulled was the Battery (BAT) Connector. Bob further explained that one of the Diodes inside the Alternator could be shorted, causing the drain on the Battery, but the Alternator would still put out 12 Volts, and therefor pass the Voltage Output Test, however it could not put out full Current, and would fail the Current Load Test, IF the Shop actually ran that Test. Bob also gave me instructions on measuring the Resistance at the Alternator Connector to determine if the Alternator had this problem.
11. "Armed" with this information, I went back to the Shop, and although it took making a CAD Drawing, on the Computer, showing the Parts, Wiring, and Connections involved, along with a detailed Written Explanation, plus talking to 6 people, including the Shop Manager, over a period of about 5 days, to finally convince the Shop that this problem could exist, they finally agreed to replace the Alternator again (as they said that they had no way to really Test for this problem, in the existing Alternator).
12. The final solution: YES - it WAS the Rebuilt Alternator, and when they replaced the Alternator again, the problem went away - full output Voltage and Current - and no more Battery drain.
