Ford distributorless ignition system

Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. He began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.

The distributorless ignition system (DIS) was introduced in the 1980s to eliminate parts that affect emissions as they wear, but it also helped improve performance and fuel mileage. Today DIS has been phased out in favor of coil-on-plug (COP) ignition, but Ford was still producing engines with DIS as late as 2008, so you’ll be servicing these systems for many years to come.

Like COP ignition, a distributorless ignition system consists of ignition coils, the crankshaft and camshaft position sensors and the control circuits. Earlier systems have some kind of ignition control module that operates the coil packs and communicates with the engine’s powertrain control module (PCM), but later systems integrated all controls into the PCM. The most prominent feature of any distributorless ignition system is that each coil fires the spark plug for two different cylinders, both at the same time.

Of all the various ignition coil designs, a DIS coil is unique. To understand why, we need to review some basics first. You probably already know that a coil is actually two windings (coils) of wire.

The primary winding is a large-diameter wire wound around a core of ferrous metal, typically a stack of steel plates cut to a specific shape. Around the outside of that primary winding is the secondary winding. This is much finer wire with about 100 times the number of windings in the primary.

In other ignition coils, both windings are connected together at the power source (ignition +). The primary connects to ground through the module that operates the ignition system. The secondary connects to ground through the body of the spark plug, but only when the voltage is high enough to jump the gap to its ground electrode.

In a DIS coil, the secondary winding is not connected to the primary winding or to the power source. Both ends of the secondary winding are connected to a spark plug. To understand how current can flow through a circuit that’s not connected to a power source, we need to remember how a coil works.

The coil is a transformer

When a coil of wire is surrounded by a magnetic field that’s moving, voltage is generated (induced) in that wire. If the wire is part of a complete circuit, current will flow. An ignition coil creates a stationary magnetic field when current flows through the primary winding.

When the primary current is switched off, the magnetic field collapses (moves), and this induces voltage in the secondary winding. Since there are more turns of wire in the secondary winding, it acts as a transformer to boost the initial (battery) voltage up to 20,000 volts or more depending on the demand (we’ll get to that later).

That voltage can’t go anywhere unless there’s a complete circuit. In a normal coil the spark plug is the secondary winding’s ground and the other end of the winding is connected to the battery, but that’s just to make a complete circuit: The voltage is generated by induction.

In a DIS coil, both ends of the secondary winding are connected to a spark plug. When the voltage is high enough to jump both gaps, current will flow through that circuit.

[PAGEBREAK]

Current flows through a circuit from positive to negative. In a DIS circuit, that means the voltage jumping the gap of one spark plug will be flowing from the center electrode to the ground electrode, and on the companion spark plug it will be jumping from ground electrode to the center electrode. You can see this in high-mileage spark plugs: The center electrode of the “positive” plugs will be worn more than the center electrode of the “negative” plugs.

Since the coil fires two spark plugs at once, obviously they must be paired so that one fires during a cylinder’s compression stroke while its companion is on its exhaust stroke. The next time that coil fires, the cylinder strokes are reversed, but the direction of current flowing across the spark plug gaps remains the same. This is a function of the way the magnetic field collapses when primary current is switched off.

Ford’s better idea

Ford began using distributorless ignition in 1989 in a system they called “low data rate DIS.” This system uses Hall-effect sensors to detect crankshaft and camshaft positions. It was used on engines that were originally designed with a distributor, with the camshaft sensor mounted in the distributor’s original location.

In 1991 Ford introduced the electronic distributorless ignition system (EDIS). Instead of Hall-effect sensors, it uses a more reliable variable reluctance (VR) sensor to detect crankshaft position. There is also a cam position sensor but it’s used for fuel injection, not ignition. This was the basic design of all Ford distributorless ignition systems before going to COP ignition.

The EDIS consists of an ignition control module (ICM), the powertrain control module (PCM), a crankshaft position sensor (CKP) and the ignition coil packs. The ICM operates the coils but only as commanded by the PCM. However, under certain conditions the ICM will operate the coils by itself. We’ll get to that later.

The crankshaft sensor on the front of the engine is similar to other passive speed sensors: A two-wire permanent magnet sensor is mounted above a 36-tooth (minus 1) tone wheel, and as each tooth rotates past the sensor it generates a sine wave or alternating-current (AC) signal. The missing tooth indicates a specific number of crankshaft degrees BTDC of cylinder number one. The sensor’s peak-to-peak voltage signal increases exponentially with speed.

The ICM reads crankshaft speed and position and reports a profile ignition pickup (PIP) signal to the PCM (Ford’s EEC-IV engine control unit). With the PIP signal and information from the engine’s other sensors, the PCM decides the timing and firing sequence of each ignition coil and issues spark angle pulse width commands (SAPW, later called the “spark angle word” or “SAW” signal) to the ICM, which then turns off the primary current to the coil to fire the spark plug.

The ICM controls primary current level and dwell (the number of crankshaft degrees the primary circuit is turned on). After turning off the primary current, it looks for the “flyback” voltage spike that indicates the magnetic field has actually collapsed as commanded. This is called the ignition diagnostic monitor (IDM), and in addition to diagnostics, it’s also used to generate a clean tachometer output (CTO) signal. If the flyback voltage is not detected, it will notify the PCM, which will then set the appropriate ignition misfire fault codes (P0300, P0301, etc).

