Using scopes to be correct, not appear correct

scope & scan scopes vehicle scope patterns diagnosing vehicles vehicle diagnostics repair shop training technician training automotive aftermarket Probably since there have been spark ignition engines, there have been ways to set and/or measure when the spark happens inside the combustion chamber. As a poor teenager, I would use a parking light bulb to time the ignition points, KOEO, on my 1964 VW Beetle. And the stroboscopic timing light was the tool of choice for much of spark ignition history.

Use of timing lights became infrequent with the advent of non-adjustable ignition systems. Those of us who still own one occasionally drag out the old timing light to check if an ignition system is firing where it should be.

Using a light shining on the crank balancer was the easiest method to check spark timing. But this method only indicates where the spark timing is occurring by the supposed correct correlation between the crank balancer, the balancer timing marks and the crankshaft. Using this method, the timing can appear to be correct without actually being correct relative to actual piston position.

The top set of waveforms shown in Figure 1 is the in-cylinder pressure and secondary ignition waveforms from a Ford Explorer that ran poorly. The tech suspected the ignition timing might be off because of the low power and high exhaust system temperature. However, when he used his timing light to check ignition timing, it showed correct timing. As you can see by looking at the top dead center (TDC) peak pressure area of the pressure waveform, the ignition event happens after TDC.

This certainly explains the technician's finding, but what went wrong with the testing? Apparently, the crankshaft balancer hub cracked and allowed the crankshaft to spin out of time with the balancer. On this vehicle, the balancer also has the crankshaft position sensor (CKP) reluctor wheel mounted to it. This balancer/reluctor wheel misalignment caused the power control module (PCM) to command the ignition event at the wrong time relative to piston movement. Using the in-cylinder pressure waveform along with the secondary, primary or ignition timing command waveform can give you actual information about where the spark event is happening relative to piston pressure.

You may ask, "Wouldn't a crankshaft versus camshaft sensor (CKP/CMP) waveform give us the same information without having to pull a spark plug?" Yes, it would if a known good waveform is available.

The lower portion of Figure 1 shows the CKP/CMP waveform along with the pressure and secondary ignition waveform. The circled areas show the signature peaks of both waves, which when compared to a known good waveform set is wrong. But what if you do not have a known good CKP/CMP waveform available? Or what if the CKP/CMP waveform is one of the "non-signature" variety?

The lower section of Figure 2 shows just such a waveform. The variable reluctance CKP sensor waveform shown in blue has no signature anomaly from which we can make a relative count against the 180-degree CMP sensor waveform shown in green. Exactly where should the 180-degree CMP mark land on the CKP? The additional 360-degree CMP waveform is also a non-signature type.

This isn't a problem. Simply use your in-cylinder pressure transducer along with an ignition waveform from the same cylinder and you can clearly see that this engine is firing out of time due to the camshaft timing being wrong and causing the in-cylinder pressure peak to occur at the wrong time relative to correct ignition timing.

Cam sensors that are mounted to an adjustable shaft are another problem when using CKP/CMP waveforms. Knowing where the CKP mounts and how the CKP reluctor wheel is mounted along with the pressure and ignition waveforms gives you absolute assurance where the spark is occurring relative to piston pressure.

We all have installed a distributor wrong. I have had many technicians with 20 or even 30 years of experience install a distributor and swear the distributor is correct only to be shown a pressure versus ignition waveform like the one in Figure 3.

This waveform set is a real time-saver on engines that appear to have low compression on the first hole tested. Without pulling another spark plug, doing a leak down test or pulling the timing cover, we know why the compression is low: it is out of time. You cannot have a mechanical defect that causes low compression and spark to occur at the wrong time in the compression cycle without being out of time.

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.