Triumph Repair: battery drains but only when engine is on, volt meter, ground cables

Question
Hello: I have a short somewhere in my 76 Spit.  I can jump start it easily enough, but once started, with a voltmeter, you can see the battery voltage drop a few hundredths of a volt/sec until the engine eventually dies. Earlier, I isolated and resolved two separate sources of parasitic draw (rusted horns and some carbon emission canister thingy whose connecting hoses had long since been cut).  But this latest bugger is different in that the source only raises its ugly head when the key is ON. There's virtually no current through the battery when the key is off (ammeter registers a few microamps; probably from the meter itself).  I recently replaced both the battery and alternator; I'd like to think those are both still good. Any ideas? Thanks!


Answer
Hi Ned,
Do these tests Exactly. Charge up the battery, with one cable connected and the other disconnected take a 12v test light and connect the clip to the cable and with everything turned off, touch the battery post with the test light probe. The test light should not light (unless you have installed some kind of high tech gadget that has a memory or a clock.
No light, reconnect the battery cable and put your volt meter across the battery posts and read battery voltage. Should be 12+v. leave the voltmeter in place and start the car. Set the idle at a fast idle and read the volt meter. should read 13.8 to 14.5 volts. Anything less and your charging system is not working and/or you have a cable problem.
To test for a cable problem, set your volt meter to a very low volt scale. Turn the engine off. Remove either of the small wires from one side of the coil. Put the neg. lead of the volt meter on the negative POST (not the cable) of the battery and the positive lead of the meter on the engine block (clean metal). Have someone try to start the engine while you read the meter. If you have a reading of anything close to a volt or more, you need to correct the ground cables. (engine to frame or battery to body or body to frame.)
Next put the positive lead on the pos. POST of the battery and the negative lead on the POSITIVE post of the starter solenoid and hit the starter again and read the voltmeter.
Any reading close to a volt and you have a positive cable problem.
A voltage draw when running that is sufficient to run a battery down to a point to kill the engine must be very large because the charging system should over-ride any minor draw.
It sounds to me like you charging system is not working at all. The above test will prove this.
Let me know.
Howard