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SKU 39224 Page 4
Operation
A compression tester can give you valuable information about the engine’s top end
(pistons, rings, valves and gaskets).
If you are not experienced in doing this type of diagnostic test, the work should
be done by a qualified technician.
NOTE: The engine must be at normal operating temperature, and the battery must be
at full charge to develop correct readings. WARNING: Wear protective gloves and
ANSI approved eye protection when working on a hot engine. Keep hands away from
the fan and other moving parts, and protect yourself from electrical shock or burns.
Be aware that working on a gasoline engine is inherently dangerous, and suitable
precautions must be taken.
1. Clean the area around the spark plugs using compressed air. It is important to
prevent foreign materials from falling into the cylinders once the spark plugs are
removed.
2. On engines with a single plug per cylinder, remove all spark plugs. Note the
position of the wires so that the correct wire may be replaced on each plug at the
end of the job. On engines with two spark plugs per cylinder, remove only one
plug per cylinder.
3. Block the throttle linkage in the wide open position.
4. Disable the ignition system. On distributor ignitions, remove the coil wire from
the distributor. On electronic ignitions, remove the connector from the coil pack.
5. For fuel injected vehicles, or vehicles with an electric fuel pump, disable the fuel
pump circuit. This may easily be done by removing the fuse for the fuel pump.
6. Install the compression gauge in the number one spark plug hole. You may want
to use any of the supplied adapters. The gauge attaches and removes using a
quick-release chuck. Be sure the installation is secure and pressure tight.
Unpacking
When unpacking, check to make sure the following parts are included.
Compression Gauge with Quick Release Chuck Molded Plastic Case
14” Vinyl Hose Extension Aluminum Tube
Extension Thread Head Angled Extension Aluminum Tube
If any parts are missing or broken, please call Harbor Freight Tools at the number
on the cover of this manual.