Catching a waveform on a scope screen is easy. It's catching the intermittent anomaly or glitch that requires a little more skill.

I have found that if you focus on the suspected problem, rather than thinking about what normal should be, it is easier to set the scope to capture the shot.

Fuel injectors are seasonally troublesome in some areas. Generally they tend to short rather than go high resistance. Many times the windings short as they reach higher temperatures. This type of failure is usually intermittent in nature and may occur in much the same way as a secondary ignition system breakdown, under load conditions. The normal diagnostic routine for this type of miss usually includes scoping the secondary ignition system first. The problem, many times, doesn't show up in the bay while doing a stall test with the scope hooked up. The next step is usually selling the customer on some routine secondary ignition system maintenance. Now you have new plugs, wires, possibly cap and rotor if it's not distributorless. You have made money and it's been a good job so far. Then comes the road test. The miss is still there. Now it's time to get down to diagnostics and figure out what the problem is.

Let's say the car is a port fuel injected GM 3.1 V6. You suspect the injectors could cause the problem but they are buried under the plenum and a resistance test of the injectors won't tell you how they perform under a load. Here is where a good current probe and scope technique can save you some time. The injector current is the key to determining if this problem is related to an intermittently open or shorting injector. These injectors are gang fired, and divided into two circuits until they reach the computer, where they connect together again and all six are fired at the same time. You can easily pick up the current anywhere from the return side of the injector circuit green and blue wires, to the ECM. You can get it from the fuse circuit also but on some models, the fuse also feeds other circuits.

Clamping the current probe around one of the injector bank wires will give you a pattern at idle of three injectors, which will look like the image in Figure 1. The voltage signal was taken from the ECM side of the injector circuit.

This is the normal pattern for one bank of injectors at 3 mS pulse width. Note that the current peak will change with pulse width and voltage level.

This comes into play especially when checking cold cranking pulses.

Figure 2 was taken from one bank of injectors while cranking the engine cold.

These injectors measure around 12 ohms out of the box. If you divide 12.0 by 3 to get the parallel resistance for one bank, you get 4.0 ohms. Multiply 4.0 ohms times the current (3.35 amps) and you get 13.4 volts. That doesn't sound too bad at first, but if you take into account that the reading was taken while the engine was cranking, when the voltage is much lower (see Figure 3), around 8 volts at the injectors in this case, the math comes out a little different.

8 volts divided by 3.35 amps = 2.38 ohms for one injector bank. With 3 injectors: multiply 2.38 X 3 = 7.16 ohms average for each injector. If just one were shorted you could take the normal current for the other two from the total current to find out how low the resistance is on the shorted one. At 8 volts, each 12 ohm injector should draw .67 amps X 2 good injectors = 1.34 amps. Subtract that from the total current 3.35 - 1.34 = 2.01 amps for the remaining injector. 8 divided by 2.01 = 3.98 ohms. The bad injector from this car measured 4.0 ohms with an ohmmeter.

If you are getting your share of CO in your shop and maybe thinking about the wolf at your door, the math may not come so easy. You may find that the comparison method of testing injectors works a little better (see Figure 4). Technicians (myself included) seem to relate better visually than textually, especially when calculating is involved.

The pattern in Figure 4 was obtained by taking a reading of each injector bank, storing it, then downloading them both to the computer program for comparison. I call this the Sesame Street Method. You know, "One of these things is not like the other ..."

The green bank of injectors is drawing nearly double its blue companion bank.

Not much math needed here to know the plenum is coming off.

This has been the standard way of testing this type of injection system for a long time. Now I would like to introduce another method called "The Current Differential Technique."

This is probably the fastest and most effective way I know to test bank-fired injection systems.

Figure 5

To do this you simply cross the injector control wires in your current probe so that the current is going through the probe in opposite directions Figure 5).

Since the current for each bank should be the same, sending them through the current probe in opposite directions will effectively let them cancel each other out. The only pattern that shows will be from the difference in the current for each bank of injectors, which should be a flat line at zero current on a good system with normal resistance injectors.

Note the telltale tail in Figure 4. This is evident usually when one or more injectors are severely shorted. It is evident only on the bank with the shorted injector.

Figure 6 is the pattern I retrieved from the Corsica with the 4 ohm injector on one bank using the differential current method.

This technique works equally well to find high resistance or open circuit injectors. With an open circuit you won't get the tail, but you will get a differential pattern of one injector if it goes open. You can set your scope to record or min max and test drive to find an intermittent short or connection problem.

Figure 7 shows what you will see if you have one injector open using the differential technique.

This looks just like what you see when you hook the probe around one injector. But here you are seeing the current for three injectors minus two injectors.

Believe me when I tell you it is a lot easier to spot this pattern in a flat line than it is to see the drop in total current from six injectors to five. As one of my old customers would say, "It sticks out like a cranberry in a snow storm."

This technique also allows you to blow up the differential enough to see slight variations in injector opening current changes (Seagull Effect), which can show you sticking injectors. These variations show up best when they act up the most (during cranking), due to the lower available voltage. The lower voltage slows down the opening time and gives a much clearer picture of the "Seagull Effect" (see AutoInc.'s Tech to Tech column from July 1999, which is now online at http://www.asashop.org/autoinc/july99/techtotech.htm).

To give a better picture of how this technique can be used, consider the following example. Figure 8 was taken from a 1989 Chevy Cavalier with a bank-fired V6. By getting a current reading of each injector bank and then stacking them you can see there is a slight difference in the current waveforms.

The scope is already set to show the largest difference in the pattern without going off the screen. Note that the injector is not shorted to the point of leaving the negative tail spike. Convincing a customer to buy an injector based on this picture would be difficult at best.

Now consider the waveform in Figure 9.

Since by using the differentiation technique you are canceling out the bulk of the current and only showing the difference from one bank to the other, it now becomes possible to switch to a lower scale and dramatize the difference to an aesthetically pleasing level.

It becomes a lot simpler to explain to a customer that this waveform is caused by the bad injector and should not be there at all. Especially when - after the job is complete - you get to show him his flat line.

This last picture (see Figure 10) is showing all three waveforms stacked.

This new technique is fun to use and when correctly implemented into your diagnostic routine can help you spot bad injectors before they become troublesome intermittent problems and enables you to sell them as maintenance items, saving both you and your customer from unnecessary headaches.

I like to go over the diagrams with the customer and explain in the simplest terms what the waveforms mean. You would be surprised how interested they are in seeing their car's "X-rays," if you will. It is not important that they have a total understanding of the waveforms when they leave, just the confidence that we do.

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is northstarguy@zoomtown.com

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