248 6 PERFORMANCE DIAGNOSTICS
12. Relative Com pression
Figure 322
Purpose
To determine if compression is too low in any cylinder
NOTE: During this test the IDM shut s off the injectors
so no fueling occurs.
NOTE: ThistestcanonlybedonewiththeEST;
MasterDiagnostics® software is required.
NOTE: This test is used in con jun ction with the
Injector Disable Test to distinguish between an
injector problem or 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 cam and
crank sensor signals to measure crankshaft speed,
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 c rankshaft rotation —
Crankshaft speed is the slowest. A cylinder w ith low
compression will have less resistance to crankshaft
rotation. Crankshaft speed will 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 recorded for each cylinder.
Example: 200 rpm (30 degrees after TDC) - 180 rpm
(TDC) = 20 rpm
The EST will display a value on the screen for each
cylinder, as typified by the following example.
Figure 323
Compare the compression values of each cylinder
with the o ther cylinder values. A cylinder with
compression lower than the other cylinders indicates
a suspect cylinder. Test value of 18 for cylinder one
indicates a suspect cylinder.
If a cylinder value is zero or a much lower than
other cylinders and this cylinder is a non-contributor
(identified in the Injector Disable Test), check f or a
mechanical problem.
Example
EGES-270-1
Read all safety instructions in the "Safety Information" section of this manual before doing any proced ures.
Follow all warnings, cautions, and notes.
©August 2008 Navistar, Inc.
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