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Chevy small block / 12v system.
Heres the problem which started a few months ago - battery is being drained as I drive. I replaced the battery with a new one as it was 4 years old. Truck ran fine for about 3 hours, then CD player started skipping, lost my tach & lights - got her home and tryed to restart - battery dead. I replaced the Alternator with a spare which tested good, charged the battery - same storey - drains it after a few hours run time. I have checked wiring for loose connections, confirmed fuses all OK. At wits end - any ideas guys ??
Thanks

The first thing I would do is get the truck started and check the voltage. You should have at least 13.2 to 15 volts if the charging system is working correctly. With the engine running and the high beams turned on, you should not have less than about 12.8 to 13 volts. If it's less than that and you're certain the alternator is good, then I would suspect something putting an excessive load on the system or an undersized alternator.

But don't forget the simple stuff. Is the fan belt slipping? Is it too loose? Are the battery terminals clean and tight? Do you have a good clean connection to ground?

Get ahold of a good ammeter and measure the current with the system turned off. If there's a current draw with the ignition off, something is wrong (unless you have a modern vehicle - then you might have minimal current drain from electronic components). Start disconnecting things, one at a time, until the current draw stops. Reconnect everything except the last thing you disconnected, and see if there's any current draw. Doing this, you can isolate the cause of the draw. Then you have to figure out why the part you've isolated is drawing current and what to do to fix it (or replace it.)

If there isn't any current draw with the ignition off, turn the ignition on and measure the current again. You should have minimal current draw.

Finally, start the truck and measure the current again. If your alternator is rated at 40 amps and the system is pulling more than 40 amps, you either need a bigger alternator or you need to find out what's using all that current and fix it.

IOW, it's time to start troubleshooting.

Hy britcon, the only things I would add to what baldeagle said, is to check the voltage at the battery once the engine is running. Also is the alternator grounded to the engine, other than through its mounting hardware, and do you have ground wires between the cab and engine, cab and frame, cab and fenders, box and frame, it is impossible to have too many ground wires. Do you have an ammeter or a voltmeter?

Thanks Guys. Alternator is not grounded directly to engine. Cab & Engine are grounded to frame. I will pick up a tester and see what I can find. Question: Could brake lights sticking on for an hour pull enough juice from the battery to cause this problem ? Switch has been acting up - noticed lights on when I got home yesterday. Thanks

Hy britcon, brake lights on for an hour will bring the battery down, but I wouldn't think it would deaden it. That certainly would not account for the battery going dead while you drive, a bad battery, HEAVY draw, or charge not reaching the battery could cause your problem. You have already replaced the battery, and the alternator, so common sense would say you have a HIGH draw somewhere, or the "charge" is not reaching the battery. Hope that helps.

are you running a gauge? the alternator needs to be excited to work and that is usually the function of the gauge. Also New batteries can be junk too. Have the parts store check it.

First, are you using an external regulated alt. or internal. You can put a voltmeter on it like the guys said & the voltage should rise when the engine is running. It's pretty certain that it is not charging from you symptoms. If the alt is good you need to check the feed wires & regulator.