Alternator Drawing From Battery

Had my battery go dead just sitting in the garage....recharged battery then go dead again. Someone told me it's a failure mode of a bad alternator. Installed new alternator....fixed!

Alternator troubleshooting for 86-93 5.0 Mustangs:

Never, never disconnect an alternator from the battery with the engine running. The resulting voltage spike can damage the car's electronics including the alternator.


Revised 15 April 2012 to add simple check for regulator failure in Engine off ignition on, battery fully charged section, item 2.

Red color text applies to cars with a 3G alternator.

Do all of these tests in sequence. Do not skip around. The results of each test depend on the results of the previous tests for correct interpretation.

Simple first step: Remove the alternator and take it to your local auto parts store. They can bench test it for free.

Engine off, ignition off, battery fully charged.
1.) Look for 12 volts at the alternator output. No 12 volts and the dark green fuse link between the orange/black wires and the battery side of the starter solenoid has open circuited.
3G alternator: Look for 12 volts at the stud on the back of the alternator where the 4 gauge power feed wire is bolted.
No voltage and the fuse for the 4 gauge power feed wire is open or there are some loose connections.
2.) Look for 12 volts on the yellow/white wire that is the power feed to the regulator. No 12 volts, and the fuse link for the yellow/white wire has open circuited.

Engine off, ignition on, battery fully charged:
1.) Alternator warning light should glow. No glow, bulb has burned out or there is a break in the wiring between the regulator plug and the dash. The warning light supplies an exciter voltage that tells the regulator to turn on. There is a 500 ohm resistor in parallel with the warning light so that if the bulb burns out, the regulator still gets the exciter voltage.
Disconnect the D connector with the 3 wires (yellow/white, white/black and green/red) from the voltage regulator.
Measure the voltage on the Lt green/red wire. It should be 12 volts. No 12 volts and the wire is broken, or the 500 ohm resistor and dash indicator lamp are bad. If the 12 volts is missing, replace the warning lamp. If after replacing the warning lamp, the test fails again, the wiring between the warning lamp and the alternator is faulty. The warning lamp circuit is part of the instrument panel and contains some connectors that may cause problems.

2.) Reconnect the D plug to the alternator
Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see 2.4-2.6 volts. No voltage and the previous tests passed, you have a failed voltage regulator. This is an actual measurement taken from a car with a working electrical system. If you see full or almost full12 volts, the regulator has failed.

Engine on, Ignition on, battery fully charged:
Probe the green/red wire from the rear of the connector and use the battery negative post as a ground. You should see battery voltage minus .25 to 1.0 volt. If the battery measured across the battery is 15.25 volts, you should see 14.50 volts

Familiarize yourself with the following application note from Fluke: See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .


You will need to do some voltage drop testing of several of the wires.

Start looking for these things:
1.) Bad diode(s) in the alternator - one or more diodes have open circuited and are causing the voltage to drop off as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.

2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Do the voltage drop test as shown in the Fluke tech note link. Measure the voltage drop between the alternator frame and the battery negative post. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.

3.) Bad regulator that does not increase field current as load increases. Remove the alternator and bench test it to confirm or deny this as being the problem.

4.) Bad sense wire - open circuit in sense wiring or high resistance. The yellow/white wire is the voltage sense and power for the field. There is a fuse link embedded in the wiring where it connects to the black/orange wiring that can open up and cause problems. Disconnect the battery negative cable from the battery: this will keep you from making sparks when you do the next step. Then disconnect the yellow/white wire at the alternator and the green fuse link at the starter solenoid/starter relay. Measure the resistance between the alternator end of the yellow/white wire and the green fuse link: you should see less than 1 ohm. Reconnect all the wires when you have completed this step.

5.) Bad power feed wiring from the alternator. Use caution in the next step, since you will need to do it with everything powered up and the engine running. You are going to do the Fluke voltage drop tests on the power feed wiring, fuse links and associated parts. Connect one DMM lead to the battery side of the starter solenoid/starter relay. Carefully probe the backside of the black/orange wire connector where it plugs into the alternator. With the engine off, you should see very little voltage. Start the engine and increase the load on the electrical system. Watch for an increase in drop as the load increases. Use the Fluke voltage drop figures as guidelines for your decisions.





Alternator wiring circuit
Notice the green wire connects to a switched power source. The circuit contains a 500 ohm resistor in series between the switched power and the alternator. Connecting it to switched power keeps the regulator from drawing current when the engine is not running. The resistor limits the current flowing through the wire so that a fuse isn't needed if the wire shorts to ground.

Also notice the sense wire connects to the starter solenoid and it is fused. It connects to the starter solenoid so that it can "sense" the voltage drop across the output wiring from the alternator.

gregandmike said:

The alternator is new and I thought I would try the tests first. On the first test I got 12 volts at each wire. On the second test the bulb did not glow and I replaced a bad bulb, which still did not glow. And when I tested the green wire I got 11.8 volts, unplugged and 2.7 volts. Does only getting 11.8 volts mean there is a break in the wiring to the lamp ? Thanks

Click to expand...

