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How to save time on your diagnostics

Tuesday, August 1, 2017 - 07:00
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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 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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