Why is a 1994 Taurus not charging the battery?

Answer 1

Battery cables corroded, alternator is defective, or the battery has a dead cell and needs replacing. Check the voltage at the battery with the engine running. It should be 13.5-16 volts. Any less and the alternator is defective. Also make sure the alternator belt is not loose.

All of those things are okay. The new alternator puts out the proper voltage, except when the new battery is hooked up to it. Then the alternator drops 4 volt output and the battery gets drainged. I have tried three new altenators and the battery chcks out. No wiring issues. What is causing this.

These kinds of problems is excessive resistance in the high amperage circuit. Loose, corroded or damaged battery cables or ground straps can choke off the normal flow of current in these circuits. And if the current can't get through, the starter won't have the muscle to crank the engine and the battery won't receive the amperage it needs to maintain a full charge. Nasty looking battery terminals that are blooming with corrosion obviously need cleaning. But many times corrosion forms an almost invisible paper-thin barrier between the battery terminals and cables. To the naked eye, the terminals and cables look fine. But high resistance in the connections is preventing the high amp current from getting through.

The same goes for battery cables with ends that have been beaten or pried out of shape, or have had the ends replaced. If the clamp isn't making good contact with the battery terminal all the way around as well as its own cable, the cable may have too much resistance and restrict the flow of current. The same goes for ground straps that have loose or corroded end terminals, or make poor contact with the engine or body.

Cranking problems can also be caused by undersized replacement battery cables. A wire's ability to pass current depends on the gauge size of the wire. The fatter the wire, the more current it can safely handle. Some cheap replacement battery cables use smaller gauge wire, which may be camouflaged with thicker insulation to make it appear to be the same size as the original cable. But the cable doesn't have the capacity to handle the current. It doesn't take much of an increase in resistance to cause trouble. Let's say a 120 amp alternator operates in a circuit that has a normal resistance of 0.11 ohms. If that resistance were increased to 0.17 ohms because of a bad wiring connection, the alternator's maximum output would be limited to 80 amps. In other words, an increase of only 0.06 ohms (almost nothing!) would reduce the alternator's maximum output by almost a third! Under light load, the drop in charging output might not even be noticeable. But in a high load situation, the alternator wouldn't be able to keep up. If you use an ohmmeter to measure across a heavily corroded battery cable or ground strap connection, or one with only a few strands of wire that make contact with the end clamp or terminal, the connection may read good because all you're measuring is continuity -- not the ability to handle a high amp current load. The connection may pass a small current, but when a heavy load is applied there may not be enough contact to pass the extra current.

So how do you find these kinds of problems? You do a voltage drop test. A voltage drop test is the only effective way to find excessive resistance in high amperage circuits. It's a quick and easy test that doesn't require any disassembly and will quickly show you whether or not you've got a good connection or a bad one.

To do a voltage drop test, you create a load in the circuit that's being tested. Then you use a digital volt meter (DVM) to measure the voltage drop across the live connection while it is under the load. Voltage always follows the path of least resistance, so if the circuit or connection being tested has too much resistance some of the voltage will flow through the DVM and create a voltage reading. If a connection is good, you should find little or no voltage drop and see less than 0.4 volts for most connections, and ideally less than 0.1 volts. But if you find more than a few tenths of a voltage drop across a connection, it indicates excessive resistance and a need for cleaning or repair. To check the alternator connections on the positive side for excessive resistance:

A. Set DVM on 2 volt DC scale.

B. Connect the meter positive lead to the alternator output stud (B+ terminal).

C. Connect the meter negative lead to the positive (+) battery post.

D. With the engine running at 1,800 to 2,000 rpm with all lights and accessories on (except the rear electric defroster), check the voltage drop reading. It should be 0.5 volts or less. If higher, the connections between the alternator output stud and battery need to be cleaned. Also, look for loose connections or undersized cables.

To check the alternator connections on the negative side for excessive resistance:

A. Set DVM on 2 volt DC scale.

B. Connect meter negative lead to alternator case.

C. Connect meter positive lead to battery negative (-) post.

D. With engine running at 1,800 to 2,000 rpm with all lights and accessories on (except rear defogger), check the voltage drop reading. On the negative side, it should be 0.2 volts or less. If excessive, the connections need cleaning or the negative cable needs to be replaced. Some alternators are mounted in rubber bushings and have a separate ground strap. If so equipped, be sure to check the voltage drop across this strap, too. This car is 16 years old and I would suspect you need to just replace both battery cables. This will more than likely solve your problem. Be sure and check the ground strap too.

THANKS!