If a short in an injector coil winding is constant, an ohmmeter will accurately identify the lower resistance. The
same is true with an open winding. Unfortunately, an intermittent short is an exception. A faulty injector with
an intermittent short will show "good" if the ohmmeter cannot force the short to occur during testing.
Alcohol in fuel typically causes an intermittent short, happening only when the injector coil is hot and loaded by
a current high enough to jump the air gap between two bare windings or to break down any oxides that may
have formed between them.
When you measure resistance with an ohmmeter, you are only applying a small current of a few milliamps. This
is nowhere near enough to load the coil sufficiently to detect most problems. As a result, most resistance checks
identify intermittently shorted injectors as being normal.
There are two methods to get around this limitation. The first is to purchase an tool that checks injector coil
windings under full load. The Kent-Moore J-39021 is such a tool, though there are others. The Kent-Moore
costs around $240 at the time of this writing and works on many different manufacturer's systems.
The second method is to use a lab scope. Remember, a lab scope allows you to see the regular operation of a
circuit in real time. If an injector is having an short or intermittent short, the lab scope will show it.
Checking Available Voltage At the Injector
Verifying a fuel injector has the proper voltage to operate correctly is good diagnostic technique. Finding an
open circuit on the feed circuit like a broken wire or connector is an accurate check with a DVOM.
Unfortunately, finding an intermittent or excessive resistance problem with a DVOM is unreliable.
Let's explore this drawback. Remember that a voltage drop due to excessive resistance will only occur when a
circuit is operating? Since the injector circuit is only operating for a few milliseconds at a time, a DVOM will
only see a potential fault for a few milliseconds. The remaining 90+% of the time the unloaded injector circuit
will show normal battery voltage.
Since DVOMs update their display roughly two to five times a second, all measurements in between are
averaged. Because a potential voltage drop is visible for such a small amount of time, it gets "averaged out",
causing you to miss it.
Only a DVOM that has a "min-max" function that checks EVERY MILLISECOND will catch this fault
consistently (if used in that mode). The Fluke 87 among others has this capability.
A "min-max" DVOM with a lower frequency of checking (100 millisecond) can miss the fault because it will
probably check when the injector is not on. This is especially true with current controlled driver circuits. The
Fluke 88, among others fall into this category.
Outside of using a Fluke 87 (or equivalent) in the 1 mS "min-max" mode, the only way to catch a voltage drop
fault is with a lab scope. You will be able to see a voltage drop as it happens.
One final note. It is important to be aware that an injector circuit with a solenoid resistor will always show a
voltage drop when the circuit is energized. This is somewhat obvious and normal; it is a designed-in voltage
drop. What can be unexpected is what we already covered--a voltage drop disappears when the circuit is
1998 Chevrolet Pickup C1500
GENERAL INFORMATION Waveforms - Injector Pattern Tutorial