|(Graph 1) This is an
oscilloscope graph of the voltage from a good working
IDM, to an injector . The vertical axis is voltage; Red
and Blue are the control signals from the PCM to the IDM
and each vertical division is 10 volts. Green and Yellow
are the injector, and each division is 40 volts.
horizontal axis is time, with 200 us (micro-seconds: .2
milliseconds, or 200 millionths of a second) per
The vertical dashed white lines are cursors on the
scope screen, used to measure time or voltage. The
triangle (delta) is the time period between the two
cursors. In the second graph, Cursor 1 is .13 ms from
the left edge of the graph, cursor 2 .62 ms from the
edge, and the time between them is .49 ms.
In the top graph, the cursors are 1.72 milliseconds
apart--the injector pulse width.
There are two main parts to the waveform: the "peak",
(Graph 2) bounded by the vertical cursors in the second graph, and
the "hold" shown in graph 3. Peak & Hold
injectors are routinely used in EFI applications.
When the injector is off, there is about .013"
between the injector solenoid and the armature, which
the solenoid pulls on. The peak is a full "shot" of
current to the injector to build up a strong enough
magnetic field to bridge this gap and overcome 25-30 lbs
of force holding the armature down in the closed
position. It takes .40-.45 ms to build up the field and
move the armature. The peak is .50 ms in duration.
Once the armature is pulled up, it is only .002" from
the solenoid, so it requires only a fraction of the
initial opening current to keep it open. Now the IDM
begins to rapidly pulse the current to the injector at a
frequency of 13-15 kHz. This greatly reduces the
required current, and reduces heat in the IDM and
injector solenoid. Note that the injector is NOT turning
on and off every time the current pulses on and off
during the hold portion of the cycle, nor during the
The resistance through the solenoid is about 3.2
Ohms; at 110 volts, 34 amps of current could flow
through the injector if the current were left "on"
continuously. The insulation on the windings in the
injector solenoids will overheat and short together
(drawing even more current) if the injector is left on
for more than a few seconds.
The solenoids are extremely well designed, with the
failure rate less than 1:1500. Most of the problems stem
from shorted glow plugs melting the engine wiring
harness or valve cover gasket, shorting the glow plug
wires with the injector wires.
The fourth graph is of the "gap", an
anomaly of sorts
in the injector wave form. All IDM's have this gap, which
is normally.32 to .36 ms. As IDM's age, this gap grows
longer, and performance suffers when it gets aver .42-.45ms
The gap always occurs at the same basic point in the
peak & hold cycle, about .90 ms after the start of
injection. It is unaffected by injection duration (pulse
width) or RPM.
After the IDM turns off the current to the injector,
the magnetic field in the solenoid collapses. (Graph 5) When the
current first flowed into the solenoid, it created a
magnetic field; now the flow of current has been cut off
so the magnetic field collapses and converts back into
current flow, a process called inductive kickback, which
generates the 198 volt spike shown in yellow in graph 5.
(Graph 6) This is an IDM 100 that has gotten weak: There are
two gaps in the hold pulse. The first one is .44 ms, the
second is .23 ms. (Graph 7)
The IDM's used in 1994 & early 1995 trucks with
"EDU-100A" on the label, are by far the worst in terms
of excessive gaps in the hold pulse.
(Graph 8) This is a solenoid that was probably
damaged by some major shorts in the injector & glow plug
wiring harness. It had 2.4 Ohms resistance. Note that
there is no peak in the pulse, and that the hold pulses
are much, much closer together--the frequency is 50 kHz
rather that the more normal 13-15 kHz..
non-existent kickback indicates that there was little to
no magnetic field built up in the solenoid due to the
(Graph 9) Another
shorted solenoid, also with 2.4 Ohms resistance. The
height of hold pulses slope down as the pulse
progresses, falling to only about 60 volts at the
end--it was drawing so much current that the IDM's main
capacitors were discharged and the power supply couldn't
keep up. (It looks like 30v here, but at a 50 micro-second time scale they were seen to be 40-70 volts.)
This solenoid had only 1.9 Ohms resistance. The moment
the IDM detected the excessive current flow, it shut the
Another shorted solenoid. This one had 2.9 Ohms, but for
some reason the PCM didn't like what the IDM
reported--after about 30 seconds of running, the PCM
would shut down the entire passenger's side cylinder bank,
i.e. all injectors on that circuit. (If this had been on
an even-numbered injector, the driver's side bank would
have been shut down.)
The peak pulse is much shorter than normal, about
.35 ms, and the voltage at the end of the hold pulse is
very, very low.
This is the pulse after the PCM shut things down; the
pulse width is .40 ms, too short for the injector to
fire. The waveform for the kickback voltage is also very
All in all, solenoid problems are very, very rare. We
might see one bad one out of every 1500-2000 injectors.
All the graphs of the bad solenoids were taken on the
output for injector 1 on the same IDM and on the same
injector. Just the solenoids were changed.
Graphs 13 through 18 are six sequential firings of
injector #1 scoped at around 2800 RPM, or about 40 ms
apart. Same IDM, same injector, same solenoid. These
waveforms are pretty typical of an older or
Although the waveforms are very similar, they are not
identical in the hold portion of the pulse. However,
these small variations from one firing to the next do
not effect the injector operation.
The critical parts of the pulse are the peak voltage and
duration, and the length of the gap in the pulse.