Testing the P-D Alternator On the Car By Juan C. Grube There has been a ¥ lot written about the | importance of | iy proper bench testing ~~ ey of the P—D alterna- Sy tors used on the lil Mazda Protégé and Miata. Improper bench testing such as using the wrong test lead will ruin the field control unit (internal regulator). But how do we test the alternator on the car without damaging it? It’s pretty simple if you remember to be extra careful about what you connect to the alternator P—D terminals. First use your normal on-the-car test procedure to test the charging sys- tem just like you do on any car. That is, verify that the charging system volt- age is acceptable. If it is, load the ve- hicle battery with your carbon pile to check for rated amperage output. If the alternator fails these two standard tests, what should you do next? Unplug the alternator P—D con- nector. Start the vehicle, run at idle, and turn on the major electrical loads (headlights, air, etc.). Measure the volt- age at the harness connector “D” ter- minal (see Figure 1). It is IMPORTANT to use a high impedance voltmeter to avoid “over- loading” the computer. Any digital volt- meter is high enough impedance. Ifyou are using an analog voltmeter it must have an impedance of at least 1,000 ohms/ volt. Do NOT use a test light! The voltage at the disconnected harness “D” terminal should be greater than 0 volts and may be as high as bat- tery voltage. If there is voltage at the disconnected harness “D” terminal the computer is doing its job, that is, tell- ing the alternator to charge. Before removing the alternator, use an “on-the-car” test lead adapter to verify that the alternator is working or not working. That way you will know for sure and not waste your time, which is money. Remember you are the automotive electrical expert in town. You know the correct way to troubleshoot a system like this without ruining a lot of expen- sive parts. Advertise that so everybody else knows to bring charging system problems to you for quick and accu- rate repair. That’s my opinion, “ex- change” yours. Juan Grube is president of JIMCO, Inc., Raymore, MO. DECEMBER, 1999 Page 1 of 1