Unison magneto troubleshooting

Unison Magneto Troubleshooting By Harry Fenton February 1999 Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12...


The E-gap gauge can also be used on older rotors, but some of the visual prompts are different. Older rotors feature a cutout area between the lamination stacks in the rotor head. The flat end of the tool is inserted against the edge of the lamination stack and the rotor is turned to wedge the tool against the left side of the frame for left rotation magnetos and against the right side of the frame for right rotation magnetos. With the tool in place, the contact points are adjusted and checked for go/no go condition using the method described above (Figure 15).

Ignition Harness
Ignition harness issues usually relate to left and right orientation, the size of the nut that fits the sparkplug and the overall fit of the harness to the engine.

First, the reference point for left and right is always from the pilot's perspective, so the left and right portions of the ignition harness should be installed accordingly. Most Lycoming engines and the TCM O-200, O-300, E-series, and IO-360 series are all configured with magnetos that attach to the rear of the accessory case. The magnetos for the TCM 470, 520, and 550 series engines are mounted on the top of the engine facing forward. The pilot's perspective rule still applies, so route the ignition harness accordingly.

Unison Magneto Troubleshooting

By Harry Fenton

February 1999

The Slick ignition harness spark plug nuts are permanently labeled for position on the engine, e.g., the nut marked "T1" routes to the top spark plug in cylinder number one.

Frequently, the installing mechanic will use the previous harness as a guide to install the replacement harness and finds that the new harness does not fit or route like the one removed. The common assumption is that the new harness was manufactured incorrectly.

But, are the old and new harnesses identical in the way that they are laid out for routing? Engine and airframe manufacturers have specified different routing schemes to address rough running, cable routing, or other issues. In fact, Lycoming issued Service Instruction 1294 to detail alternate ignition cable routing for their four cylinder engines. Ultimately, Slick manufactures ignition harnesses to the engine manufacturer's specifications, so the engine OEM has determined the best lead lengths and routing for the leads. The bottom line: Route the replacement Slick harness as it is marked for left and right location and spark plug placement.

Also, be careful of using the cylinder numbers embossed on the case of the engine for lead placement. Sometimes the baffling can obscure an embossed number causing the ignition lead to be installed on the wrong cylinder. The TCM O-200 is a classic example of an installation that suffers a disproportionately high number of problems due to the placement of the embossed number on the engine case (Figure 16).

Choosing between an ignition harness configured for 5/8-24 or 3/4-20 sparkplugs can cause some confusion, too. Slick spark plug insulators provide the easiest clue: green insulators denote 5/8" plugs and red insulators are used on 3/4" plugs (Figure 17).

There are also some general rules associated with spark plug configuration: a 3/4" wrench fits 5/8"-24 ignition lead nuts, and a 7/8" wrench issued to remove 3/4"-20 nuts. If Champion spark plugs are used, the part number embossed on the spark plug will begin with REM for a 5/8' plug, and RH for a 3/4" plug. Auburn used SR to denote 5/8" and HS to denote 3/4" plugs.

Most Lycoming four and six cylinder engines leave the factory with 5/8" plugs, and the turbo'd engines are configured with 3/4" plugs. The TCM O-200, O-300, and GO-300 engines typically use the 5/8" plug. All TCM 360, 520 and 550 series engines use the 3/4" plug. The wild card is with the 470 and E-series engines. There seems to be a 50-50 chance between the two plug types.

If possible, always visually check the engine for plug type as virtually all piston aircraft engines are approved for either 5/8 or 3/4 plugs.

Documentation
Many customers call for information or clarification of documentation issues.

The applicability and approval documentation is the most frequent issue beyond the topics of mechanical troubleshooting. One of the grand paradoxes of the aviation industry is that the FAA demands that aircraft comply with the type certificate, but sometimes the information available in the Type Certificate Data Sheet does not reflect currently manufactured products.

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