DIAGNOSING COMPUTER-CONTROLLED CHARGING SYSTEMS

Motor, Sep 2007 by Marinucci, Dan

Even though most have a computer controlling the output, popular GM and Chrysler alternators can still be accurately diagnosed. Specific tests will quickly rule in or rule out the alternator as the cause of charging problems.

Sometimes we goof by making a job more complicated than it really is. Later, we look back and realize we cost ourselves time, money and grief by making the task tougher than it needed to be. Whoa! We have enough aggravation already, don't we?

Charging system diagnosis fits into that category, and I believe it should be kept as simple as possible. The way to simplify always has been to divide and conquer. In odier words, divide a vehicle's charging system by carefully isolating a suspect alternator. Then replace the alternator if it still doesn't work or work correctly.

Diagnostic Premises

The single, most common charging system symptom is a dead battery. First, always recharge or replace the battery as needed, then test the charging system. Suppose the system fails the routine volts and amps tests-especially an alternator that won't charge at all. At this point, the diagnostic choices used to be a bad alternator or wiring failure. Naturally, a thorough voltage-drop test will identify any wiring problems and/or bad connections.

Unfortunately, many modern charging systems have a third failure possibility-a computer! This combination of possible problems is one convincing reason why isolating the alternator is more appropriate than ever before. Another compelling reason is the time required to R&R some of these alternators. If you're spending that time, you want to know the effort is justified.

Yes, you must access the alternator and connect something to it in order to isolate it. But you probably were heading toward that suspect alternator anyway. And once you make the proper hookup, this approach completely separates the alternator from the computer and its wiring. Now if that alternator still doesn't charge normally, you're confident that you must replace it and retest.

However, if a so-called isolated alternator charges normally, the trouble must be somewhere outside the alternator. This means you need troubleshooting time to methodically track down the problem. Politely explain your diagnostic format to the customer. If he or she won't authorize even a minimal amount of diagnostic time, collect for what you've done thus far and move on to the next vehicle. Why entangle yourself in a potentially lengthy troubleshooting procedure if you won't get paid for it?

Let me give the ol' scan tool its due here before I proceed. Many scan tools have an extremely helpful selection of active or output tests. These may include the ability to command the alternator to charge. But suppose you command the alternator to charge and it doesn't respond. In a pinch, is the root problem the scan tool or its software? Or is it an ECM failure, a wiring problem or a bad alternator?

Here I'll concentrate on nailing no-charge conditions on popular General Motors and Chrysler systems. Fords are a topic for another time. I'm working on the assumption that you'll also consult the appropriate shop manual and wiring schematic for the vehicle in question.

GM's Terminal L

The overwhelming majority of General Motors products have used alternators with the same basic terminals since 1986. The most popular alternators have either terminals P-L-I-S or P-L-F-S. Remember that a harness connector for a P-U-S alternator physically interchanges with one for a P-L-F-S unit. For the purposes of our isolation tactic, we're concerned only with terminals P and L.

Earlier alternators with these terminal layouts have rectangular harness connectors; later ones have ovalshaped connectors. The lion's share of GM products coming into your bays have the oval alternator connector, which was used until the mid-2000s.

The neat thing is that regardless of connector shape, the first pin is always terminal P. I'll discuss P momentarily. The second pin is always L, which stands for lamp or charge indicator light. The key to operating these alternators is energizing or exciting terminal L. A typical P-L-I-S or P-L-F-S alternator will operate on terminal L alone. Indeed, zillions of the GM vehicles that have passed through your shop had a single-wire alternator connector-a lone wire connected to terminal L.

The electrical systems on the most common GM vehicles apply either 5.00 or 12.00 volts to terminal L after the engine starts up. Remember that these alternators shut off immediately if L loses its power supply! Equally important, it takes only a very small amount of constant current to energize L and keep the alternator operating. I've measured current flow on many L circuits; typically, it's only 5 to 8 milliamps (.005 to .008 amp) with the alternator operating normally.

GM's Terminal P

Terminal P, which stands for phase, is a stator terminal. The typical P-L-I-S or P-L-F-S alternators have functioning P terminals, but it's rare to see a wire connected to P on the vehicle. Suppose tiiere's a good circuit from the alternator output terminal (BAT) over to the positive bat- tery terminal and the alternator is properly grounded. In that case, start the engine and carefully backprobe P. Stator voltage on a healthy alternator is one-half charging voltage, plus or minus about .50 volt. Therefore, if voltage at BAT is 14.00, P should be 7.00 volts or extremely close to it. Experience shows that when a stator or diode failure occurs, stator voltage is way above or below the one-half value.