Ignition Control Module (ICM or Igniter) and Coil Replacement
Bench testing ICMs and coils is virtually useless. It does not come close to simulating the extreme heat that often causes these components to fail. I recommend replacing these critical and problematic components every 120,000 miles or 10 years, whichever comes first.
1. Disconnect negative battery cable, recording any radio codes first.
2. Remove hex-head machine screws (3), securing distributor cap to housing, using an 8mm nut driver.
3. Move distributor cap and wires off to the side.
4. Remove machine screw securing rotor to shaft, using a precision #2 Phillips head screwdriver. A 1/4-inch drive magnetic bit holder (e.g., Wiha 52650) with a #2 Phillips head bit works really well. It may be necessary to "hit" the starter once or twice, in order to rotate rotor for access to mounting screw.
5. Remove rotor and leak cover.
6. Unfasten ICM wires, remove coil mounting screws (on distributors with a coil inside), and set coil aside. Trick: Removing the coil first, on distributors with a coil inside, improves access to the ICM.
7. Remove both screws securing heat sink to housing, using a precision #2 Phillips head screwdriver.
8. Remove heat sink from distributor and unfasten both screws mounting ICM to heat sink, using a precision #2 Phillips head screwdriver.
9. Clean connections with #240 abrasive cloth or steel wool.
10. Coat the back of the new ICM (or old ICM, if reusing) with a thin, even coat of silicone heat transfer compound. This special silicone grease increases heat transfer to the heat sink. Failure to apply silicone heat transfer compound will cause the ICM to quickly fail. Arctic Silver 5 is recommended by an AMHD member, which is available at Radio Shack.
If the ICM and the heat sink are simply placed together, the small air gaps (insulator) that naturally exist between them will inhibit heat transfer. By filling these gaps, the compound allows a direct path through which heat can travel. Failure to apply this grease is one reason why some Hondas suddenly die. Attention to detail makes a big difference with modern day electronics.
11. Mount ICM to heat sink and reinstall ICM, ICM terminal wires, coil, coil wires, leak cover, rotor, and distributor cap. Ensure female ICM connectors fit snugly--crimp with pliers, if necessary.
Conceptually, the ICM is a large output transistor (switch) controlling current through the primary coil. ICMs replace points and condenser in older vehicles. In the primary coil, current builds to 5 to 12 amps, generating lots of heat (due to inductance). Then, the ECM controls when the ICM "switches off" current to the primary coil, causing amperage to go to zero. When this happens, 12 to 14 volts in the primary coil is "stepped-up" to 30,000 volts in the secondary coil. This process creates a "type" of alternating current required for the coil (step-up transformer) to work. In a properly working ICM, timing is precisely when needed to fire each spark plug.
Bottom Line: ICMs and coils take a beating.
Note: Distributor cap contacts can oxidize or corrode, creating high secondary resistance, which can result in rotor and ICM failure, reduced mileage, CEL coming on, catalytic converter failure, or failed emissions test. The secondary ignition system will destroy the ICM and coil, if you do not properly maintain the ignition system. This means replacing the distributor cap, rotor, and spark plug wires every 60,000 miles.
WARNING: Intermittent ICM failure can allow fuel to build-up in the exhaust system. Detonation can then blow the exhaust system off the vehicle.