DO NOT REMOVE THE BATTERY CABLE! Assuming you mean the battery negative cable and not the battery charger cable; you didn't say. On older cars, it was common for mechanics not knowledgeable in electrical stuff to pull a battery cable off while the engine was running. If it stayed running, it proved the charging system was working. The problem with this trick is the battery helps regulate the system voltage. Without the battery in a working circuit, the alternator's output voltage will go up a little. That same voltage supplies the alternator field winding, so the electromagnet gets stronger, and output voltage goes higher. It's a vicious circle of voltages going higher and higher. The voltage regulator tries to cut the field current back, but there's just so much it can do.
On older cars, there was a danger of popping any light bulbs that were turned on when the battery cable was removed. The voltage will rise even higher and faster if the engine is sped up. It is not unheard of for the voltage to hit over 30 volts! Computers can't handle that and will be destroyed. Your car has computers for the engine, transmission, body, air bag, HVAC, and possibly anti lock brakes, auto load leveling, remote keyless entry, and electronic vehicle information center. Can you imagine the cost if they all popped due to high voltage?
Now, assuming no damage occurred, lets troubleshoot this system the right way. It's actually very simple and easy to diagnose, but the diagrams in the service manual make it look really complicated. To start with, use a digital voltmeter to measure the voltage between the two battery terminals while the engine is running. It must be between 13.75 and 14.75 volts. Lower than 13.75 and it won't keep the battery fully charged. Over about 15 volts and it will start to boil the water out of the battery. You must make these measurements when the problem is acting up. If the voltage is a little low after repairs are completed, but slowly rising, that's ok. The voltage will slowly rise to normal as the battery becomes charged.
Assuming you find about 12 volts or less, the charging system is dead. Don't waste your time with any relays. The alternator field current comes through the Automatic Shutdown (ASD) relay. This relay is turned on by the engine computer for only two seconds when you turn on the ignition switch, and then when the computer sees engine rotation, (cranking or running). You can be sure the relay is working because it also supplies the current for the injectors, coils, fuel pump or pump relay, and O2 sensor heaters. If the relay didn't work, the engine wouldn't run.
There are three wires on the back of the alternator. The large one bolted on must have full battery voltage all the time. One of the small ones will have full battery voltage when the ASD relay is on, so it's easiest to measure the voltage while the engine is running. The second small wire will have less than battery voltage, but rarely less than 4 volts. The lower the voltage on the second wire, (the bigger the voltage difference between the two small wires), the bigger the electromagnet and the higher the current the unit is trying to produce.
The most common results when you have a problem will be battery voltage on one terminal and 0 volts on the other. Only two things can cause 0 volts. That wire is grounded, such as rubbed through on a sharp metal bracket, or the brushes in the alternator are worn, (open). A grounded wire would cause the alternator to charge wide open. You'd be popping light bulbs and the sides of the battery would be bulging. The battery would be hot too. The more very common cause of 0 volts is the brushes. You can verify this by stopping the engine, then using an ohm meter to measure continuity between the two small wires. Normal is around 4 ohms. Good brushes can also measure open circuit due to small bits of carbon breaking off the brushes and getting stuck under one of them. This isn't a problem when it's spinning. When you find an open circuit, irritate the drive belt a little while watching the meter. If it continues to read open circuit, you need to replace the alternator, or if parts are available, you can replace the brush assembly. You can do that without removing the alternator from the engine if there's room to remove the rear cover.
In the rare event you find 0 volts on both small wires while the engine is running, you have a break in the feed wire between that terminal and the splice connecting to the injectors and coils. More likely there is corrosion or rust between your meter probe and the point you're measuring. Just poke a little harder.
If you find the exact same full battery voltage on both small wires, there is an open circuit, (break in the circuit), between the alternator and the voltage regulator in the engine computer. It's rare to find a defective regulator. More common would be a corroded pin in a connector. If you do indeed find equal voltage on both pins, you can go to the engine computer connector and ground that wire for a few seconds as a test. The results will determine if the regulator or wiring is the problem. We'll cross that bridge if it becomes necessary.
99 percent chance the brushes are the problem. Let's hope for a cheap fix before talking about the expensive stuff. You can try finding a brush assembly at the local parts store. Cost should be around ten bucks. They might be available from the dealer too. I get mine from a local rebuilder. You should have a Nippendenso alternator. After removing the wires, three or four nuts and the rear stamped steel cover, the brush assembly is held on with three philips screws. Having the new assembly in hand will give you a better idea of where to find the screws.
Be sure to watch the terminal on the heavy bolted-on wire that it doesn't touch any metal engine parts. Disconnecting the battery is safest, but you will lose the radio clock and station presets, and you'll lose fuel trim data stored in the engine computer. The engine also will probably not idle on its own without holding your foot on the accelerator pedal. That's normal until you relearn minimum throttle. Again, we'll handle that later if necessary. These are just inconveniences compared to arcing that wire with your wrench or ratchet.
Monday, November 30th, 2009 AT 11:13 PM