Any time you perform a compression test it is important to disable both the Fuel and Ignition systems. In this class you will also want to disable only the fuel system to observe cranking vacuum and cranking ignition.
To disable the fuel system you can turn off the fuel pump, the fuel injectors, or sometimes the entire engine control computer. Vehicles have many different controls for the fuel pump or injectors. On some you can pull a fuse, on others it is easier to disconnect a relay, or you might simply unplug a wiring harness connector. Disconnect any fuse to the engine control computer as a last resort and be sure to check for codes first. Unhooking this fuse may cause the computer to lose diagnostic codes and data.
After disabling the fuel, it is important to verify the fuel is truly shut off. Try starting the engine. If you turned off the fuel pump there is often residual fuel pressure that may cause the engine to briefly run even though the fuel pump is turned off. Simply allow the engine to run until it dies. If the engine runs for more than a few seconds, the fuel pump is still operating and you must find another way to disable the engine.
When performing a compression test you MUST disable the ignition.
CAUTION: If you unhook a spark plug or coil wire without disabling the ignition and crank the engine, you can damage the ignition coil.
If you accidentally damage this coil the vehicle may run fine in the repair bay, but misfire when the engine is under hard acceleration, or driving up a steep hill.
The best way to disable an ignition system is to unhook the primary circuit. This may be a simple as removing a fuse, or you can find the ignition coil primary wires and disconnect them.
All ignition coils have a primary and secondary circuit. The primary uses 12 to 15 volts with normal vehicle wiring. The secondary ignition will create over 40,000 volts (40kV) and has very thick insulation on the wires. These are known as spark plug, or coil wires.
To verify the ignition system is disabled, unhook only one spark plug wire and connect a spark tester to that plug wire and a good engine ground. Then crank the engine. If you see sparks at the tester, or the engine starts, your ignition system is not disabled.
To find out what is the easiest way to disable a fuel or ignition system it helps to have a wiring diagram. When looking for a wiring diagram showing both the fuel and ignition systems with ShopKey5, look under Wiring Diagrams>>SYSTEM WIRING DIAGRAMS>>ENGINE PERFORMANCE you may need to also look under Engine Performance>>Wiring Diagrams - Engine Performance.
Once you have an engine performance wiring diagram, look for a fuse, relay, or connector that goes to the fuel pump, the fuel injectors, or the ignition coil.
Graphical version of Phil Krolick's Website: cf.linnbenton.edu/eit/auto/krolicp/