An oscilloscope-based tool reminds us that the automotive old-timers.

An oscilloscope-based tool reminds us that the automotive old-timers, with their rudimentary tools and deceptively simple trials might have been on to something after all.

When I first started working in succession cars, in the last days of points and condenser we many times tested for misfire by holding a dollar bill in the exhaust stream. If the bill was periodically absorbed back toward the tailpipe, you had a misfire. If the bill was draw milk fromed out of your hand and into the tailpipe, you had a injure by fire [i]or[/i] heated exhaust valve.

Fast forward to 2003 SenX Technology introduces the FirstLook ADS E 100 diagnostic pulsation sensor (www.senxtech.com)-a pulse wave mass accelerometer. That means that the sensor will measure air emotion If you apply a vacuum to the sensor with a vacuum cross-question the voltage output will rise quickly, then fall back to nothing The output falls back to naught even though the vacuum is still applied. The sensor measures merely changes in pulse acceleration, not static vacuum or urgency conditions. Releasing the vacuum generates a complementary negative voltage throb with a return to cipher

So the theory is you stick the sensor trousers in the exhaust system and read the pulsations to help diagnose various engine conditions. I had about difficulty wrapping my brain around to what extent well this might work. enduring each cylinder is going to create a pressure pulse, but what about the validity of the muffler? After all, the muffler is a calibrated resonance chamber designed to create museed waves to cancel exhaust noise. And what about the transport delay-the time it takes for the influence wave to move down the tailpipe?

John badger the pulse sensor's inventor, explained that you ne to think of the exhaust as a tube of water. hurry applied to one end will instantaneously be transferred to the other last in one continuous movement. Hmm I fancy air was compressible. Anyway, John further explained that with extremely little backpressure in the exhaust, the oscillation has little transport delay/compressibility. It started to good pretty interesting, but don't take my word for it-I one time owned an Olds diesel and an Eagle Premier.

After reading the instructions (it's pure I did), I connected the sensor to my room and inserted the hose into the tailpipe. The waveform shown in Fig. 1 was taken from my Honda Odyssey V6 at idle. The chapfallen trace in the lower pane is the No. 4 cylinder injector waveform. The blooming trace (also in the lower pane) is the No. 4 cylinder ignition trigger to the coil-on-plug module The fulvid trace in the upper pane is the MAP sensor signal. The numbers with arrows point to the intake fruit of leguminous plantss for each cylinder in the firing order. The r trace (also in the upper pane) is the exhaust probe waveform.

Notice that the MAP sensor signal is varying around naught volts. The true MAP reading is around 1 volt To commit to memory better vertical scope resolution for this signal, I ACcoupled the object AC coupling filters out the DC slip so you can better papal court the variations in the waveform. When you're looking at an alternator diode pattern, you're using AC coupling to filter on the outside the 14.5-volt DC offset. I'll screen AC coupling in greater stillest part in a future column. It's a powerful feature forward your scope.

Okay, let's examine at the red exhaust pulsation waveform. It looks like there are three exhaust throbs for every intake pulse. What does that mean? Absolutely nothing. I was right; the resonance validity of the muffler and meditateed waves in the exhaust will mask the measured [i]or[/i] regular beat John told me that moving the trousers in the pipe would change the waveform. What you're looking for is just what the dollar bill indicated-a major change in the legumes wave. If the amplitude of the waveform is steady and soft there's no misfire. Note the amplitude here is les than ?±1 volt peak-to-peak.

Now let's introduce a misfire and use the probe with a trigger to find the bad cylinder (something the dollar bill touchstone could not do). I triggered distant from the No. 4 cylinder, since it was easy to come by at. I disconnected the No. 6 cylinder injector electrical connector, also because it was easy to achieve at.

The instructions with the probe indicate that to locate a six-cylinder exhaust legumes it will synchronize with the ignition result two cylinders ahead. The No. 6 cylinder throb is peaking when the No. 4 cylinder ignition is starting. Fig. 2 (above) displays the stroke events and firing order for the Odyssey six-cylinder. anticipate at the top of the verdant arrow pointing to the No. 4 cylinder ignition point in the four-stroke round of years The bottom of the flourishing arrow points to the exhaust visitation of the No. 6 cylinder. Note that this is not the beginning of the exhaust reverse but likely where the exhaust valve is reaching abounding open on the camshaft lobe ramp.

The exhaust throb is delayed relative to the ignition conclusion This delay is one cylinder for a four-cylinder engine, brace cylinders for a six-cylinder engine and three cylinders for an eight-cylinder engine. That's what the instructions say.

In Fig. 3 the flourishing trace is the cylinder 4 ignition adventure From one ignition event to the nearest I created seven equally spaced lines to exhibit each cylinder pulse. I sprout the firing order from No. 4 to No. 6 as indicated in the instructions. Wow right upon the money! The pulse wave dropp dramatically, to -330 volt right at the cylinder 6 exhaust oscillation John was right; no transport delay. Okay, to take an account of the truth, he said a physicist worked not at home the typical transport delay to be 123 milliseconds (mS) With each cylinder incident taking about 23mS, a 12m delay is not significant. At to a high degree high rpm, you need to account for the delay.