Power Transistor Unit
Removal, Inspection, and Installation
on the
DOHC 6G72 engine
in the Mitsubishi 3000GT and Dodge Stealth
by Jeff Lucius
Introduction
The ignition system's purpose is to deliver sufficient
energy to the spark plug at the correct time during the compression stroke. 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 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. In contrast, for example, Toyota often incorporates dwell, timing
control, and spark distribution in their igniter units.
The two diagrams
below show a very simple and partial view of the DOHC 6G72 engine's ignition
circuit close to the PTU. The NPN transistors in the PTU both amplify and switch
the current to the primary windings in the igniton coil. The ECU applies a small
current to the transistor base switching the transistor to "on". This allows
current to flow from the transistor collector to the transistor emitter and
therefore through the coil's primary windings. Using an internal resistor, the
ECU controls the current to the transistor base to limit the current in the coil
to 6 amps. When it is time to fire the spark plugs, the ECU very quickly
withdraws current from the transistor, switching the transistor to "off". As
described on my web pages 2-ignitionsystem.htm
and 2-ignitioncoils.htm,
this produces an electric field in the coil's secondary windings.
Three ignition coils are used in the distributorless
ignition system on our cars. 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. The spark during the
exhaust stroke is wasted and the basically inert exhaust gas does not combust.
This is called a wasted spark ignition system. Inside the PTU there is a
power transistor and circuit for each of these coils. The engine speed check
connector (2-checkconns.htm) taps
into the circuit for transistor and ignition coil "C". 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 same ignition power transistor unit is used on all years and
models of the DOHC 6G72 V6 (turbo and non-turbo). It is part number MD152999. I
paid $125.31 plus shipping for MD152999 at Tallahassee Mitsubishi
(1-888-825-5648 or 850-575-1238).
These instructions supplement the service manual's instructions for
removing, inspecting, and installing the power transistor unit. Please read all
of these instructions before beginning this operation. 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. 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.
Removal
I replaced the PTU at the same time I replaced the ignition
coils and these instructions reflect that. You should be able to remove the PTU
while leaving the coils in place. However, it may be easier if the coils are
removed first.
1. Battery ground cable. With the ignition off,
remove the negative battery cable from the battery. Be sure you have security
codes for any devices that might need them before you do this.
3.
Intake hoses/pipes. I removed my ARC2 MAS and air filter, my aftermarket
Y-pipe, the "T" hose that the MAS connects to, and my aftermarket metal intake
pipe that goes to the front turbo. I place clean rags into all openings (IC
pipe, intake hose, turbo, throttle body). This gives pretty good access to the
coils and power transitor unit, which is right below the coils. Better access
can be had if the upper coolant hose is removed. If you want to remove this hose
you can do it the easy, messy way (disconnect the hose at the radiator and catch
maybe a quart or two of sticky fluid in rags and containers) or the harder,
correct way (remove undercovers and maybe some active aero stuff and loosen the
petcock at the lower driver's side corner of the radiator to drain fluid out of
the drain hose into a container).
4. Capacitor (if removing
coils). Remove the screw that attaches the capacitor to the lower front
corner of the bracket. I used visegrip pliers to loosen the screw. The capacitor
is attached to the wiring harness. I did not find a part number for it in CAPS
or my hardcopy 3000GT Parts Catalog. The picture below shows the screw that
attaches the capacitor (yellow arrow near capacitor) and the two mounting screws
(yellow arrows at either end of the bracket).
5. Coils harness and ignition wires (if removing coils).
Disconnect the wiring harness connector from the bracket by pressing in the
lever (underneath) and pulling on the connector. Remove the ignition wires from
the coil terminals. Note that the cylinder numbers for each terminal are stamped
into the top of the bracket. Toward the back of the bracket a piece of the
wiring harness is attached to the bracket using a plastic snap-in piece. I
simply cut the plastic piece to disconnect it from the bracket. I did not
replace it nor do I know the part number for it. I imagine the dealerships have
bins of these things laying around.
6. Front turbo coolant pipe
bracket (if removing coils). Remove the 10-mm screw on the bottom of the
coils bracket. This attaches a bracket that holds the small coolant pipe for the
front turbo center housing rotating assembly.
7. Coils with bracket
(if removing coils). Remove the two 12-mm mounting screws that attach the
coils and their bracket to the front cylinder head. The shorter one goes in
front. Extract the coils and bracket together through the top. This would be
much easier if the top radiator coolant hose is removed.
8. Wiring
harness. There are two harness connectors on the PTU. The big one, toward
the front, has a lever on top. Push down on this lever and pull the connector
away from the PTU. You may have to wiggle it a little. The small connector is
secured to the PTU with a wire clip, similar to that used on the throttle
position sensor harness connector. The wire clip is removed, and installed, from
below. It is almost like Mitsubishi tried to make it as difficult as
possible to disconnect this connector. Use the pocket knife blade or similar
tool to barely slide the clip loose from the connector. Grasp the wire clip with
a forceps or similar tool or by hand as you remove it completely. There really
is no need for this type of connection. A simple lever-type latch would suffice.
