Diagnosing the RIGHT Way

If you were one of the many visitors who stopped by my booth and watched a diagnostic demonstration, you have a general idea of scope waveform analysis. ALL those humps, bumps, dips and squiggly lines displayed on a grid represent changes (electricity, pressure, temperature, etc.) over time in real time. Scope diagnostic has significant advantages over former diagnostic test methods in that the tests are performed LIVE during operating conditions. To increase the odds for a successful diagnostic, it’s important to test under identical conditions in which the failure occurs. If the problem only occurs when the engine is not running and everybody is resting under the shade of the closed hood, then by all means go ahead and go old school. Take the components off one by one and test them on a work bench. If not, the tests should be performed when the engine is running and the components are hot, tired and stressed. Dynamic testing for dynamic failures I always say.

Let’s use electronic fuel injector testing as an example. With today’s engines, the odds that you can even see a fuel injector, let alone the one you want to test, are low. Many fuel injectors live under or inside the intake manifold. If you remove the intake manifold and all the garbage attached just to gain access to test, you are going to be at it for a while so don’t forget your lunch. When you finally do reach the injector, you won’t be able to start and run the engine. You will be limited to static testing. Good luck with that. Oh, don’t forget: You are going to need a new set of intake manifold gaskets, minimal.

With a Lab Scope, the fuel injector component testing is done at the fuel injector fuse. We replace the fuel injector fuse with a loop of copper wire. By the way, my wire loop is fused to protect the circuit. Using this diagnostic method, EVERY fuel injector is tested and ALL at the same time. Now, before the electrons can complete their journey from the battery to the fuel injectors they must first pass through my wire loop. Whenever electrons flow, they produce an electromotive force called current. Current is measure in amps. The greater the flow the greater the current. I simply place a low current amp probe around my wire loop and measure. I also measure the available voltage at either end of my wire loop. Using two channels of my Lab Scope, I display all current and voltage values in parade fashion (two squiggly lines) during running conditions. The last thing I need to do before I start testing is look up the fuel injector resistance value specification.

The only thing I remove was one fuse so I can start and run the engine. Now comes the math: Ohms Law. Resistance = Voltage divided by Current. For example, if my injector resistance specification is 2 ohms, and my available voltage is 14 volts (remember the engine will be running) the current HAS TO BE 7 amps. I know what my waveforms should look like before I even start testing. If I see a current value different then 7 amps, my resistance is NOT 2 ohms. If the current (amperage) is higher, the resistance has to be less than 2 ohms. This condition typically indicates a short across the coil winding or a short to ground. The resistance is lower because the electrons take a short cut and do not pass through the entire injector winding (resistance). If the current is lower, the resistance has to be higher. This condition typically indicates an open or gap in the circuit making it more difficult for the electrons to pass.

A typical electronic fuel injector current waveform looks similar to the outline of a dorsal fin of a shark. The leading edge of the waveform gradually curves into a horizontal position as it reaches its max height. It’s not a straight slanted line, like a motorcycle ramp; it’s rounded. That leading edge represents the building up (ramping) of an electromotive force (magnetism). When the magnetism is strong enough to overcome the fuel pressure, a metal pintle inside the fuel injector is lifted off its seat allowing fuel to pass into the cylinder. This fuel injector opening event is displayed as a dip and then a flattening out of the leading edge ramp. That dip should be located about 2/3 of the way up the ramp. The dip in the waveform not only indicates the injector opened, but when it opened. Too early would create a rich condition. Too late would create a lean condition.