PRESSURE POINTS: PRESSURE TRANSDUCER DIAGNOSTICS

Motor, Feb 2007 by Thompson, Bernie

A pressure transducer converts an engine's operating characteristics into oscilloscope-ready electrical signals. Skillful interpretation of these 'vital signs' can save valuable diagnostic time.

Black with a 2-in. brim and a red satin liner, it looked like a normal top hat any man might wear at a formal gala. The man placed the hat on the table. An instant later, he reached in and pulled out a white rabbit! How did the rabbit appear? Was it magic? To members of the audience, of course, it remains a mystery.

Mechanics have been trying to solve the "mysteries" of the internal combustion engine for over a century. In that time', many tools have been developed to help with this process. With the advent of the modern automobile have come high-tech diagnostic tools. Let's pull our own rabbit out of the hat and examine the "magic" behind one such high-tech diagnostic tool: the pressure transducer.

A pressure transducer takes a physical quantity and changes it into an electrical signal. A pressure transducer can measure physical quantities such as oil and fuel pressure, engine compression, plus exhaust, intake, crankcase and radiator pressures, to name just a few. By viewing this electrical signal on an oscilloscope, a large amount of information can be conveyed to the technician quickly. This device will change the way modern technicians diagnose problems in the internal combustion engine.

We'll begin by examining a Dodge Caravan with a 3.OL V6 engine and overhead camshafts. This vehicle was brought into the shop because it was exhibiting a rough idle condition. The complaint was verified and the PCM codes were pulled. There were no pending or mature DTCs recorded, and all of the monitors had run. A pressure transducer was placed into the exhaust tailpipe (photo 1 on page 36).

This is a special type of transducer called a differential pressure transducer, and it can read the exhaust pulses from the tailpipe. For years, technicians have used a hand or a dollar bill to feel or see these exhaust pulses in order to de termine whether they were even, to help them diagnose the engine. If differential pressure transducer is connected to an oscilloscope, these exhaust pulses can be viewed as a waveform, which will help tremendously in engine diagnosis.

This waveform can't be understood without a trigger to locate the exhaust pulsations. It the ignition is used as the trigger, the exhaust pulsations can be related to individual cylinders. To accomplish this, the firing order must first be known (Fig. 1 on page 36). There will also be a timing issue when applying the trigger to the exhaust waveform.

In a four-cycle engine, the ignition spark occurs at the end of the compression stroke. During the compression and power stroke's, both tlie intake and exhaust valves are closed. When the spark ionizes the spark ping electrodes, the air/fuel mixture is ignited. In turn, the burning air/fuel mixture creates an expanding force that drives the piston away from the cylinder head. As the piston approaches the bottom of its stroke, the exhaust valve opens. The high pressure inside the cylinder moves to the low-pressure area outside the cylinder, which creates a outside as it moves through the exhaust pipe.

The piston now starts to move toward the cylinder head on the exhaust stroke pushing out the remaining content of the cylinder into the exhaust system. Il you're using the ignition as the trigger for the exhaust pulse the're will be a delay 4between the spark ionizing the spark plug electrodes and the exhaust stroke. To compensate for this delay, the trigger must be moved from cylinder 1 to cylinder 3. By moving the trigger two firing events after the firing event in cylinder 1, the exhaust pulse for the No. 1 cylinder will align with the trigger event. Therefore, on a 4-cylinder engine, the trigger is moved one cylinder after No. 1. On a 6-cylinder engine the trigger is moved two cylinders after No. 1. On an 8-cylinder engine, the trigger is moved three cylinders after No. 1.

In Fig. 2 on page 38, the yellow trace is the waveform produced by the differential pressure transducer, the red trace is the waveform pnxluced by an inductive clamp around the cylinder 3 spark plug wire and the green trace is the waveform produced by the ignition coil primary signal. The addition of the ignition triggers will divide the exhaust waveform into individual cylinders. Once the waveform can be isolated into individual units, it can he analyzed to determine where the problem cylinder or cylinders are located.

The firing order must he known at this point, so an association can he made hetween the exhaust pulse and the cylinder that created it. When examining the exhaust waveform, two things must he checked-the amplitude of the signal and the timing placement of the exhaust pulse. Of the two, timing placement Ls more important.

When analyzing Fig. 2, the peak amplitude (vertical) on cylinders 1-3-5 is greater than the peak amplitude on cylinders 2-4-6. Now check the timing placement of the peaks on cylinders 1 and 2. The peak fin cylinder 1 comes very close in time (hori/.ontal) to the green primary ignition turn-on signal. The time hetween the cylinder 1 peak and the green primary falling edge is 1.69 milliseconds (mS). On cylinder 2, the peak is much further away from the green primary falling edge, at 6.76mS. Now check the other cylinders.