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1. De-Mystifying Mazda RX-7 Ignition Operation, John Thornton,
Underhood Service, October 1998
• Rotary Engine Principles
• G & Ne Signals
• Poorly Idling '88 RX-7
• Finding TDC
The Mazda RX-7 is an infrequent visitor to our shop. On average, we may see
three to five of these vehicles a year for driveability concerns. It used to be that
much of our diagnostic time was spent re-learning the engine's support systems
such as fuel, emissions and especially the ignition system. I can still remember
carefully studying crowded ignition schematics while trying to determine what
wire goes where and what its purpose was.
This month's Diagnostic Dilemmas will try to explain the Mazda RX-7's ignition
system operation, and how the ignition systems (yes, systems - these vehicles
have two) can be evaluated in an efficient manner. We'll focus on late 1980s and
early 1990s RX-7s.
Rotary Engine Principles [go to top]
The best place to start this discussion is with a review
of rotary engine principles. As you know, there are two
rotors in each rotary engine. Each triangularly shaped
rotor has three faces, or three combustion area
surfaces. Each face of the rotor goes through an intake
event, a compression event, a power event and an
exhaust event. Therefore, in one revolution of a rotor,
each rotor face has gone through all four events
(intake, compression, power and exhaust). Said
another way, there are three power events, or pulses,
in one rotor revolution. Here's where things get
interesting.
Each rotor is turned by an eccentric shaft; we can call it
a crankshaft. See Figure 1. The eccentric shaft has two
offset eccentrics (one for each rotor) mounted 180
degrees apart. This shaft allows the rotors to move in
the shape of an epitrochoidal curve.

2. Figure 3 does a nice
job of showing the
relationship of the two
rotors and how they go
though the four
combustion events.
What does this all
mean? Let's
summarize with some
key pieces of
information.
• In one rotor
revolution, there
are three firing
events or power
pulses.
• The eccentric
shaft makes
three revolutions
for every rotor
revolution. This
means the
eccentric shaft
rotates three
times faster than
the rotor. This
allows the rotors
to follow the
shape of an

3. epitrochoidal
curve.
• In a two-rotor
engine, the
eccentric shaft
rotates through
three revolutions
(1080 degrees)
for six firing
events or power
pulses.
Therefore, and
this is important,
in one eccentric
shaft (crank)
revolution, an
ignition event
occurs every
180 crank
degrees.
• The front rotor
leads the rear
rotor by 60 rotor
degrees or 180
eccentric (crank)
shaft degrees.
The above information becomes important when we start to examine the
relationships between ignition signals.
We also know that each rotor has two spark plugs: a leading plug and a trailing
plug. The leading spark plug fires at 5 degrees ATDC and the trailing spark plug
fires at 20 degrees ATDC. This 15-degree difference at the crank translates into
5 degrees at the rotor. Do you remember why? The crank turns at a 3:1 ratio to
the rotor. The purpose of the trailing spark plug is to continue the burn for more
complete combustion and reduced emissions. The leading plug is the lower plug
in each housing, and the trailing plug is the top plug in each housing. Fortunately,
each plug hole is marked with an "L" or a "T."
Starting in 1986, a crank angle sensor replaced the distributor. The crank angle
sensor looks like a distributor, but instead of a cap with wires at the top, it has a
removable inspection plate. Because of the crank angle sensor and loss of the
distributor cap and rotor, these ignition systems are known as distributorless.

4. Our next step in understanding this system is to review the schematic. Figure 4 is
a partial schematic from an RX-7. What is not shown are the crank angle sensor
inputs. However, Figure 4 shows all the wires between both igniters (leading and
trailing) and the PCM. We can see that there are four wires connected between
the igniters and the PCM, but what are their functions? How are the coils
connected to the igniters? These are just some of the questions I've asked
myself when troubleshooting these systems. Let's simplify and continue.
G & Ne Signals [go to top]
Figure 5 is a much simplified schematic. The G and Ne pick-up coils are located
inside of the crank angle sensor. Their output is sent directly to the PCM. IgT is
the ignition timing signal (trigger) sent to the igniters from the PCM. The igniters
will not toggle the primary without this signal. The leading coil (mounted in the LF
corner of the engine compartment) is a true distributorless coil. Both leading
plugs fire together, even though the rotors are 60 rotor degrees apart or 180
crank degrees apart. The two trailing coils are mounted in the LR corner of the
engine compartment.
This is not a
typical
distributorless
system. You'll
notice that
there is only
one IgT signal
sent to the
trailing igniter
from the
PCM. Well,
how does the
trailing igniter
know when to
fire each coil
with only one
signal? That
is the purpose
of the Select
signal. The
Select signal
tells the
trailing igniter
which coil to
fire. We'll see
how, shortly.

