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Waveform Pattern

Fuel Injector Waveforms

Read saturated and peak-and-hold injector waveforms: pull-down, on-time, pintle bump, inductive kick, and current ramp. Learn the shapes for shorted windings, stuck pintles, and driver faults.

Why the waveform matters

A scan tool can command an injector on/off and time the pulse width, but it can't tell you whether the pintle actually moved or the coil is losing turns. The voltage and current waveforms show both — a five-second capture per cylinder finds the dragging injector, the shorted winding, or the failing driver that fuel-trim data only hints at.

Saturated vs peak-and-hold — know your driver

Saturated (most modern port injectors)

The driver pulls the injector low for the entire on-time and releases. High-impedance coils (~12–16 Ω) limit current, so no fancy switching is needed. Waveform: one pull-down, flat on-time, one inductive kick.

Peak-and-hold (older TBI, many diesels, some GDI drivers)

The driver pulls hard to snap the pintle open (peak current ~4 A), then chops to a lower hold current (~1 A) to keep it open without cooking the coil. Waveform: two pull-downs, two inductive kicks, a chopped-looking on-time.

What you'll need

  • Lab scope with at least two channels (voltage on one, current on the other, ideally)
  • Back-probe pins for the injector connector's ground side (the switched wire, not the 12 V feed)
  • Low-amp current clamp — 20 A range with 100 mV/A output is ideal
  • Timebase 500 µs–2 ms/div depending on pulse width; 5 V/div and 500 mA/div starting points

Step-by-step capture

  1. Warm the engine so injector on-time stabilizes.
  2. Back-probe the injector ground — never pierce the wire. Set channel to DC 5 V/div.
  3. Clamp the current probe around the same ground wire (or the 12 V feed). Zero the clamp.
  4. Trigger on the voltage channel's falling edge (the driver pulling to ground) so the pulse lands mid-screen.
  5. Capture at idle, then at 2 000 RPM, then during a snap-throttle for a wide-open-throttle sample.
  6. Repeat on every cylinder and compare. The outlier is the suspect.

Reading the voltage waveform

Healthy saturated pattern

12–14 V at rest, sharp vertical pull-down to near 0 V, flat on-time at 0.2–0.4 V (the driver's saturation voltage), a small pintle bump partway down the on-time, then a sharp inductive kick of 35–80 V when the driver releases, before settling back to battery voltage.

Rounded pull-down

The driver's turn-on is slow — usually a failing driver transistor or a high-resistance ground path. Compare to another cylinder on the same PCM driver bank; if only one is rounded, the driver is going bad.

Low or missing inductive kick

Shorted primary winding. The collapsing magnetic field can't build voltage across shorted turns. Replace the injector. Confirm by measuring coil resistance — should be within 0.5 Ω of the good injectors.

Missing pintle bump

The coil is being energized but the pintle isn't lifting — carbon buildup or a stuck plunger. Try a cleaning cycle first; if the bump doesn't return, replace the injector.

Never releases (voltage stays at 0)

The driver is stuck on — the injector is dumping fuel until the fuel pressure crashes or the fuse pops. Replace the PCM driver stage / disconnect the injector immediately.

Never pulls down (voltage stays at 12 V)

The driver is stuck open or the command signal isn't reaching it. Check the PCM's driver-side wiring before condemning the module.

Reading the current ramp

Current is the truth-teller. On a healthy saturated injector:

  • Ramps smoothly from 0 to ~1 A over the on-time
  • Shows a small notch or inflection at pintle-lift (about a third of the way up the ramp)
  • Collapses sharply to 0 when the driver releases

Faults look like this:

  • Missing notch: pintle isn't moving — mechanical stuck injector
  • Ramp too steep, too high peak: shorted winding (low impedance = high current)
  • Ramp low and flat: open or partially-open winding
  • Ramp with steps: shorted turns within the coil

Worked example: cyl 2 low fuel trim on a 4-cylinder

2016 four-cylinder, LTFT on bank 1 running −18% at idle. Injector voltage looks normal on all four cylinders. Current ramp on cyl 2 shows a peak of 1.7 A vs 1.0 A on the others and a much steeper slope. Coil resistance on cyl 2 measures 9.4 Ω vs 12.6 Ω on the others — a shorted winding. Injector was leaking extra fuel because the pintle was being held slightly open by the excessive current. Replaced, trims returned to ±3%.

Common mistakes

  • Piercing the wire instead of back-probing — creates a corrosion point that shows up as a fault later
  • Clamping around both injector wires — the fields cancel and current reads zero
  • Comparing on-time between banks on a V-engine without checking fuel-trim balance first
  • Skipping the current ramp — voltage-only capture misses shorted-winding faults
  • Testing at idle only — some faults only appear at high pulse widths under load

Frequently asked questions

How high should the inductive kick be on a saturated injector?

35–80 V is normal. Under 30 V points to a shorted injector winding — the collapsing field can't build voltage because the turns are shorting together. Over 100 V is unusual and often means the driver's clamp diode has failed.

What does the pintle bump look like and why does it matter?

As the pintle finishes lifting inside the injector, its motion induces a tiny voltage 'bump' about halfway down the on-time slope. Seeing the bump proves the pintle physically moved. A missing bump on an otherwise-normal waveform means the injector is stuck closed even though the coil is being commanded.

Saturated vs peak-and-hold — how do I know which I have?

Saturated injectors show a flat 0 V on-time with a single inductive kick at release. Peak-and-hold shows two distinct pull-downs (a hard 'peak' pull to open the pintle, then a lower 'hold' phase to keep it open) and two smaller kicks. Most modern port injectors are saturated; older TBI, high-impedance GDI drivers, and many diesels are peak-and-hold.

Why capture injector current instead of voltage?

Current shows what the coil is actually doing. A shorted winding pulls too much current early; an open one draws nothing. Current also reveals mechanical events — you'll see a small 'notch' in the current ramp exactly when the pintle lifts. If the notch is missing, the pintle isn't moving even though voltage looks normal.

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