Maintaining Marine Ignition Systems 7-13 on page 113 show my favorite spark tester in use, but more on that later.
Engine misfires. Check the spark plugs and leads, rotor and distributor cap, and ignition coil for loose connections at the coil and ignition switch. Also check the engine firing order, plug wire routing, and the engine timing.
Engine fires when cranked but stops when key is released. Check the ignition switch and related wiring.
Engine cranks but doesn’t start. Check for spark at the plugs; check the coil and bypass circuit to the coil positive terminal; and check the wiring. Also check the engine timing and plug wire routing, and the tachometer and related wiring.
Some of the items mentioned here, such as engine timing and tachometer wiring, have not yet been discussed, but I will cover them in the following sections. This list is a guide for inboard and inboard/ outboard (IO) engines only. Use it for sorting out your thoughts as you approach a problem. You must work with the specific information for your ignition system if you expect to be successful as an ignitionsystem diagnostician. I will discuss two of the most common inboard engine electronic ignition systems, the MerCruiser Thunderbolt IV and Thunderbolt V. Other widely used systems with similar design and features are made by Prestolite and Delco. Use this chapter as a primer on electronic systems in general, and refer to the workshop manuals for the specific information you need to troubleshoot these other systems.
MerCruiser Thunderbolt IV and Thunderbolt V Systems With about 75 to 80 percent of the market at this writing, MerCruiser is the largest producer of gasolinefueled inboard engines in the world. The Thunderbolt series of ignition systems has been quite popular over the years, and for the last 15 years the Series IV and Series V systems have been the mainstay of the MerCruiser line. The Thunderbolt IV system
comes in two variations, one with a remotely mounted ignition module (located on the port side exhaust elbow) and the other with the ignition module mounted on the side of the distributor body. Repair procedures for both variations are the same. Ignition-module replacement will be slightly different. Service procedures for this system are not too difficult. Besides the generic procedures already mentioned for ignition systems, some specific system information follows.
Distributor-Cap Service The distributor cap needs to be kept clean and dry. MerCruiser recommends periodically removing the cap (annually will be fine) and giving it a thorough inspection. Be sure to mark the high-tension wires with tape and a marker before removing them from the cap so you can be certain they go back in the same order. Loosen the four screws that hold the cap in place and carefully lift the cap off its seat on the distributor. There should be a gasket between the cap and the edge of the distributor housing; be careful not to damage it. If it’s damaged in any way, it will have to be replaced. This gasket is an integral part of the ignition protection for the distributor. Look closely and observe the alignment tab molded into the cap and the corresponding indent on the body of the distributor housing. Upon reassembly, make sure this tab and groove are aligned; otherwise, damage to the distributor cap and rotor can occur the first time the engine turns over, regardless of whether it starts or not. Figure 7-5 on page 108 shows the cap orientation with the position of the tab indicated. Once you have the cap off, clean it thoroughly with warm soap and water and dry it (use compressed air if you have a supply). Next, carefully inspect the cap looking for excessive burning or corrosion. Check the center contact for deterioration. Minor corrosion, which is not unusual, can be removed with the tip of a straight-bladed screwdriver. Look for any signs of carbon tracking on both the inside and outside of the distributor cap. Carbon tracks will show up as random, fine lines etched into 107
