If there is any doubt as to the maximum peak voltage level being within the scope's input range (refer to scope lid) an attenuator must be used. If using an attenuator the appropriate built-in probe must be selected from within the channel options.
The waveform being monitored is the supply voltage to the coil. The supply is at the battery or charging voltage of 12 volts or more. When the coil's primary circuit is switched on, the voltage drops and remains at a low level until the current in the primary circuit reaches the target of 5.5 amps, at which point the supply voltage is allowed to spike for ignition.
The example waveform above shows the current-limiting circuit in operation. The current switches on as the dwell (ECU controlled ground) period starts and rises until approximately 5.5 amps is reached in the primary circuit. At this point the current is released at the point of ignition. The length of time from the initial switching-on point to the moment the current is released depends on engine speed. The lower the engine speed, the shorter the current ramp; then the ramp lengthens with increasing engine revs.
Historically, the supply voltage was present as soon as the ignition switch was turned to the 'on' position. Modern systems, however, do not provide a supply until the key is turned to the 'crank' position and the engine turns. A simple fault such as a non-functioning crank angle sensor may result in a loss of supply voltage, simply because the electronic control circuits do not recognize that the engine is rotating.
The example waveform shows the current in the primary circuit switches on as the dwell period starts, and rises until a level of 5.5 amps is reached.
As the engine speed increases, the dwell angle expands to maintain a constant coil saturation time and therefore constant energy. The coil saturation time can be measured by placing one time ruler at the beginning of the dwell period and the other at the end of the current ramp. The distance between the rulers will remain exactly the same regardless of engine speed.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
30 A (low amps) current clamp
20 A / 60 A DC (low amps) current clamp
Connector breakout lead kit A
Back-pinning Probe Set
Flexible Back-pinning Probe
Small Crocodile/Gator Clips
PicoScope Sprung Hook Probe
PicoScope Battery Clip
Large Dolphin/Gator Clips
Premium Test Lead: BNC to 4 mm, 3 m
Premium Test Leads: Set of four leads 3 m (TA125 - TA128)
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May 08 2017
This guided test is incorrect for Channel A. You wouldn’t connect Channel A to the positive side of the coil to see a primary voltage waveform. You would connect it to the negative side of the coil. All you will see is a slight voltage drop if you remain connected to the positive side.