GxT Ferret 63 DIAGNOSTIC ENGINE ANALYZER - Manual (Page 26)

“CRANKING ANALYSIS” Key
ELECTRONIC COMPRESSION
The cranking-amps-per-cylinder readings of this test give
an indication of engine cylinder compression uniformity.
This is possible because starter motor amperage is
related to the cranking torque, and the cranking torque
varies as cylinders go into compression and then spring
back .
If all peak amps readings from this test are within a 5%
range, compression is probably within the typical speci-
fication of 75% of the highest gauge reading. If at least
one amps reading is more than 8% lower than the high-
est, it is likely that there is a problem.
When a leaky cylinder springs back less than a sealed
one, the cranking amps of the following cylinder will read
higher than it would otherwise. The test is pretty good for
catching 20% to 70% compression cylinders, but totally
dead cylinders may not show well because they have
neither compression or springback torque. Totally dead
cylinders will usually be apparent by the engine sound
anyway.
Engines with 4 cylinders have a good readings-to-cylinder
correspondence. Six cylinder engines have a readings
overlapping effect, and 8's overlap so much that the low-
est amps reading may correspond to bad compression in
the preceding cylinder. This can make the interpretation
of these readings complicated.
The way to interpret this test is to first look for a uniform
set of readings which indicates that compression is not
a likely engine problem. Then look for at least one cylin-
der which is notably low, thus suggesting doing a gauge
compression test so that the actual pressures/cylinder
are measured before extensive work is prescribed. In
between these decisions is the gray area where the
decision to go to the gauge test must be made with the
weight from other engine symptoms such as correspond-
ing weak performance in the Power Balance test, an
indication of lean burn for a cylinder in the Spark Burn
test, poor emissions output, a noisy valve, a popping or
spitty exhaust pulsation, a steady jerking as engine load
increases, or other mechanical signs.
The Cranking Peak-Amps per cylinder test procedure for
a standard engine is as follows in the left-hand column.
When there are test complications, variations may appear
which are described in the right-hand column.
STEP ONE
Press CRANKING ANALYSIS to begin.
Using SELECT, there is a choice of "Cranking History"
or running only the "Compression" test.
SELECT "Compression" test.
Press START.
The following display will appear:
Press START.
Close Doors, then START.
-24-
AVERAGE
AMPS
CYLINDER
SEQUENCE
PEAK
AMPS
1 2 3 4
190 165 200 190
LOW
CRANKING PEAK AMPS WAVEFORM
WARNING
THE ENGINE MAY START DURING THE TEST.
SELECT: Compression
TEST then START
CRANKING ANALYSIS
Engine Warm, Brakes Set.
Switch ACCESSORIES off!
Close Doors, then START