As noted earlier, the PCM commands the ICM to fire the coils, but it’s the ICM that controls primary current. Depending on the engine, primary current is limited to 6.0 to 6.5 amps. If the SAPW signal from the PCM is lost, the ICM can operate the coils by itself using just the crankshaft position sensor. In this case, ignition timing is fixed at 10 degrees BTDC.

On most models, if engine speed is less than 1,800 rpm, the PCM will command the spark plugs to fire two or three times during each firing event. This multi-strike feature improves idle quality, but at higher speeds there isn’t enough dwell time to reliably generate multiple sparks.

The ignition coils are made in four-cylinder and six-cylinder packages, and some include the ICM. Resistance on the primary circuit will range from 0.5 ohms to about 2.0 ohms. Secondary resistance will vary significantly from one model to the next, but it can be upwards of 16,000 thousand ohms. When checking coil resistance, check the specs and pin location diagrams carefully; if primary resistance is off by even one-tenth of an ohm, the engine may not start.

In 1996, Ford introduced the integrated electronic distributorless ignition system, and it remained in production through 2008. As the name implies, it’s exactly the same as the EDIS but with the Ignition Control Module built into the PCM. That means the crankshaft sensor connects directly to the PCM, and a lot of other wiring has been eliminated. While this reduces cost and complexity, the coil drivers are now inside the PCM, so the PCM must be protected against the coil primary flyback voltage (up to 400 volts). It also means that dealing with a failed coil driver requires replacing the whole powertrain control module.

[PAGEBREAK]

Known failure modes

Probably the most expensive failure that can happen in a DIS system is when high voltage from the coil’s secondary winding finds a path to ground through its primary winding. This can damage or destroy the coil drivers in the ICM or the PCM. It’s not difficult to create this kind of failure, but fortunately it’s also not difficult to avoid, because most DIS failures are caused by lack of proper maintenance.

Worn spark plugs are the obvious place to start. The voltage required to jump the spark plug gap peaks just before current begins to flow, and the size of that gap has a major influence on the resistance in the secondary circuit. Higher resistance creates a higher voltage peak, often called the “kV demand.” Resistance is created by the spark plug wire, the spark plug itself, cylinder compression (it takes less voltage to jump the gap in the companion cylinder on its exhaust stroke) and by the size of the spark plug gap.

Spark plugs ware because a few molecules of metal are displaced with each spark, departing from the positive electrode into the plasma (the spark itself) and finally leaving with the exhaust gasses. Ware can be slowed by a “button” of precious metal attached to the positive electrode, either platinum or iridium.

When installing such spark plugs, make sure they have the precious metal on both electrodes: Remember, current flows “backwards” through half of the spark plugs, from the ground electrode to the center electrode.

As the spark plugs wear and the gap grows wider, resistance increases and higher voltage is required to jump the gap. Some coils are capable of generating 80,000 volts or more, and that voltage will find a path to ground somewhere. The coil failure noted above is one possibility, but there are others. Here’s one that seems to be unique to DIS.

The photos show two spark plugs from the same engine that suffered a steady misfire when the car was climbing a hill. One spark plug has a carbon track showing the path the voltage took to ground when load (and therefore kV demand) increased. New spark plugs solved the problem, but only for a few days.

When the car came back, the plugs were removed and the same carbon track was noted on the new spark plug from the same cylinder. That’s because there was a matching carbon track in the spark plug boot. A new spark plug wire solved the problem.

Another picture shows where voltage was leaking to ground through the wall of a hard plastic boot that fits into a deep spark plug well. Early DIS used hard boots but when Ford switched to all rubber boots, this problem became almost non-existent.

As noted earlier, there is a Camshaft Position Sensor (CMP) on these engines but it’s not used by the ignition system. That sensor is only used by the PCM for controlling sequential fuel injection. The engine will start and/or continue running if that signal is lost (P0340), but it will not affect the ignition system.

If the CMP signal is good but not synchronized with the crankshaft signal, there will be no codes and the engine will still run, but not smoothly. This is one of the more difficult driveability problems on a Ford because both the fuel injection and ignition systems are working properly.

There is only one moving part in Ford’s DIS; the tone wheel for the CKP sensor. If a properly installed CKP sensor shows signs of contact with that wheel, there’s a mechanical problem that needs to be fixed first. On engines that use a Woodruff key to locate the crankshaft balancer, if the bolt isn’t torqued properly the keyway can become distorted, allowing the balancer/tone wheel to move on the crankshaft. This will retard the ignition timing and cause low power, and the problem may be intermittent as the balancer moves back and forth on the crankshaft.

Also, if a tooth is broken off from the tone ring, there will be two missing teeth instead of one. Depending on where the tooth is missing, this can cause no-starts, backfiring and other problems. Neither of these mechanical problems will set a fault code. When installing the CKP sensor, use a feeler gauge to make sure the gap is correct.

You can test the coil drivers in the ICM or PCM by connecting a noid light, test light or PowerProbe between the coil primary and the battery and cranking the engine with the fuel system disabled. The light should flash as the driver grounds the circuit. If it stays on or doesn’t light at all, there’s a bad driver or bad circuit between the coil and control unit. Be careful with this test: Accidentally shorting the primary side of a coil pack to ground will destroy the winding in short order.

And finally, if you find one bad part in the system, consider how it has made all the other parts work harder, especially in the secondary ignition circuit. Don’t be afraid to spend the time to examine everything. ●

Crankshaft sensor specifications