Disconnect the D shaped connector from the alternator and ground the green wire to the engine block or alternator housing. With the ignition switch in Run, the bulb should glow if the bulb and wiring is good.

gregandmike said:

I grounded the green wire and the bulb did not glow. Is my option now to run a new wire from the dash to the alternator ?

Click to expand...

The instrument cluster has two spring type connectors that seat against a flexible plastic sheet that acts as the circuit board for the instrument cluster. The flexible plastic sheet is plated with copper strips that act as the wiring. The odd are that one of the spring clips isn't making a good connection with the plated copper strips.

I don't advise you to make any changes or repairs to the plastic sheet circuit. It is too easy to melt something and almost impossible to fix if you do. The one possible exception is the resistor that is in parallel with the battery warning light. Check it with the bulb removed: it should measure about 500 ohms +\- 50 ohms. If it is bad and the bulb is inoperative, the alternator will not charge. You can replace it with a resistor from Radio Shack or other electronics supply companies if you are very good at soldering small things. It will take a small 25 - 50 watt pencil type soldering iron, don't use big soldering gun or you'll melt things. A Radio-TV repair shop can do it for you if you don't have the proper tools or don't feel comfortable doing it.

To check out the wiring between the instrument cluster connector and the alternator, disconnect the D connector from the alternator and ground the green/red wire. Disconnect the instrument cluster connector from the instrument cluster.
Then measure the resistance to ground with one ohmmeter lead connected to a good ground and the other on the green/red wire on the instrument cluster connector. You should see less than 2 ohms.

See http://www.stangnet.com/tech/cluster87-93.pdf for a drawing of the instrument cluster wiring connector. Keep in mind that there are two sections of the drawing, one for 90-93 model cars and another one for 87-89 model car. Be sure to choose the correct drawing, since the wiring is very different between the two groups.

See the link that I posted to Stangnet showing the instrument cluster. Your answer is dependent on the year car you have. The diagrams for both models 90-93 and models 87-89 are in there in there.

The .4 ohms is good. That means the wire to the alternator is OK.

Have you verified that the 500 ohm resistor is in the 450-550 ohm range with the cluster disconnected from all the wiring?

If so, the problem is ether in the cluster itself or the quality of the connections to the cluster.
Disconnect the D connector from the alternator and ground the green/red wire to the engine block.
Set your meter to measure 12 volts and ground one lead. With the cluster connected to the wiring harness, turn the ignition switch to Run. Measure both sides of the resistor for 12 volts: one side should read 12 volts or whatever the battery voltage is. If there is no ground path, both sides will read 12 volts. That means the cluster connections are bad or the plastic sheet circuit has a break in one of the copper traces plated on it.

gregandmike said:

With the cluster disconnected and the bulb removed I connected to the resistor and got a reading of .510 ohms

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Looks good, except the decimal point is in the wrong place...

Using a Multimeter to measure resistance
Almost every meter has a different method for showing an open circuit. An open circuit is one with a break in it somewhere. That break can be a switch turned off, a fuse blown, a lamp burned out, a bad connector, a damaged circuit board or a cut or burned wire.

Without reading the instruction manual that came with your meter, I would not try to guess what your readings mean.

Step 1.) Find the instruction book that came with your Multimeter. Read it and familiarize yourself with how it works and how use it. If you lost the book or didn’t get one with it, do a Google search on the web to find the manufacturer’s web site & download a copy of the manual. Remember that while some meters auto-range to find the correct voltage range, the Ohms function ranges are usually set by the selector switch. Most of the resistance testing done in automotive troubleshooting uses the lowest Ohms range possible.

Step 2. ) Make sure that you know what test lead plugs into which jacks on the Multimeter. There are usually several different jacks on most Multimeters, and they have different functions. Make sure that your battery(s) in the Multimeter are good: if you have any doubts, replace the battery(s).

Step 3.) Once you are sure that the Multimeter is functional and you have the leads plugged into the jacks for Ohms ( the Step 3.) Once you are sure that the Multimeter is functional and you have the leads plugged into the jacks for Ohms ( the Ω symbol), do some simple measurements to make sure that you know how to use it correctly. Set the switch to the lowest range and touch the leads together: you should not see “nothing” but you should see 0.3-1.0 ohms. Measure a 60 watt light bulb: cold it will measure about 17.5 Ohms. It you measure it while it is hot, the reading will be greater.

symbol), do some simple measurements to make sure that you know how to use it correctly. Set the switch to the lowest range and touch the leads together: you should not see “nothing” but you should see 0.3-1.0 ohms. Measure a 60 watt light bulb: cold it will measure about 17.5 Ohms. It you measure it while it is hot, the reading will be greater.

Step 4.) Make several test measurements using the ohms function and the DC volts function. Remember all resistance measurements must be done with the power off the circuit. This avoids false readings and possible damage to the ohmmeter.. Repeat steps 3 & 4 until you are sure that you can do it without making any mistakes.