Chapter 7: Ignition System; Troubleshooting The Ignition System - MTD P90 series Shop Manual

Troubleshooting the ignition system

Spark tester
Figure 7.1
NOTE: It only takes 1,000 volts to jump a 0.025" air gap in open atmosphere, it takes 10,000 volts to jump the
same gap at 120 psi; therefore, an open air spark test is not valid.
NOTE: The spark should be a minimum of 10 Kv (10,000 volts) at pull over speed.
3. Move the throttle lever to the full throttle position.
4. Turn the ignition key to the start position. If sparks can be seen in the spark tester, the ignition system is work-
ing.
NOTE: If there are sparks present in the spark tester, install a known-good spark plug and prime test the
engine. If the engine does not start, check the fly wheel key. If the fly wheel key is intact, the problem is
not in the ignition system. Check the engine's compression.
5. If no sparks or weak sparks are seen in the spark tester, check the module air gap.
NOTE: If the module air gap is correct, further testing is required.
6. Unplug the wire that connects the ignition module primary windings to a ground for engine shut down (the con-
nector is behind the dipstick tube).
7. Re-test for sparks.
• If there are no sparks, the module is bad or the flywheel magnets have lost their magnetism (very rare).
• If there are sparks, troubleshoot the electrical system of the equipment that the engine is mounted to.

CHAPTER 7: IGNITION SYSTEM

The purpose of the ignition system is to provide a spark
in the combustion chamber at the proper time to efficiently
ignite the fuel/air mixture. The steps in troubleshooting the
ignition system are:
1.
NOTE: It is convenient to check the compression when the
2.
NOTE: Do not connect the spark tester to the spark plug
! CAUTION ! CAUTION
Examine the spark plug(s) by following the steps
described in the spark plug section of this chapter.
spark plug is removed for examination.
Connect a spark tester between the spark plug wire
and a good ground point on the engine.
See Figure 7.1.
when testing for ignition.
Never remove the spark plug and
hold it against the cylinder head to
test for spark. The fuel/air mix com-
ing out of the spark plug hole will catch on fire.
Ignition System
73