GxT Ferret 54 ENGINE DIAGNOSTIC CENTER - Manual (Page 43)

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:
~ ELECTRONIC COMPRESSION
Cyl 1 3 7 2 6 5 4 8
% Max 97 98 98 100 96 92 98 99
% Low 3 2 2 0 4 8 2 1
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. When a leaky cylinder springs
back less than a sealed one, the cranking peak of the following cylinder will be more than it
would be otherwise. 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.
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