Ignition Primary Voltage Waveform Analysis


Primary Ignition Waveform


  1. Coil is Switched ON
    • The switch internal to the PCM closes. Current rushes into the coil and begins to build, which is why voltage drops close to ground and essentially remains there until the firing spark.
  2. Saturation
    • The coil is now saturated with electricity, as indicated by the jump in voltage. The coil is no longer charging up thanks to the ICM/PCM.
  3. Voltage Spike
    •  The PCM switch opens, unleashing all the built-up current. Amps drop like a rock and voltage skyrockets.
    • Expect spark plugs with plug gaps of 0.045-0.060 inch to require 8 to 12 KV to jump the gap at no-load idle. If firing voltage is higher than 2 or 3 KV, this decreases the duration of the spark, known as firing time.
  4. Burn-time
    • The spark line indicates the length of the spark event at the plug.
    • Shorter spark times often indicate a weak ignition coil, high resistance, lean mixture, lower compression.
    • In times where spark time is short and firing voltage is typically high, this indicates that something is forcing the ignition to expend all its energy on the initial spark, leaving nothing left for its duration.
    • The slight upward spike in the end of the spark line is normal.
    • Lean systems spike up right after the firing spike (during throttling up).
    • Good spark line is 1ms or more.
    • 📖 Spark Line Explanation
  5. Oscillations
    • Oscillations occur when the spark has ended because the energy necessary for the spark has been extended. There is still some energy left in the ignition coil, but not enough to continue the spark event. So, the remaining power oscillates up and down, effectively ringing itself out.
    • Sometimes, some ignition systems have one coil oscillation.
    • Fewer and lower coil oscillations indicate an open circuit in secondary, e.g. fouled spark plug, open coil tower.
    • 📖 Oscillations Explained
Spark Type
  • Waste Spark
Spark Plugs
  • Rich mixture may cause spark plugs to fail. Replace spark plugs if car runs worse and worse, and then try to fix rich condition before they fail again.
  • Tip: clean the spark plug hole to prevent carbon buildup on the threads to cause misfire after putting a new spark plug.



Examples using oscilloscopes are here and here.


     No Spark or Weak Spark
    • Very weak spark (orange spark) and sometimes no spark (2006 GMC Sierra 4.3L)
    • Bad Distributor (Secondary Ignition Waveform Comparison)
      • Secondary Ignition Spark Waveform. Bad Distributor.  Secondary Ignition Spark Waveform. Good.


      Bad Coil
      • Tip. Good ignition current is above 5 – 6 Amps.
      • 📎 Ignition Current
      • 📎 Misfire Under Load | Bad Coil Pack | 2001 Vauxhall Astra
      • 📎 Poor Performance | Bad Coil | 1991 BMW 325i
      • 📝  Poor Performance Under Load | Bad Coil | 2014 Mini Cooper
        • Would run fine in all rpm/speed modes; except when cruising at a higher speed, say 50mph, and then suddenly pressing the accelerator, it would start misfiring. New coil fixed the problem.
        • Why would it misfire only under those conditions? I didn’t bother checking the spark on that cylinder, but generally speaking blue spark is strong and red/yellow spark is weak. Compression, temperature, and high amount of fuel can resist the spark ignition (jumping between electrodes). I think in my case under load computer has been adding more fuel which in turn resisted the spark to the point of dead misfire.
      • 📎 Misfire | Shorted Coil | Chrysler Concord
      • 📝 Backfiring | Wrong Coil | 1999 Grand Cherokee
        • Wrong coil might have reversed polarity and make spark in a little different time, thus backfiring.
      • 📝 No Start, P0353 | Bad/Disconnected Coil | 2007 Toyota Sienna
        • Р0353 Toyota – Ignition Coil C Primary/Secondary Circuit
        • Bad ignition coil caused engine to cut off immediately after DTC has been set.
        • When trying to start the car, it starts and cuts off. Even disconnecting a bad coil didn’t allow engine to start.


      Bad Coil Wiring
      Bad Distributor
      Bad ECM
      • 📎 Misfire | Bad ECM | Fiat Punto
      • 📝 No Start | Bad ECM | 1999 Subaru Impreza 2.2L
        • Ignition testing revealed constant spark on all spark plugs with just the ignition ON!
        • Cam and crank signals were present only with the engine cranking and absent with ignition ON.
        • Power and ground to the PCM checked out to be good.
        • Injectors were constantly switched on with ignition ON. 
        • New PCM fixed the problem. Old PCM didn’t have any sign of circuit damage.
      No Compression  (secondary ignition waveform)


      Fuel Trims. Ignition Misfire (false lean)
      • Total fuel trim ~ +24% (2000 Lexus RX300 with a wideband O2 sensor)
      • Total fuel trim ~ +20% (2004 Dodge Neon with a narrow band O2 sensor)
      • Disconnected fuel injector creates leaner condition than disconnected spark plug on that cylinder.
      • Disconnected spark plug in one cylinder creates less lean condition than disconnected fuel injector.
          • Total fuel trim ~ +8% (2000 Lexus RX300 with a wideband O2 sensor)
          • Total fuel trim ~ +7%-8% (2004 Dodge Neon with a narrow band O2 sensor)
      • Rule of Thumb. If STFT has changed less than 15 percent positive and the downstream oxygen sensor has voltage higher than 650mV that you can with confidence look for an ignition related concern to be the cause of the misfire.
        • Why downstream O2 shows rich condition when its lean?
          • The downstream O2 will show an increase in voltage and go into the 900mV to even 1,000mV range.  This happens because the cat has to try and clean the exhaust and so it uses all of the stored O2 to attempt to oxidize the unburned HC entering from the misfiring cylinder. It is unable to oxidize all of the HC and uses most to all of the excess O2 that came with the HC from the cylinder that had the misfire. The cat is trying to clean the exhaust and therefore the downstream O2 sees a rich exhaust stream, low O2. It will output a voltage above 650mV and usually above 900mV during the event.
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      Carlos lopez
      Carlos lopez
      5 months ago


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