Project Overview

Desoldering Components

IDM Models

Printed Circuit Board Details

Component Identification

Circuit Tracing & Component Symbols

Completed IDM Schematic

Sub-Circuit Analysis

Injector Waveforms



(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.

The horizontal axis is time, with 200 us  (micro-seconds:  .2 milliseconds, or 200 millionths of a second) per division.

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 gap.

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 or so.

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..

The virtually non-existent kickback indicates that there was little to no magnetic field built up in the solenoid due to the shorts.




(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.)




(Graph 10) This solenoid had only 1.9 Ohms resistance. The moment the IDM detected the excessive current flow, it shut the injector off.







(Graph 11) 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.



(Graph 12) 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 abnormal.

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 higher-mileage IDM

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.

Graph 1

Graph 1--Injector Waveform


Graph 2

Graph 2--Peak Pulse


Graph 3

Graph 3-Hold Pulses


Graph 4

Graph 4-Pulse Gap


Graph 5

Graph 5-Inductive Kickback


Graph 6

Graph 6-.44ms Pulse Gap

Graph 7

Graph 7-.23 ms Pulse Gap


Graph 8

Graph 8--Shorted Solenoid 1

Graph 9

Graph 9- Shorted Solenoid 2

Graph 10

Graph 10- Shorted Solenoid 3

Graph 11

Graph 11- Shorted Solenoid 4A

Graph 12

Graph 12 - Shorted Solenoid 4B


Graphs 13-18

Graph 13

Graph 14

Graph 15

Graph 16

Graph 17

Graph 18