9. Mounting screws. The PTU is secured to the engine using
two screws. There is a 10-mm screw underneath near the middle of the bracket and
a 12-mm screw at the rear of the bracket. You can see the two holes for these
near the bottom of the picture above of the new PTU. To avoid dropping sockets,
extensions, and screws I tape the socket to the extension using masking tape. I
also take a long piece of masking tape and with the sticky side up push it into
the socket and then wrap the rest of tape after giving it a twist around the
socket. This way the screw head will stick inside the socket when removed from
the bracket. Grasp the PTU and remove it from the engine bay.
Inspection
The service manuals, in section 16 for the 3000GT manuals or
in section 8 for the Stealth manuals, offer a few troubleshooting hints plus
some service and inspection tests. Under troubleshooting the manuals suggest
that a malfunctioning PTU can lead to the following symptom: insufficient or no
spark. This suggests that either the PTU works and you get pulses to the coils,
or it does not work and the coils do not get pulses. With an analog volt-ohm
meter (VOM) you can perform the tests shown below directly on the PTU. These are
easy to perform with the PTU off the engine. However, you should be able to
perform the tests with the PTU in place and harness disconnected.
The
measurements below are designed to test the three power transistors inside the
PTU. Looking at the partial circuit diagram above, we will be testing
the collector-emitter continuity of each transistor. If the NPN transistor is
operating normally, when sufficient voltage is applied to the base (a minimum of
0.7 volts for a silicon transistor) current will flow between base and emitter.
At the same time current can also flow from the collector to the emitter.
For the tests, the ground terminal, noted by "GND" on the PTU case and
by the number "4", is the emitter side of all the transistors. The base of each
transistor is connected to the terminals labelled "IB1", "IB2", and "IB3" on the
PTU case or by the numbers "3", "2", and "1", respectively. The collector of
each transistor is connected to the terminals labelled "OC1", "OC2", and "OC3"
on the PTU case or by the numbers "13", "12", and "11", respectively. We test
for continuity between the collector and emitter by applying +1.5 volts (using a
D or C cell "flashlight" battery) to the base terminals with the battery's
negative side connected to the PTU ground (terminal 4).
Use an
analog-type circuit tester for the three tests. Cut a 14" piece of 18-ga
insulated multi-strand wire into two unequal length pieces, such as 8" and 6"
(this helps to avoid a short circuit). Strip the insulation off the ends about
1/4 to 1/2 inch. Tape an end of one wire to the positive end of the battery and
an end of the other wire to the negative end of the battery. Prop or brace the
PTU so that you have good access to the terminals. Clip one of the jumper wires
to the end of the wire connected to the positive side of the battery. Clip the
other jumper wire to the negative wire from the battery and to terminal 4 (GND)
on the PTU. When we clip the positive jumper to IB1, IB2, or IB3, this setup
will apply 1.5 volts to the base of the transistor with current through the
base-emitter junction. If the transistor is functioning currectly there will
also be a complete circuit from the collector to the emitter (ground). When we
do not apply a current to the base the connection between collector and emitter
should be broken.
1. Power transistor for no. 1 - no. 4 coil (coil "A"). Set
the VOM to measure Ohms and touch the negative lead (black) to terminal 13 (OC1,
the collector) and the positive lead (red) to terminal 4 (GND, the emitter). You
should see some resistance. Now clip the jumper wire connected to the batttery's
positive terminal to terminal 3 (IB1). Measure the resistance between terminals
13 and 4 as before. The resistance should be much less or close to zero. The
service manual states there should be continuity with the battery connected to
terminal 3 (in fact, the resistance will be very low) and no continuity with the
battery not connected (in fact, the resistance will be high). Note on an analog
VOM that zero ohms is on the right side of the scale.
2. Power transistor for no. 2 - no. 5 coil (coil "B").
Repeat the above test except connect the battery positive to terminal 2 and the
VOM black lead to terminal 12.
3. Power transistor for no. 3 - no. 6 coil (coil "C").
Repeat the above test except connect the battery positive to terminal 1 and the
VOM black lead to terminal 11.
Installation
Installation is basically just the reverse of the removal
steps. It may be difficult, especially if you left the upper radiator hose in
place, to start some of the screws. To avoid dropping sockets and screws I tape
the socket to the extension using masking tape. I also take a piece of masking
tape and place the sticky side against the head of the screw. I then insert the
screw into the socket, twisting and wrapping the masking tape around the outside
of the socket. This way the screw does not fall out of the socket as I start it
by hand. Once the screws are started several turns, remove the masking tape and
use the socket wrench to finish tightening them. When I reconnected the small
PTU harness, I slid the wire clip on from the top instead of from the bottom. I
then used some electrical tape to secure the clip. Connect the negative battery
cable last. Start the engine and be sure it runs smoothly.
Page last updated September 22, 2003.