1998 isuzu trooper 3.5L
alternator gain control wire has ground KOEO ( alternator connector disconnected). Supposed to have battery voltage. Is this a bad PCM.
Fuses are good. OCV battery is 11.9 and vehicle running I get 12.4 volts. the charging light is off but the alt is undercharging.
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It isn't a good idea to disconnect the battery while it is running as the resulting voltage surge can damage electronics. Obviously if it isn't charging there won't be much of a surge...
Some alternators are mounted in rubber to cut noise and vibration. This type needs a ground wire which is sometimes forgotten during replacement and sometimes the ground wire just breaks.
A standard alternator only needs two or three wires and sometimes a ground wire. If the direct connection to the battery is good, the ignition light wire intact and working and if needed a battery sensing wire (though this wire isn't often needed in the last couple of decades) the alternator, if it is any good, will charge the battery at the regulated voltage of about 14.5.
If you are sure the alternator is a good one and it isn't charging the connecting wires must be at fault and need to be checked.
There is one other possibility and that applies only in the case of smart charging systems. If your car has a smart charging system the alternator will have several extra wires, usually plugged into the side of the machine.
The alternator works in a similar way to the standard unit except instruction from a separate control unit is capable of reducing or suspending charging. The smart charge black box talks with the engine management black box and charging is suspended during acceleration to cut fuel consumption or improve acceleration. A fault in the smart charge system can prevent the alternator charging.
Disconnecting the additional wiring will allow the alternator to perform its normal function without interruption. The ignition, charge or battery light is controlled by the black box and can do strange things that mean nothing when the smart charge is disconnected.
Your charging system is not working correctly. Unlike most other vehicles, the alternator in yours does not have a built in regulator circuit. Some troubleshooting must be done to find the defective component. Most of this can be done with the vehicle still assembled.
First, check that you don't have a blown fusible link. In most newer cars, these will be in the underhood fuse panel, and look like giant fuses. If good, you will read very nearly zero volts and zero ohms when measuring between the positive battery terminal and the alternator output terminal. If either measurement doesn't read zero, find the loose connection, bad wire, or blown fusible link.
Next, check the ground connections to the engine, battery, and car body to make sure you don't have a bad wire or connection somewhere. Again, measure voltage and resistance between the negative battery terminal and all associated grounding locations. For a quickie test, you can rig a jumper cable between the battery negative terminal and the alternator case. If there is a spark when you make the connection, you have a bad ground somewhere.
If your battery is well connected to the alternator, the problem may lie in the control circuit. Most Chrysler alternators have two control wires that control the field coil. Some control the battery current and have constant ground supplied, and some have battery voltage applied and control the ground current. To test, disconnect the control connector and measure the voltage of both terminals with the ignition in the "off" and "run" positions. Make a chart of each terminal and its corresponding voltage.
Start the vehicle and measure both terminals again. If the terminals tend to be zero volts, except when the vehicle runs you have voltage on one terminal, you have a system where the computer controls battery voltage. If the terminals tend to be 12 volts when the ignition is on, then one terminal goes low when the engine runs, the computer controls ground current. If the voltages of one terminal change but not the other, this suggests a bad computer.
To do a go/no go test of the alternator unit, you can connect the control pins on the alternator with the control connector disconnected. Simply connect jumper wires to the two control terminals of the alternator. Connect one jumper to a known good ground, leaving the other jumper loose. Connect a voltmeter to the battery terminals or to the output wire on the alternator. Start the vehicle and briefly connect the second jumper wire to the positive battery terminal while watching the voltmeter. While the jumpers are both connected, the battery voltage should rise dramatically.If it doesn't, this suggests a bad alternator.
If all this sounds too complicated or dangerous, a good mechanic can perform a similar diagnosis in under an hour with definite results. Good luck.
step one remove battery neg lug . the above video (of book?) is wrong. the belt tension-ing step was skipped.
remove belt steps remove alt. wiring. remove alternator (bolts) it falls free. replace it , reverse steps above belt back tension belt. battery cable back. test run. charge/BAT lamp off? see wrench below, turn it CW. this releases the spring tension and belt comes off to put back, do same and put on belt and release wrench.
The usual cause for this to happen is either a loose or worn belt; or that the battery cables are dirty and need cleaned. To clean properly take the cables off and clean the ends and post with a wire brush or sand paper; then reattach and tighten.
Make sure the ground wire going out of the voltage regulator is grounded and the alternator ground is good.Check all ground wires to there are the ones that stop the over charging.Theres also is a small little capacitor that should bolt right next to the regulator that holds alot of the over charging..