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.

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 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.

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.

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.

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.

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 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 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!

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