MILEPOSTS Garage - The Online Classic Car Magazine
Everything from Hood Ornaments to Tail Lamps Autolite Batteries sign circa 1969
Return to Index

Vol. 3, No. 2
January 26, 2005

Diagnosing A Battery Drain
by Ken Wilson
Automotive Mileposts Restoration

Image: MILEPOSTS Garage

Image: Troubleshooting/Tech Tips Series

The electrical systems on old cars have caused frustration among restorers for years. All kinds of things can go wrong with them, and the symptoms don't always give you a straight shot back to the source of the problem. Among all of the electrical system complaints I get, the one that seems to anger people the most is having a dead battery. Especially if the battery, battery cables, alternator, and voltage regulator are new. Few things are worse than turning the key, only to be met with silence.

Fortunately, determining the cause of a battery drain is pretty easy. Did the headlights get left on? Are the battery cables tight? Is the battery fully charged? Is the battery holding a charge? Answers to these questions are the beginning of a process that will reveal what is at fault.

If nothing was left on, and the battery cables are tight, and the battery is able to hold a charge on its own, something else in the electrical system is draining the battery. How do you determine if the battery is holding a charge on its own? That's easy. Start by making sure it's fully charged. Using a voltmeter or multimeter, put the red probe on the positive terminal of the battery, and the black probe on the negative. Make sure you have the meter set at 20 volts. Check to see what the battery voltage reading is. It should be somewhere in the 12.4-12.8 volt range, depending on the ambient temperature and how recently the battery was used or charged. If it's below this voltage range, charge it on a slow charge until it's in this range. Then disconnect all the cables from the battery, and let it sit overnight.

Check the voltage again in the morning. Don't connect the battery cables, just use your meter. If the voltage is still in the proper range, chances are the battery is still good. If it's dropped substantially, it might be going bad. Many auto parts stores will test batteries for you for free, including doing a load test to see how the battery performs. Failing a load test is often one of the first signs a battery is on its way out.

If everything checks out OK, reconnect the positive battery cable (or whichever isn't the system ground - usually negative is ground). Now with the negative (ground) cable off, use your meter to determine if there is a drain on the battery. Do this by placing one probe on the battery grounding terminal, and the other probe on the battery grounding cable. There should be no voltage showing.

There is one exception to this rule, however. If your car has an electric clock, it will use voltage briefly when this connection is made to wind up the clock mechanism, provided the clock works. If the clock doesn't work, it could be the source of your drain. If you have a digital clock (for instance, the kind used in 1974-1976 Cadillacs), or an electronic clock (a car with a clock display in the radio or elsewhere), or a radio or sound system with electronic tuning, all of these will require a small amount of voltage to retain memory, even with the key off. Unplug this equipment before continuing the test, as it will be impossible to determine where the drain is otherwise.

With this accomplished, check to see if there is voltage showing between the battery ground terminal and the ground cable. If there is, start unplugging fuses one at a time. After each fuse is unplugged, check to see if voltage is still showing. If you notice a voltage drop, make a note of which fuse was removed to create that drop. Continue removing fuses one at a time until no voltage is showing on your meter. Whichever fuse was removed that resulted in no voltage showing is the circuit causing the drain.

Autolite Battery Group 27FJust to be on the safe side, start installing fuses, one at a time, in the same order you started removing them. Check for voltage after each fuse is reinstalled. If you get all the way back to the last fuse you removed without any voltage indication, that means all other circuits are OK, and there is likely just one circuit causing the problem. If you start to see voltage before you get to the last fuse, that means you have problems in more than one circuit. Usually this is not the case, but it's best to know for certain only one circuit is at fault.

If you remove all the fuses, and there's still voltage showing, it indicates a diode in your alternator has gone bad, and it's time for an alternator rebuild or replacement.

Once you've identified the circuit at fault, determine which electrical items are powered by that circuit. Check your owner's manual, shop manual, or wiring diagram for these items. If you don't have this literature, it's best to obtain it before beginning this project, as it is very difficult to determine which electrical items are on a circuit without them. Sometimes, an electrical item may be on more than one circuit, for instance a radio might be on one circuit to retain memory, and on another to power up. This allows the radio to turn off with the key, while its memory functions retain power.

Once you know which electrical items are on the suspect circuit, start unplugging them until the voltage reading stops, and that would be the component causing the problem. Just to be on the safe side, plug back in all the other components on that circuit just to make sure there was only one problem.

If it's something like a radio, you're best to just remove it and take it to a professional for repair. Same goes for electric motors. If you're not an expert, or if you don't have access to one, you almost always save time and money by having a pro make the repairs. Quite often the equipment needed to make a repair exceeds the cost of the repair itself. Unless you plan on making a hobby out of it, don't spend the money on stuff you'll likely never use again. Garages all across America are full of stuff only used once, or purchased with good intentions, but never taken out of the box.

It is possible to have everything unplugged, yet still have a voltage draw. This normally means a wire is crossing with another somewhere. This is where your role of playing Sherlock Holmes becomes challenging. It could be a connector, a wire with chafed insulation touching another connection, a bad ground somewhere in the car causing something that normally doesn't serve as a ground to become a grounding point, etc. Normally, it will not be a direct short to ground, though, as those pull lots of voltage, and would fry wiring or blow fuses before slowly draining the battery.

You may need to unwrap wiring harnesses, unplug connectors and check them for internal damage, or check for continuity one wire at a time before the problem reveals itself. Just do this in an organized manner, and work your way from the power source to the components in question. Normally, this will be from the fuse panel to the component, but depending on the circuit involved, it could start at the battery.

Check the most obvious places first. Check where wires bend around corners, pass through grommets, or have retainers clamped around them. Often these are the areas for the worst wear, as vibrations from thousands of miles of travelling can wear on wiring like sandpaper. Also check areas where harnesses move, especially around door hinges, deck lid hinges, etc. Years of opening and closing doors can also create problems.

Just remember to take it slow, be thorough, and be careful and gentle as you do your work. These cars are older than they were ever designed to get, and wiring and connectors can be delicate, brittle, and costly to replace if damaged. In most cases, the problem reveals itself fairly soon, but I've been known to spend several days tracing down an elusive electrical problem, often only to find in the end it was something a previous owner did in an attempt to make a repair, but he caused another problem by doing so. And this would be something to keep in mind: if you aren't aware of the history of the car, look for signs that someone else has been there before you. The factory knew what they were doing in most cases, and although these cars were never intended to last as long as they have, the fact so many are still around proves the manufacturers built in a lot of reserve.

All the best to you, and I hope this article will help you get on down the road with your classic car!

Copyright © 2005 Automotive Mileposts, Inc. All rights reserved.
If you have advice, tips, technical ability, or just know a secret or two about old cars, and you'd like to contribute, click here and tell us about it. We'll help you write it, and give you the credit for it! It's the perfect way to help out your fellow enthusiasts in the old car hobby. Image: AutomotiveMileposts.com
Mileposts Garage Index | Automotive Mileposts

Recent Updates | Main Contents | AUTOPOSTS Forum

Vintage Car Care | Parts | Books | Magazines