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How to save time on your diagnostics

Tuesday, August 1, 2017 - 07:00
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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!

Figure 2 - The high amp pattern (blue) and the two AC Coupled patterns are somewhat similar, aren’t they?

Take a look at Figure 2. The blue trace is a conventional starter cranking current test using a high amp current clamp at the battery. The green trace is also directly at the battery but is a voltage reading that is AC coupled, showing the AC ripple produced by the alternator. The third trace, the gold one, is at the Diagnostic Link Connector by way of a breakout box (or “BOB”), and the leads are connected to pin #5 for ground and pin #16 for B+.

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.

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