5. We will now look at these signals in more detail and we'll study some problems
I've come across with these engines.
If you were to remove the inspection plate
or cover of the crank angle sensor, you
would see two teeth at the very top and 24
teeth just below the top two. The two-tooth
reluctor provides the G signal, which is
rotor position information. The 24-tooth
trailing coil firing reluctor provides the Ne
signal, which is rpm information and
trailing-coil firing synch. Like a
conventional distributor, the crank angle
sensor turns at half crank speed. One
crank angle sensor degree is equal to two
crankshaft degrees, since the crank turns
twice as fast.
Known good G and Ne signals are shown
in Figure 6. Channel 1 is Ne. Ne frequency
is approximately 12 times as fast as G.
This should make sense, since G
produces two signals to Ne's 24 signals
per crank angle sensor revolution. These
signals were captured at approximately
1,360 rpm.
Poorly Idling '88 RX-7 [go to top]
What do you think is going on in Figure 7? Something doesn't look right, does it?
When the inspection plate was removed on this crank angle sensor, I discovered
one chipped tooth on G and two chipped and slightly bent teeth on Ne. I'm not
sure how this happened, but I suspect someone was trying to adjust something
with a screwdriver. This pattern is from a 1988 non-turbo RX-7 with about 82,000
miles. It had a very poor idle, and the engine acted like it had a miss under load.
Both G and Ne are critical inputs; they should be checked if one suspects
primary problems or injector trigger problems.

6. Figure 8 compares a leading ignition primary to IgT. Notice when the 0-5 volts
IgT pulse goes low, primary dwell ends and the coil fires. For every IgT pulse
sent to the leading igniter, the coil will fire. This is a very good way to evaluate
the leading igniter's performance. (Compare input to output.)
By the way, the leading ignition system is critical for performance. You can
disable the trailing ignition system and the engine will start and run. As a matter
of fact, based on my testing, there won't be any performance concerns with it
disconnected. So, if you suspect an ignition issue, focus on the leading system.

7. But, since the
trailing system is
important for
emissions
reduction, it should
be tested for proper
performance. A few
paragraphs ago, I
briefly mentioned
the Select signal
used by the trailing
igniter. Fifteen crank
degrees after the
leading plug fires,
the trailing plug will
fire. How does this
igniter know which
trailing primary coil
to fire? There is only
one IgT signal for
two coils. The
Select signal sent
from the PCM to the
trailing igniter
determines which
primary (trailing plug
#1 or trailing plug
#2) will fire. Please
study Figures 9 and
10.
Figure 9 compares
the primary of
trailing coil #1 to the
0-5 volt Select
signal.
Figure 10 compares
the primary of
trailing coil #2 to the
0-5 volt Select
signal. What is
different between
these two?
Focus on the Select

8. signal. Trailing
primary #1 fires
when the Select
signal goes from 0
to 5 volts, while
trailing primary #2
fires when the
Select signal goes
from 5 to 0 volts. So
while there is only
one IgT signal for
both coils, the
trailing igniter knows
which coil to fire
based on the rising
or falling edge of
the Select signal.
Certainly, an
interesting and
unique approach.
Finally, the last trailing
igniter signal to
discuss is IgF. This is
a feedback signal from
the trailing igniter to
the PCM. IgF is only
produced when trailing
primary #l fires. There
must be actual primary
firing!
Figure 11 shows IgF
on channel 1 and
trailing primary #1 on
channel 2. Look okay?
Well, this about wraps
things up, or does it...?
Take a close look at
that primary pattern in
Figure 11. What is
going on in that spark
line? Figure 12 will
give you a better view.
Have you ever seen a

9. spark line look like
that? About 1 msec
into the spark line,
extreme turbulence
seems to occur,
lasting for about 0.8
msec, and the spark
line remains flat. Both
trailing primaries have
the same pattern. The
leading primaries look
normal. Overall engine
performance was very
poor. I have only seen
this pattern twice, and
both vehicles were
RX-7s. The cause:
retarded ignition timing
due to improper crank
angle sensor position.
Finding TDC [go to top]
Now this next discussion is a real diagnostic dilemma. Properly installing a crank
angle sensor is pretty straightforward. The eccentric shaft (crank) marks are first
lined up, and a mark on the crank angle sensor driven gear is aligned to the
crank angle sensor housing. The crank angle sensor is then slid into its bore in
the engine. Everything seemed to be okay. But with the engine running, idle
quality and performance could drastically be improved by advancing (turning
clockwise) the crank angle sensor. If I advanced the crank angle sensor's
position by one tooth, performance was better. A few phone calls to a tech line,
two Mazda experts and a little research said I needed to find TDC.
With a new TDC identified, I installed the crank angle sensor. Did it work? I'll let
Figure 13 answer that question. Figure 13 is the trailing primary #1 pattern. The
spark line is back to normal, as is engine performance. As I mentioned, I've run
into this situation twice. I'm still not sure what causes the timing to change in the
first place. It must be something in the engine, but what?
Well, I hope I've shed some light on these very interesting ignition systems. While
most of us may not see these vehicles regularly, it can be very helpful to have a
good understanding of how known good ones work. This knowledge can reduce
our diagnostic time drastically. Good luck in your own Diagnostic Dilemmas!
Here is the procedure I used:

10. 1. Remove both rear rotor spark plugs.
2. Rotate the crank until you can see an apex seal (tip of
the rotor) in the trailing spark hole. The trailing plug
hole is restricted, making the seal somewhat difficult
to see. Use a flexible light and a mirror. When the
seal is seen, mark the crank.
3. Rotate the crank until you see the same apex seal in
the leading spark plug hole. This plug hole is not
restricted, which makes it easy to see the seal. Mark
the crank.
4. Halfway between your two marks is TDC for the front
rotor (#1) rotor.