Waveform Pattern
Fuel Injector Waveforms
Read port-injector voltage and current waveforms: pull-down, on-time, inductive kick, pintle opening notch and closing hump. Learn the physics behind shorted windings, sticking pintles, and driver faults.
Why the waveform matters
A scan tool reports the commanded pulse width. It can't tell you whether the pintle moved, whether the coil has shorted turns, or whether the driver switched cleanly. The voltage and current waveforms show all three, cylinder by cylinder.
Know the driver type
| Type | Injector | What the driver does | Voltage signature |
|---|---|---|---|
| Saturated (most port injection) | High impedance, roughly 12–16 Ω | Pulls to ground for the whole on-time; the coil's resistance limits current | One pull-down, one inductive kick |
| Peak-and-hold (older low-impedance port and throttle-body injection) | Low impedance, roughly 2–4 Ω | High current to open, then regulated lower current to hold | One pull-down, a small kick where current drops to hold, a final kick at release |
| Direct injection | Very low impedance | Boosted supply, high peak current, then hold stages | Different enough that it needs its own analysis; don't judge it by port-injector values |
This guide covers port injectors.
What you'll need
- Lab scope, two channels: injector voltage on A, injector current on B.
- Back-probe pins for the injector's control (driver) wire.
- Low-amp current clamp, 100 mV/A.
- An attenuator or a ±100 V range for the inductive kick.
Step-by-step capture
- Verify supply voltage. Injector current follows supply voltage (I = V / R), so record it with the capture.
- Warm the engine to operating temperature.
- Voltage: back-probe the control wire (the side the driver switches to ground). Range ±100 V, DC coupled.
- Current: clamp the injector feed or control wire, 1 A/div. Zero the clamp with no current flowing first.
- Timebase: 1–2 ms/div; sample rate at least 100 kS/s.
- Trigger on the voltage channel's falling edge.
- Capture at idle, at 2,000 RPM, and during a snap throttle.
- Repeat on every cylinder with identical settings. The outlier is the suspect.
Reading the voltage waveform (saturated)
In order along the trace:
- Rest: system voltage (12–14.5 V).
- Pull-down: a sharp drop to near 0 V (typically under 1 V) when the driver switches on.
- On-time: flat near 0 V. The driver holds it there, so no pintle motion can show on the voltage trace during on-time.
- Inductive kick: when the driver releases, the collapsing field produces a spike, often clamped flat-topped at 35–80 V by the driver's protection.
- Closing hump: a small bump on the decaying voltage just after the kick. This is the pintle closing: its motion changes the coil's inductance and induces a small voltage.
- Return to system voltage.
| Voltage symptom | Physical cause |
|---|---|
| Slow, rounded pull-down | Weak driver, or high resistance in the driver's ground path |
| Pull-down doesn't reach near 0 V | Resistance in the injector control circuit or driver |
| Low kick (compared with the other injectors) | Less stored energy at release: low current (resistance) or shorted turns |
| Kick far above the others or unclamped | Failed driver clamp protection |
| Closing hump missing, or later than on other injectors | Pintle not closing normally: sticking or slow |
| Never pulls down | No command, open circuit, or failed driver: check the circuit before the module |
| Never releases (stays near 0 V) | Driver stuck on, or control wire shorted to ground: injector is flooding the cylinder — disconnect it |
Reading the current waveform (saturated)
Current rises during on-time as:
i(t) = (V / R)(1 − e^(−t/τ)), where τ = L / R
- It heads toward V / R: 1.17 A for a 12 Ω injector at 14 V.
- It reaches 63% of that at t = τ.
- Partway up, a notch or inflection appears: the pintle opening. Moving iron changes the coil's inductance, and the current ramp shows it.
- It collapses when the driver releases.
| Current symptom | Physical cause |
|---|---|
| No opening notch | Pintle not lifting (stuck closed) |
| Notch later than on other injectors | Pintle slow to open (deposits, sticking) |
| Steeper ramp and higher final current | Shorted turns: lower R and lower L |
| Lower final current and slower ramp | Added resistance in the coil, connector, or wiring |
| No current while commanded | Open coil or open circuit |
Worked example: shorted turns found by comparison (illustrative)
A four-cylinder port-injected engine at 14.0 V, all injectors captured with identical settings:
| Cylinders 1, 3, 4 | Cylinder 2 | |
|---|---|---|
| Measured resistance | 12.6 Ω | 9.4 Ω |
| Expected final current (14.0 V ÷ R) | 1.11 A | 1.49 A |
| Captured current ramp | Normal | Steeper, higher, matching the lower R |
The 34% higher current on cylinder 2 matches its lower resistance, confirming shorted turns. Fuel trims may show little: a shorted coil still opens the injector, and a slightly faster opening changes fuel delivery only a little. The risk is the extra load on the driver, so replace the injector before the driver fails.
Note what a shorted winding cannot explain: a large rich fuel trim. Once the driver releases, current stops and the spring closes the pintle. A large rich trim from one cylinder points to a pintle that isn't sealing mechanically, or to a different cause.
Common mistakes
- Using 5 V/div (or a range that clips the kick): the kick and closing hump are lost.
- Piercing the wire instead of back-probing.
- Clamping the feed and control wire together: the fields cancel.
- Comparing injectors captured at different supply voltages.
- Voltage-only capture: shorted turns and opening problems show in current.
- Testing at idle only: some faults appear only at longer pulse widths.
- Reading a direct-injection trace against port-injector values.
Frequently asked questions
How high should the inductive kick be?
Many drivers clamp it at about 35–80 V, so a flat-topped kick is normal. Compare injectors on the same engine: a kick much lower than the others means less stored energy (low current or shorted turns); one much higher or unclamped points to the driver's protection.
Where is the pintle signature?
Opening shows as a notch in the current ramp during on-time. Closing shows as a small hump on the voltage trace just after the kick. During on-time, the driver holds the voltage near 0 V, so no motion can appear there.
How do I tell saturated from peak-and-hold?
Saturated: one pull-down, one kick, and current that rises smoothly toward V / R. Peak-and-hold: current rises to a high peak, then drops to a regulated hold level, with a small kick at the transition and a final kick at release.
Why capture current as well as voltage?
The voltage trace mostly shows what the driver did. The current trace shows what the coil and pintle did: shorted turns, added resistance, and pintle opening all appear in the current.