General Technologies TD101 SmarTach + D: Gas and Diesel Tachometer with Ignition Voltage Tester - Manual (Page 10)

Common Causes of Low Voltage Readings
Low voltage readings could be caused by any one of the fol-
lowing problems, or a combination of more than one of these
problems:
•
Shorted or dirty spark plug gap.
•
Spark plug gap too small.
•
Dirty spark plug insulator allowing spark “tracking” across
insulator .
•
Shorted spark-plug wire.
•
Broken core in spark plug wire. Break is located between
the distributor (or DIS coil) and the test point.
•
Short circuit, disconnection or partial disconnection
between the distributor (or DIS coil) and spark-plug wire .
•
Rich mixture. Most likely due to leaking fuel injector.
•
Low cylinder compression pressure. Could be caused by
leaking valve(s), worn piston rings, blown cylinder head
gasket, etc.
Common Causes of High Voltage Readings
High voltage readings could be caused by any one of the fol-
lowing problems, or a combination of more than one of these
problems:
•
Spark plug gap too wide.
•
Spark plug wire cap loose on spark plug terminal or totally
disconnected.
•
Broken core in spark plug wire. Break is located between
the plug and the test point.
•
Lean mixture. Most likely due to inoperative injector or
restriction in fuel injector.
•
Lean mixture due to vacuum leak.
Special considerations:
Carburetor Engines
Differences between cylinders due to rich or lean mixtures are
unlikely in single carburetor engines except for the possibility
of a lean mixture caused by a leaking gasket between the
cylinder head and the inlet manifold. However, in multi-carbure-
tor engines, an imbalance between carburetors could cause a
difference in readings between the groups of cylinders served
by each carburetor.
Spark-Plug Wires
Due to variations in insulation and core resistance of spark-
plug wires from different manufacturers the spark analyzer
cannot be used to accurately read the exact voltage in a spark-
plug wire. What it can do, is show by comparison, which wires
are operating at a higher or lower voltage than the remainder of
the wires in the engine. However, it can only do this when the
spark wires are identical. If an engine has been fitted with a
variety of dissimilar wires the usefulness of the spark analyzer
will be reduced.
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