![]() However, it may be easier if the coils are removed first.ġ. You should be able to remove the PTU while leaving the coils in place. I replaced the PTU at the same time I replaced the ignition coils and these instructions reflect that. If you want to inspect the PTU you will also need an analog or digital volt-ohmm meter, 14" of 18-ga insulated multi-standed wire, a 1.5V C or D cell battery, two jumper wires with 1" aligator clips on the ends, and electrical tape. The suggested tools include: Phillips and slotted screwdrivers, a 10-mm and 12-mm sockets with 6" extension and wrench, 12-mm open/box end wrench, visegrip-type pliers, regular pliers, a small pocket knife or similar tool, and masking tape. Please read all of these instructions before beginning this operation. These instructions supplement the service manual's instructions for removing, inspecting, and installing the power transistor unit. I paid $125.31 plus shipping for MD152999 at Tallahassee Mitsubishi (1-88 or 85). The same ignition power transistor unit is used on all years and models of the DOHC 6G72 V6 (turbo and non-turbo). While there actually is one pulse per engine revolution through this circuit (the coil fires every revolution), the signal from this check connector must be multiplied by three when monitored by a handheld tachometer (set to 6-cylinder) to determine engine speed. The engine speed check connector ( 2-checkconns.htm) taps into the circuit for transistor and ignition coil "C". Inside the PTU there is a power transistor and circuit for each of these coils. ![]() This is called a wasted spark ignition system. The spark during the exhaust stroke is wasted and the basically inert exhaust gas does not combust. Each ignition coil fires a pair of spark plugs simultaneously: one with the piston before TDC on the compression stroke and the other one with the piston before TDC on the exhaust stroke. Three ignition coils are used in the distributorless ignition system on our cars. As described on my web pages 2-ignitionsystem.htm and 2-ignitioncoils.htm, this produces an electric field in the coil's secondary windings. When it is time to fire the spark plugs, the ECU very quickly withdraws current from the transistor, switching the transistor to "off". Using an internal resistor, the ECU controls the current to the transistor base to limit the current in the coil to 6 amps. This allows current to flow from the transistor collector to the transistor emitter and therefore through the coil's primary windings. ![]() The ECU applies a small current to the transistor base switching the transistor to "on". The NPN transistors in the PTU both amplify and switch the current to the primary windings in the igniton coil. The two diagrams below show a very simple and partial view of the DOHC 6G72 engine's ignition circuit close to the PTU. In contrast, for example, Toyota often incorporates dwell, timing control, and spark distribution in their igniter units. The PTU, which can also be called an ignition module or igniter, in our electronic, inductive-discharge ignition system has ony two basic functions to perform: (1) to provide switches, controlled by the ECU, to supply then remove current to each of the three ignition coil primary circuits, and (2) to provide a tachometer signal every time a spark plug fires, which is every 120 crankshaft degrees. For our DOHC 6G72-engined Mitsubishi 3000GT and Dodge Stealth cars the engine control unit (ECU) provides both dwell control (to generate sufficient energy) and timing control (crankshaft and camshaft position information goes directly to the ECU), relieving the power transistor unit (PTU) of this task. The ignition system's purpose is to deliver sufficient energy to the spark plug at the correct time during the compression stroke. In the Mitsubishi 3000GT and Dodge Stealth ![]() Stealth 316 - Power Transistor Unit Power Transistor Unit ![]()
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