The Rules Have Changed…Again! Battery Location Dictates Testing Procedure COLUMN BY JUAN C. GRUBE “Hot or mild sauce?” “Bride’s or groom’s side?” “Ground or air shipment?” “Four or six cylinder?” Sometimes asking questions is the only way to get that one piece of information you really need. For a rebuilder, the right question can lead to a profit—or a warranty loss. Today, if you’re dealing with a late model General Motors vehicle, one of the most important questions to ask is: “Where’s the battery?” At this point, you may want to give the guy a break by further asking him, “Is it under the hood, or under the back seat, or in the trunk?” His answer is critical for testing the alternator or troubleshooting the vehicle. If the battery is under the hood, then treat the alternator like a “normal” GM alternator. No problem. If the answer is “T don’t know where it is but it’s not under the hood” then there are new rules. A remote battery vehicle is a “ther- mistor sense” vehicle. In this vehicle the regulator “S” (SENSE) terminal is not a direct battery connection but one that has a thermistor in series with the con- nection. A thermistor is a resistor that changes resistance when its temperature changes. The thermistor is located in the sense wire near the battery. The colder the battery, the higher the resulting reg- ulator set point. The warmer the battery, the lower the regulator set point. This is true whether the alternator is a CS, CS130D or AD series. A thermistor sense alternator is regulator part number specific. The test procedure for this alternator on the vehicle and on your test bench is differ- ent from other GM applications. If you put the wrong regulator part number in the alternator it may “test” okay but not work correctly on the vehicle. If you test a thermistor sense alternator with a test lead for a conventional alternator the results will be erroneous, period. Figure | is a diagram of a thermistor sense alternator. Figure 2 is a diagram of a non-thermistor sense alternator. Compare the two. The procedure for bench testing this alternator is simple: just use a test lead with the provision for thermistor sense. There are two methods for checking the thermistor circuit on the vehicle. 1. Disconnect the regulator plug. Check for an ohm reading from the “S” (SENSE) terminal in the plug to the bat- tery positive (+) cable on the battery. The value should be approximately 1800 ohms. This simple procedure is actually difficult to do because of the distance involved. It takes two people and long lead wires. 2. The other procedure sounds more complicated but is really easier to do. Disconnect the regulator plug and measure the voltage at the “S” (SENSE) terminal in the plug while loading the circuit with an 1800 ohm resistor. The reading should be right at one half of the battery voltage at the alternator (B+) ter- minal. For example, If the battery (B+) voltage is 12.6 volts, your meter should read 6.3 volts. If there’s a problem in this circuit it’s either going to read 0 volts because the circuit’s open or read 3 to 4 volts. Use of an 1800 ohm resistor is critical to this test. Refer to figure 3. NOTE: Since this is a “thermistor sense” vehicle, the results of either circuit test procedure will be affected by the temperature of the thermistor. Remember, ask questions. If the answers don’t make sense, ask more. That’s my opinion, “exchange” yours. Juan Grube is president of JIMCO, Inc., Raymore, MO.