In-Vehicle Testing Prevents Misdiagnosis COLUMN BY JOHN THORNTON How many times have you heard someone say, “the alternator is bad?” ~~ Have you ever i responded by say- ing, “what’s wrong with it?” This is usually followed by the ever- informative, “it doesn’t work!” I’ve always subscribed to the theory of testing that helps determine why a system, circuit and/or component failed. This technique helps ensure accurate diagnosis. Instead of saying “the alternator is bad,” I believe it’s preferable to say one of the following… « the field winding is open *the regulator power transistor is open or failing * the rectifying diodes are allowing excessive AC voltage into the charging system. The purpose of this article is to share with you some of the in-vehicle testing techniques we use at our shop when evaluating Ford JAR _alternator- equipped vehicles. This style of testing prevents misdiagnosis. It’s easy to jump to conclusions in a busy and hectic shop environment. A car comes in with a charging problem. Load testing the alternator shows little or no output. Must be a bad alternator, right? Maybe, maybe not. A planned, specific testing approach will verify the diagnosis. Let’s start by examining and dis- cussing Figure 1, page 22. This drawing shows the typical IAR wiring and component relationship. Terminal “T’ at the regulator plug is the ignition turn-on. Terminal “S” is the stator signal input to the regulator and terminal “A” is both sense and the field current source. Though Figure 1 doesn’t show it, the regulator is grounded through its case and mounting fasteners. At the back of the regulator are ter- minals (fasteners) “A” and “F’’ “A” is the same as “A” in the regulator plug sense and the field current source. “F” is the field connection point in the A-circuit regulator. There’s not much I can add regarding the highly publicized B-B-S output plug connector. Outside of the dotted line is the battery, ignition switch, fuse and the warning lamp. Depending on the appli- cation, the warning lamp can come on when sense voltage (terminal “A’’) is too low or too high, and/or if there is a sta- tor pulse problem at the “S” terminal. Please study Figure 2, page 22. This is my simplified rendering of what is inside an IAR regulator. We will use Figure 2 as our roadmap for many of the circuit checks. The first group of tests is performed with the ignition key turned off. Measure the voltage at terminal “A” of the regulator plug (plug connected to the regulator). What do you expect to find? That’s right! Battery voltage which should be approximately 12.6 volts. Where else could you make this meas- urement? Terminal “A” on the back of the regulator. This is usually where I go first. A careful look at Figure 2 shows the connection between these two points. Next, go to terminal “F” on the back of the regulator and measure voltage at this point. What do you expect to find here? Let me give you a clue; with the key off, the field power transistor is turned off. If that’s the case, there will be approximately 12.6 volts at “F,” since the field circuit is open at the power transistor. Suppose you measured 12.6 at “A” and one volt at “F2’ What do you know, the field circuit is turned on. Should it be on with the ignition key off? No, it shouldn’t. Something has tumed the field on and kept it on. Sounds like a regulator problem, doesn’t it? Be aware that Ford has issued a bul- letin regarding this very condition. Ford bulletin no. 94-9-11 affects 1993 and 1994 Rangers. If the blower motor is on when the ignition key is cycled off, an induced voltage will be backfed into the regulator through the alternator warning lamp circuit, specifically fuse number nine in the fuse box. This will leave the regulator turned on with the key off. Obviously, this current draw on the bat- tery will affect starting performance. The fix? An inexpensive kit that changes the alternator warning lamp feed from fuse number nine to fuse number seven. Aside from these oddball situations, with the key off the field circuit should be turned off. If it’s not, suspect a regu- lator problem. Suppose you measure 12.6 volts at “A” and no voltage at “F’’ What have you found? Looks like there is an open in the rotor or a problem with the brushes. The next test to perform with the key off is a resistance measurement of the rotor and brushes. This will verify the previous scenario. Disconnect the regu- lator plug from the regulator. Put your ohmmeter across “A” and “F” on the back of the regulator. Ford’s specifica- tion (spec) is 2.4 ohms minimum. Note: I’ve tested good alternators that had resistance readings in the 2.2 and 2.3 ohm range. The second group of tests is per- formed with the ignition key on, engine off (KOEO). Using a “T-pin” or a “J-hook” measure the voltage at the “T’ (lamp) terminal of the regulator plug. Please remember, the connector is plugged in. The warning lamp should be turned on. Readings typically fall in the 2 to 3 volt range; o