How to save time on your diagnostics

Aug. 1, 2017
Becoming efficient, at least in my mind, is more about taking knowledge and applying it to a given situation. This may be system-specific, or it may be one of technique, or a combination of both.

For most of us sweating away in the service bay, being fast and efficient means earning a bigger paycheck by the end of the week. I’ve always maintained that “fast” comes with experience and encourage younger techs to first focus on “right”. But becoming efficient, at least in my mind, is more about taking knowledge and applying it to a given situation. This may be system-specific, or it may be one of technique, or a combination of both. Knowing what you’re doing, no matter how many years you have in the business, goes a long way in making the job go smoothly.

This month, I’d like to share a few tips that may help you add efficiency to your diagnostics, and as a result – add some cash to your wallet.

Testing Engine Mechanical Condition

I learned from being bitten once too often that insuring the engine is mechanically strong before looking elsewhere for the cause of a drivability concern was a good idea. However, testing compression conventionally with a mechanical gauge is time-consuming and certainly not efficient! Better is to use the relative compression test, using your scope.

Figure 1 - This screen save is an example of excessive AC ripple that would be hard to see on most scopes without using the “AC coupling” feature.

Now, we’ve written and discussed the process for relative compression testing many times in the pages of this magazine so I’m not going to rehash it all here. Typically, though, the contributors that have demonstrated this test method always use a high amp clamp to capture the starter current pattern – one basis for this test method.

But it’s not the only one, and owning an amp clamp is not a prerequisite to performing the test. All you need is a single channel of any scope that allows you to select “AC Coupling” as an input for the voltage scale. What does that do? It eliminates the DC component from the waveform and allows you to see just the AC component of the voltage signal you’re attached to. This comes in handy, for example, when inspecting the alternator diodes for failure by measuring the amount of AC voltage present in the DC output. Figure 1 is an example of AC ripple. The leads are attached directly to the battery and if the normal DC signal were present, we would see a somewhat fuzzy line across the scope at around the 13.5v level. But to zoom in and see the reason for the “fuzz” would be nearly impossible for most scopes. Notice, too, how the AC signal remaining is passing over the “0” line, providing us with a measurement of just how much AC ripple there is. Anything over 0.50 VAC is too much and indicates a failed diode.

But I’m getting off track a bit. There is some AC ripple present even on healthy charging systems and since the alternator is being turned by the engine, we can use that signal the same way we use the starter current. And we can track the signal from a variety of sources to make our lives even easier!

Notice how similar they are? What, we’re not done yet.

When most of us perform any kind of current test using our scopes, we tend to orient the pattern so any current “draw” appears as a positive reading up from the “0” line. Actually, it’s a negative number, isn’t it? But that would be harder to relate to, for me anyway, and I hate making anything harder than I have to. I have enough trouble as it is!

Figure 3 - By inverting the voltage leads (neg-pos, pos-neg), we’ve reversed the AC Coupled patterns. Look how similar they are to the current pattern!

Rather than get used to a whole new way of viewing a relative compression pattern, let’s cheat and simply reverse the leads for the two AC Coupled channels and see what happens. That’s what you see in Figure 3.

Note now that all three patterns are nearly identical.

So next time you do a relative compression test, try this method. And if you have never performed a relative compression test because you don’t own a high amp clamp – your welcome!

Backprobe Or Pierce?

Continuing on the scope theme, let’s talk about acquiring a basic signal. The first thing we need to do is decide on how to connect to the circuit we want to test. Typically, that boils down to two choices; backprobe or pierce.

I was recently told by a GM factory employee that GM is telling their dealer technicians that backprobing is a “no-no” and should not be performed. And there are some drawbacks to the technique. First is proper placement of the probe. Take a look at an example of a bad placement in Figure 4. Notice how the probe is puncturing the weather seal, between the seal and the connector body. This is no different from piercing the insulation and left as it is, will allow moisture to enter the connector and cause problems down the road. It is imperative to place the probe carefully by following the outside of the wire you want to connect to, pushing the seal aside as you press through to the connector.

Figure 4 - It’s a bit hard to see, but in this example the back probe is on the outside of the weather seal – not good.

This also helps insure that you come into contact with the connector you intended to, and that leads to potential drawback #2. What I mean is, on some connector blocks it is easy to place your probe in pin #13 but end up skewed a little sideways and actually come into contact with pin #14 or #12. On some rare cases, you could even short two together and that could be a very bad thing!

A third drawback to backprobing a connector is dependent on what you are testing the circuit for. If you are looking for a source of voltage drop, or otherwise testing the integrity of the connection at the connector, backprobing could allow you to bypass the internal connection and lead you to a false test result.

Piercing is certainly easier but has drawbacks, too. First, make sure you only apply enough pressure with your piercing probe to come into contact with the wire strands underneath as shown in Figure 5. No need to crank it down! If you do, you could actually break some of the wire strands and that adds a source of voltage drop to the circuit.

Figure 5 - It is only necessary to pierce the insulation enough to make contact with the wiring. Too much pressure could cause some of the wire strands to break. Figure 6 - These breakout leads allow you to tap into a circuit without harm to the wiring or the connection (as long as you select the correct lead, that is).

Second, you are obviously going to leave a hole in the insulation when you’re done. It is critical that you seal the hole you made with some liquid electrical tape. In a pinch, nail polish will also work.

No matter what technique you use, there is a risk of damage to the wiring and with so many circuits under ECU control, carrying vital information on low current lines, any introduction of added resistance could spell trouble. There is, however, an alternative.

My friends at AESWave.com sent me a set of breakout leads (made by Pico) that can be used to tap into most circuits with no damage to the wiring insulation or the connector. The kit contains a variety of sizes to accommodate most any terminal design in use and they are simple to install. I grabbed my test headlight to give you an idea of how they work – take a look at Figure 6.

You can even make your own. Long ago, I made a set of six test leads with replaceable male/female connector ends so that I could adapt to whatever connector I wanted to tap into. When I ran into one that I didn’t have the ends for, it was a simple matter to make them.

So, if your intent is the testing of a component or acquiring a signal, the use of breakout leads will make life easier for you and less painful for the wiring. Like any other technique we’ve shared, it is ultimately the focus of your test that will determine the best process to use.

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