GxT V055-01 Diagnostic Analyzer - Manual (Page 13)

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If a DIS ignition was selected during Engine Setup, the
EngineLink will prompt you to disable the fuel supply to
prevent the engine from starting during cranking. This
is usually done by unplugging the power wire or fuse to
the fuel injectors or fuel relay. Many late model vehicles
will disable the fuel if wide-open throttle is applied before
cranking.
The engine may start unexpectedly, so be prepared to release the key and the pedal.
On a distributor engine, the EngineLink will suppress the ignition during cranking.
Ignition power must be kept on so the analyzer will sense each cylinder’s ignition pulse. Keep the vehicle’s
doors closed during the test.
Crank the engine. If the engine fires during cranking, the data will be invalid. Holding the engine intake
manifold throttle wide open will help prevent some engines from firing. Data saving will begin when cranking
is sensed.
On a distributor engine, when the ignition has been suppressed, the engine will start when the Diagnostic
Center releases the ignition suppression at the end of the cranking period, otherwise the STOP CRANKING
message will appear. On a DIS engine, or if you were prompted to disable fuel on a distributor engine, a
message will appear reminding you to restore the fuel supply.
Then the results of the test will appear.
The % Max row shows the amount of cranking draw required to pull each cylinder through compression,
as a percent of the largest cylinder’s draw. One (or more) cylinders will show 100%, and the rest should
be within a few points of 100%. The % Low row shows how much below 100% each cylinder was.
If all readings from this test are within a 5% range, compression is probably within the typical gauge
specification of 75% of the highest pressure reading. If at least one reading is more than 7% lower than the
highest (% Low is greater than 7%), it is likely that there is a problem. In that case, a message may suggest
a gauge pressure test. Four-cylinder engines have good correspondence between display readings and
cylinder performance, while six cylinder engines may have overlapping readings. The readings on eight
cylinder engines overlap enough that the lowest peak reading may actually correspond to the preceding
cylinder.
Compare the results with those obtained in the Power Contribution, Power Balance and Ignition Secondary
tests. Those results, along with other symptoms, may suggest a gauge compression test to determine the
actual pressures/cylinder before extensive work is prescribed.
Note that temperature and friction of the engine affect the electrical draw. A very hot or cold engine,
or one that has new piston rings and bearings may draw higher than normal, thus tending to cover up
compression variations.
Cranking Draw readings give an indication of engine cylinder compression uniformity. This is possible
because starter battery draw is related to the cranking torque, and the cranking torque varies as cylinders
go into compression and then spring back.
On double-end DIS ignitions, it is likely that the message "Results Not in Sync" will appear. The tests
results will be accurate, but may not start with Cylinder #1. If readings are uniform, sync will not matter.
If one or more cylinders are low, a gauge test will be recommended to verify the actual compression in
each cylinder.
Redo the test when in doubt about the test results.
Electronic Compression