GxT Ferret 54 ENGINE DIAGNOSTIC CENTER - Manual (Page 35)

DRIVER MODULE / POINTS VOLTAGE
“Driver” is the voltage during the dwell time.
For mechanical points it should be less than
0.3 volts. Electronic modules typically have
0.5 to 1.5 volts. Ignitions made alike should
compare within 0.2 volts. In this test, a higher
voltage indicates a failing (high resistance)
coil driver output transistor, bad points, or a
high resistance connection to ground.
DWELL
Press SELECT to display dwell in degrees,
percent, or milliSeconds. Readings in
degrees and percent are useful for setting
mechanical points. (If dwell is displayed in
degrees, it is scaled according to the number
of cylinders selected in
Engine Setup .)
MilliSeconds are useful for checking the
charging time for current-limited electronic
ignitions, which take a fixed time to build up
the coil amps. A typical HEI coil charges in
3.5 mS at idle RPM, and 4.5 mS while
cranking.
This Diagnostic Center measures dwell
according to the interval that the points are
closed, or the transistor driver is fully on. The
coil amps hold time that sometimes follows
amps buildup is excluded in these readings.
VARIATION
Variation gauges the engine’s control stability.
Dwell Variation
This is the difference in degrees between the
longest and shortest dwell period. On
electronic ignitions, it could be from an
unstable coil drive module. On point contact
ignitions, mechanical sloppiness in the
distributor shaft bearings and cam shaft drive
is the prime cause. At higher speeds, on
point contact ignitions, the problem can be
floating contacts due to weak springs and
poor lubrication. Readings should be less
than 3 degrees.
Timing Variation
Variations in timing are associated with
fluctuations in engine speed. This is the
difference in engine shaft degrees of the
longest and the shortest times between
ignition firings. On pre-computer engines,
variation meant looseness in the mechanical
drive from the crankshaft to the points cam.
This is still generally true for distributor
located ignition sensors, but sometimes
timing is intentionally varied by engine
computers.
Irregular idle RPM from O2 feedback fuel
metering can cause timing variation also.
Operating the engine at higher RPM should
reduce the variation to under 3 degrees. Be
alert to erratic timing variation, which points
to sloppiness in the gears, bearings and
chains leading to the ignition timing sensor.
Confirm timing variation with a Timing Light
triggered from the coil-distributor wire by
watching the paired marks.
Read variation from 1000 to 3000 RPM with
the RPM held steady. During acceleration or
deceleration, the Timing Variation is not a
valid measurement.
NOTE:
GM made some six cylinder engines built
from a shortened 8 cylinder block that have
a normally high variation since firing occurs
on an 8 cylinder pattern with 2 missing
cylinders.
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