3 DIAGNOSTIC SOFTWARE OPERATION 91
Relative Compression
NOTE: During this test the IDM shuts off the injectors
so no fueling occ urs.
NOTE: The Relative Compression test can only be
done with the EST; MasterDiagnostics® software is
required.
NOTE: This test is used in conjun ction with the
Injector Disable test to distinguish between an injector
problem o r a mechanical problem.
The Relative Compression test provides the
difference between the fastest and slowest crankshaft
speed during the power stroke of each cylinder.
As the engine is cranked, the IDM uses the CMP and
CKP sensor signals to measure crankshaft s peed,
as piston reaches two points: Top Dead Center
(TDC) compression and about 30 degrees after TDC
compression.
When the piston approaches TDC, crankshaft speed
should be slower because of compression resistance.
As the piston passes TDC, compression resistance
dissipates and crankshaft speed increases.
At TDC compression, the cylinder reaches its highest
compression and resistance to crankshaft rotation -
Crankshaft spe ed is the slowest. A cylinder with low
compression will have less resistance to crankshaft
rotation. Crankshaft speed w ill be faster than normal.
About 30 degrees after TDC, crankshaft speed should
be fastest because compression has dissipated. On a
cylinder that has low compression, crankshaft speed
will be close to, or less than crankshaft speed at TDC.
At TDC of each power cylinder, and about 30 degrees
past TDC, the IDM collects data for crankshaft speed.
NOTE: If not cranked long enough to collect data, the
EST will display 255. 255 represents an erroneous
rpm value
The TDC value is subtracted from the value about 30
degrees after TDC and is recorded for each cylinder.
Example
200 rpm (30 degrees after TDC) - 180 rpm (TDC) = 20
rpm
The EZ-Tech® will display a value o n the screen for
each cylinder.
Example
Compare the compression values of each cylinder
with the other cylinder values. A cylinder with
compression lower than the other cylinders indicates
a suspect cylinder. Test value of 18 for c ylinder one
indicates a suspect cylinder.
If a cylinder value is zero or a much lower than
other cylinde rs and this cylinder is a no n-contributor
(identified in the Injector Disable Test), check for a
mechanical problem.
Example
If TDC rpm is greater than rpm 30 degrees after TDC,
the EST will disp lay 0.
If the test value for a power cylin der is 0, the cylinder
is suspect.
If the test value for a power cylinder is significantly
below 15 rpm, the cylinder is suspect.
Test value 5 for cylinder 1 indicates a suspect cylinder.
Test value 0 for cylinder 6 indicates a suspect cylinder.
When the Relative C ompression test is done, the EST
indicates, stop cranking the engine, and will display
test values.
Test data displayed in this test should be compared
with data collected from the Injector Disable test.
EGES-270-1
Read all safety instructions in the "Safety Information" section of this manual before doing any procedures.
Follow all warnings, cautions, and notes.
© August 2008 Navistar, Inc.
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