678 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. 1500 rpm. Connect a spark tester (wide gap) on the ignition coil output. With a coil pack, you will have to test each coil’s output voltage separately. Just because one coil passed its tests does not mean the others will. By using the coil pack fi ring order and the secondary pattern, you can tell which coil should be tested. With the spark plug tester in place and grounded, a tall fi ring line should stand out from the others. Look at the scope scale on the side of the screen. Read the voltage that is even with the top of the spike. Th is value should equal the safe voltage capacity of the ignition coil. Caution: A few electronic ignitions may be damaged by disconnecting spark plug wires while the engine is running. Be sure to check manufacturer’s directions. With older electronic ignitions, coil output voltage should range between 30,000 volts to 45,000 volts. However, some electronic ignition coils are able to produce up to 100,000 volts. Warning: Even though ignition coil or coil pack current is too low to normally cause electrocution, the high voltage could injure you or cause a potentially deadly heart attack. If the ignition coil voltage is below specifi cations, do not condemn the coil until completing further tests. Low coil output could be due to low primary supply voltage, leak- ing secondary wires, or similar problems. Eliminate these as sources of the problem before replacing the ignition coil. Load Test A load test, or acceleration test, measures the spark plug fi ring voltages when engine speed is rapidly increased. When an engine is accelerated, higher voltage is needed to fi re the spark plugs. While a defective component may not produce an abnormal scope pattern at idle, it may not oper- ate properly under load. To perform a load test, set the scope on parade and idle the engine between 1000 to 1200 rpm. While watching the fi ring lines on the scope, quickly open the engine throttle and release it. Th e fi ring voltage should increase, but it must not exceed certain limits. The highest firing line should not be more than 75% of actual coil output. Typically, voltage should not exceed 20 kV in an electronic ignition. The upward movement of the firing lines during the load test should be the same. If any of the firing lines are high or low, a defect is present. Fuel Injector Scope Patterns Th e two common types of electronic fuel injector cir- cuits are saturated switch and peak-and-hold. Other elec- tronic fuel injection circuits are the PNP fuel injector driver and the Bosch peak-and-hold injector. Each produces it own unique scope waveform which can be analyzed. Caution: Never use jumpers to apply full battery voltage to an injector winding. Excess current flow will quickly overheat and ruin the fuel injector. Saturated Switch Fuel Injector Waveform A saturated switch fuel injector waveform will show a voltage trace that almost drops to zero as the injector is turned on and a 30-volt to 50-volt inductive voltage spike as the injector is turned off . Th is injector circuit has more than 12 ohms resistance to limit maximum current draw by the injector windings. Refer to Figure 34-35 as the waveform for this type of injector is explained. Th e start of the trace (point A) shows that voltage is applied to the injector, but the circuit is not grounded through its control circuit. At point B, the electronic con- trol unit (ECU) grounds the injector circuit and the voltage drops to almost zero due to the small resistance in injec- tor circuit. Voltage remains low (point C) as current is fed through the injector windings. At point D, the ECU opens the circuit to stop current fl ow to the injector. Th is causes the magnetic fi eld across the injector coil windings to collapse, inducing an inductive voltage spike (point E). Figure 34-35. Analyze all of the parts of this saturated switch injector circuit waveform. Time Injector On Time Inductive Voltage ECU Opens Circuit Pintle Snaps to Closed Position ECU Grounds Circuit Injector Voltage Drop Voltage Supplied to Injector E D F B C A Voltage