Solving the mysteries of Chrysler ignitions

Motor, May 2001 by Masterman, Jeff

Although not radically different from other designs, Chrysler's newer ignition systems can throw you one nasty diagnostic curve if you're not up on some of their idiosyncrasies.

Chrysler ignition systems are different enough from others out there to seem almost mysterious to the average tech working in a multibrand shop. Because of this, some common failures tend to result in lots of wasted diagnostic time. This article will focus on the design of Chrysler's newer systems and give you tips on how to solve some common problems. Before we begin, note that a late-model Chrysler could have one of three types of ignition systems-DI (conventional distributor), El (no distributor, with coil packs) or EI-COP (no distributor, with individual coils).

All of these systems have some commonality. For instance, Chrysler uses a transmission-mounted crankshaft position sensor (CKP) for the ignition trigger signal as well as misfire detection. A part called an ASD Relay provides power for the coil and other essential circuits. This relay is activated only for a couple of seconds when the key is first turned on and remains on as long as the PCM receives a crank signal. On all domestic production Chrysler engines, the ignition primary circuit is switched by the PCM directly; there is no "module."

The location of the cam position sensor (CMP) varies with the type of engine and ignition system. On DI systems, the cam sensor is integrated into the distributor; on other applications, it's on the front of the engine and reads off the cam sprocket.

The PCM sends approximately 5 volts to a Hall effect sensor, which is required to operate the Hall effect chip and the electronics inside the cam sensor. The ground for the sensor is provided through its return circuit.

The input to the PCM occurs on a 5-volt output reference circuit. The CMP sensor operates in the same manner as the CKP sensor, except that with the CMP sensor, there are only two edges to detect. The PCM identifies camshaft position by registering the change from 5 to 0 volts or 0 to 5 volts, as signaled by the cam sensor.

Coll-On-Plug Ignitions

In Chrysler's coil-on-plug ignitions, there's one coil for each cylinder, and it's attached to the engine directly above the spark plug. A rubber boot contains a stainless-steel spring, which serves as the secondary conductor between the coil and the plug.

The coils are powered by the ASD Relay when the key is first turned on and the PCM is receiving a crank signal. The ground side is switched by the PCM. There's also a capacitor on each bank of the engine for RFI suppression. Note that each coil fires only once per ignition event, unlike DIS waste-spark systems, which fire twice.

The PCM grounds the proper coil based on signals from the CKP and CMP sensors. When spark is required, the PCM opens the ground circuit, causing the coil's magnetic field to collapse and generate high secondary voltage. The PCM also monitors the primary circuit of each coil and will set a DTC if there's a circuit fault.

Misfire Monitor

Misfire is detected by analyzing crankshaft speed fluctuations. The crank sensor detects engine speed, and an rpm value is established for each cylinder for comparison. When a sufficient difference in rpm occurs among cylinders, a misfire count is recorded. The rpm varies slightly because of different input variables.

Two types of misfires can be detected. A two-trip misfire, which occurs about 2% of the time, will result in an emissions increase. A one-trip misfire, also called a catalyst-damaging misfire, occurs in about 10% to 20% of the events.

To detect a misfire, the PCM must first learn the spacing of the crank target windows. This is done when the engine is in the Deceleration Fuel Shutoff mode. To learn the crank target spacing, three closed-throttle decelerations from 55 mph to rest must be performed. The misfire monitor will not be enabled until this spacing is learned.

The PCM also checks the machining tolerances within each group of slots on the flywheel. The PCM calculates the variation among each group of slots. This calculation is called rpm error. The variation in rpm readings between cylinder pairs must be less than 5% in order to run the misfire monitor.

Once the PCM has detected a misfire for two consecutive trips, the MIL will be continuously illuminated and a permanent DTC will store. If the PCM detects a catalyst-damaging misfire, the MIL will flash. If the active misfire ends, the MIL will change to continuous illumination. The MIL will remain on for more than one trip, but will go out if the conditions that set the DTC are not found on subsequent trips.

Common Problem Areas

The following problems are commonly found on Chrysler ignition systems, but there are ways to solve them:

ASp Relay. When you perform a diagnosis for a no-start condition on a Chrysler vehicle, remember that the ASD Relay will not activate without a crank signal. Many techs waste time trying to figure out why there's no power at the injectors, coils and other components without manually activating the relay. The relay can be activated using a scan tool and Chrysler's Actuator Test Mode (ATM). In fact, a scan tool also can be used to verify the system's ability to produce spark.