feed circuit if you see big shifts and repair as necessary.
Note that circuits with external injector resistors will not be any different because the resistor does not affect
open circuit voltage.
Point "B" is where the driver completes the circuit to ground. This point of the waveform should be a clean
square point straight down with no rounded edges. It is during this period that current saturation of the injector
windings is taking place and the driver is heavily stressed. Weak drivers will distort this vertical line.
Point "C" represents the voltage drop across the injector windings. Point "C" should come very close to the
ground reference point, but not quite touch. This is because the driver has a small amount of inherent resistance.
Any significant offset from ground is an indication of a resistance problem on the ground circuit that needs
repaired. You might miss this fault if you do not use the negative battery post for your Lab Scope hook-up, so it
is HIGHLY recommended that you use the battery as your hook-up.
The points between "B" and "D" represent the time in milliseconds that the injector is being energized or held
open. This line at Point "C" should remain flat. Any distortion or upward bend indicates a ground problem,
short problem, or a weak driver. Alert readers will catch that this is exactly opposite of the current controlled
type drivers (explained in the next section), because they bend upwards at this point.
How come the difference? Because of the total circuit resistance. Voltage controlled driver circuits have a high
resistance of 12+ ohms that slows the building of the magnetic field in the injector. Hence, no counter voltage is
built up and the line remains flat.
On the other hand, the current controlled driver circuit has low resistance which allows for a rapid magnetic
field build-up. This causes a slight inductive rise (created by the effects of counter voltage) and hence, the
upward bend. You should not see that here with voltage controlled circuits.
Point "D" represents the electrical condition of the injector windings. The height of this voltage spike (inductive
kick) is proportional to the number of windings and the current flow through them. The more current flow and
greater number of windings, the more potential for a greater inductive kick. The opposite is also true. The less
current flow or fewer windings means less inductive kick. Typically you should see a minimum 35 volts at the
top of Point "D".
If you do see approximately 35 volts, it is because a zener diode is used with the driver to clamp the voltage.
Make sure the beginning top of the spike is squared off, indicating the zener dumped the remainder of the spike.
If it is not squared, that indicates the spike is not strong enough to make the zener fully dump, meaning the
injector has a weak winding.
If a zener diode is not used in the computer, the spike from a good injector will be 60 or more volts.
Point "E" brings us to a very interesting section. As you can see, the voltage dissipates back to supply value
after the peak of the inductive kick. Notice the slight hump? This is actually the mechanical injector pintle
closing. Recall that moving an iron core through a magnetic field will create a voltage surge. The pintle is the
iron core here.
This pintle hump at Point "E" should occur near the end of the downward slope, and not afterwards. If it does
1998 Chevrolet Pickup C1500
GENERAL INFORMATION Waveforms - Injector Pattern Tutorial