Using Oscilloscopes on Vehicles
Q: How will the oscilloscopes be used to diagnose a stalling vehicle?-Customer Inverview
A: From the outset, engine stalling (like all diagnostic procedures) needs to be evaluated by an interview with your customer as they have all the answers and more importantly the history/sequence of events
A basic inspection must never be overlooked before any diagnosis as it is all too easy to attack vehicles with an array of test equipment when in fact our concern could be contaminated fuel!
Confirmation of the customers concern is paramount and ideally with a scan tool attached to gather valuable data during the fault condition. This is not always possible as faults are often intermittent and dependent on driving conditions and so again, the customer interview will assist
-Road Testing
Diagnostic trouble codes are always the first port of call along with any serial data from your chosen Scan tool. Road testing the vehicle with your scan tool connected whilst trying to simulate the condition will also assist to provide target areas for further diagnosis and testing and this is where oscilloscope becomes invaluable.
-Measure Components with an Oscilloscope
Armed with all the information above the technician will be required to follow a fault code flowchart if a relevant fault code is present, or, armed with his knowledge of the system will begin to test and measure components of the engine management with an oscilloscope that could be responsible for the customers symptom.
To give a typical example, let’s take a gas engine stalling when stopping at junctions but no fault codes evident. A description of the engine idle control system is required and a list complied of possible components that could produce the staling. From this list the technician may decide to test the, oxygen sensor, airflow meter, and throttle position sensor. The order of tests depends on the ease of access to these components but with a 4 channel scope all these inputs can be measured and monitored in real time on one screen.
Using the oscilloscope gives the technician the advantage to monitor (in real time) the actual live signals and compare them to the processed data from the scan tool.
Q: Why use an oscilloscope not just a scan tool?
A: Once connected to the components the technician is able to look at the cause and effect of various sensors on the idle control system. A wiggle test is such a simple yet effective technique when looking at signals that may be interrupted due to wiring failures, connection issues, and heat damage. Only the oscilloscope can respond fast enough to display these “glitches” where the scan tool would miss this vital “drop out” in signal integrity so leading the technician on a potential goose chase. The oscilloscope can also be used during road tests when most faults occur, allowing the technician the same high speed capture ability under the operating conditions reported by the customer. The oscilloscope can record the signals above and when the fault occurs the technician is able to pause and playback the captured data, zoom, measure, and evaluate the signals to conclude a diagnosis as to why the vehicle may have stalled
Q: What can I expect from a technician using an oscilloscope?
A: From experience, oscilloscope users are dedicated professionals who have invested both time and money (often their own) to improve their diagnostic capability in the ever increasing complexity of today’s vehicles. You can expect your vehicle repaired in a realistic time backed with proof of component failure with supporting literature of before and after fix that will deliver confidence in any repair.?
Q: Why a Scope If You Have a Scan Tool?
A: Very few shops have a scan tool for every vehicle, and using a scope for drivability problems is an alternative. The scan tool might spot a misfire, but the scope does more than just find the location. It identifies under what conditions (speed or load) the problem occurs and provides a chance to evaluate the cause. Because you can compare each cylinder under the same load or driving condition, you can show the customer potential future misfires and prevent comebacks.
The Secondary Ignition Waveform-1The voltage required to fire the plug is determined by the greatest gap in the circuit. It is not a measure of all the gaps in the system. A better terminology would be"dominating gap," because kilovolt (Kv) demand increases with spark gap under compression. If a 0.045-inch gap outside the combustion chamber requires 2 Kv tojump the gap, this same gap under compression may require 10 Kv for the plug toionize, providing fuel is present to help conductivity.
Edited by WisdomAugust at 2017-8-3 08:43
The Secondary Ignition Waveform-2
Secondary Scope Pattern
Edited by WisdomAugust at 2017-8-3 08:42
The Secondary Ignition Waveform-3
A cylinder lost compression and we see less kilovolt demand increasing the firing time. The opposite is true when the kilovolt is high. We learn that kilovolt is a necessary parasite, robbing from the burn time in the combustion chamber. Kilovolt does not burn fuel, firing time does! A lean fuel mixture, (reduced HC) causes the kilovolt demand to increase even more in proportion. Therefore, an insulation breakdown causing a misfire is most often the result of unusual high kilovolt demand created by any combination of worn spark plugs and lean mixture.
The Secondary Ignition Waveform-4
Low Compression
Edited by WisdomAugust at 2017-8-5 08:07
Replacing a part to see if the misfire goes away without knowing the cause is a fruitless exercise. Crossfire occurs when kilovolt is driven high beyond the capacity of the insulation. Then the spark is enticed to find a path to ground outside the combustion chamber. When that happens, there is no HC present outside the combustion chamber to sustain conductivity resulting in high resistance creating shorter firing time at a higher kilovolt level.
For practical purposes, there is no current flow until the plug fires as indicated by the firing time. When the coil output runs out of energy, the residual energy dissipates into oscillations. Because hydrocarbons (fuel) are conductors, they not only assist in the ionization process, but also affect the conductivity during firing time.
How Do We Know It Is a Plug Wire?Let us assume a cylinder at idle showed abnormally high kilovolt, and the firing time seemed to be shorter compared to the other cylinders. We need to find out where that high kilovolt is created, internally or externally. Play with the throttle, accelerate and decelerate and watch the response. If the kilovolt stays high, this indicates that the greatest gap was external. How should the scope respond to rpm increase? At 2,000 rpm, the timing advance is between 30 and 40 degrees BTDC. At that time, the piston barely starts the compression stroke resulting in less compression when the plug fires. Therefore the reduced kilovolt tells us that the timing advance is functional and there was compression. We have verified the greatest gap is indeed in the combustion chamber and not external.