Learning how to test for electronic component malfunctions can save do-it-yourself mechanics much frustration and money, as they search for the cause of their sputtering engines. The Mitsubishi 3000GT sport car's power transistor unit -- or PTU -- is usually the weak link in the ignition system. It should be tested instead of simply replacing it with an expensive new unit, which may not correct the engine problem.
Access the PTU -- which is located on the passenger side of the engine, bolted to the block beneath the ignition coils -- so you can test the unit without removing it. Take the negative lead off the battery post. Disconnect the upper and lower radiator hoses (engine block side only), and wedge them in an upright position to retain most of the radiator coolant. Position a pail under the engine block to catch any coolant escaping from the block. Disconnect any other obstructions such as turbo or air intake hoses.
Remove the two harness plugs from the PTU. The larger plug is disengaged by pressing on the visible clip on top of the plug and gently wiggling the plug loose. The smaller plug is locked in place by a wire clip located on its underside. Snag the locking wire -- with a 90-degree angle pick, a very small slot-head screwdriver or a pocket knife blade -- and slide it away from the connector. Remove the wire lock with needle-nose pliers, and disconnect the plug.
Test the PTU. The unit is tested by placing a current from a 1.5-volt battery through each of the PTU's three transistors and measuring continuity on the ohm scale of an analog multimeter. If the transistors are good, the meter will register "0" or near "0" resistance (located on the right side of the scale), indicating a completed circuit. A bad transistor will show up as an open circuit with the ohm scale needle swinging to the left side, indicating high resistance.
Zero the ohm scale of the meter by touching the two probes together and aligning the needle on "0" with the adjustment screw. Cut a 2-foot, stranded, 18-gauge, insulated wire in half, and strip the ends of both wires. Attach small alligator clips to each wire, and tape the other ends to the positive and negative poles of a flashlight battery.
Apply current to the first transistor by attaching the clip from the negative battery lead to the ground terminal of the PTU, marked "GND" on the casing. Connect the positive battery lead clip to the emitter terminal of the first transistor identified as "IB1" (next to the ground connector).
Check continuity (a completed circuit) between the first transistor's emitter and the other side of its circuit, called the "collector." Do this by touching the positive (red) probe of the meter to the PTU's ground terminal and the negative (black) probe to the first transistor's collector terminal -- marked "OC1" on the casing of the smaller plug.
Test the second and third transistors by moving the positive battery lead clip to the "IB2" and "IB3" connectors and the black (negative) meter probe to "OC2" and "OC3." If any of the three readings shows high resistance, the power transistor unit is faulty and must be replaced.
