A few months ago, while teaching a seminar in Kansas City, an attendee approached me during a break. He posed a question, not about how to fix a car, but rather, how a car had fixed itself.
The vehicle in question was a Ford product that had been setting diagnostic trouble codes for the manual lever position (MLP) sensor. The technician obtained passing results on all the wiring and, following Ford’s diagnostic trouble chart, replaced the faulty MLP. Then he cleared the code and returned the vehicle to the customer.
It came back three weeks later with the same trouble code.
Casting the trouble chart aside and arming himself with a wiring diagram, the technician opted to backprobe the MLP connector and operate the vehicle while watching the voltages on a scope. After an extended test drive, the issue never returned.
He delivered the vehicle to the customer and the complaint hasn’t returned for weeks. The technician’s question: “Why didn’t the intermittent issue return?” My answer: “You fixed it by backprobing the connector.” Of course, my answer was only based on experience and speculation. In addition, the “fix” was by no means permanent.
The most likely issue was poor pin tension because the female end of the connector was spread slightly. Forcing a backprobe pin into the connector obtained a reading, but also inadvertently bent the female connector so it gripped the terminal tighter. This testing method “fixed,” or more appropriately, hid, the cause of the DTC.
Here’s a thought you may find frightening: If the technician had pierced the wire instead of backprobing the terminal, he’d have found the problem and the vehicle wouldn’t have “fixed itself.” Of course, this means I advocate piercing wires for testing and I do so on a regular basis.
They recommended backprobing to check the signal while maintaining the integrity of the insulation.
Maybe they did this to preserve the wiring harnesses of the college’s vehicles that were being poked and prodded daily. I personally followed this practice for years until connectors got smaller and tension issues became more common. Besides, they also taught me how to repair wires, right?
A highly respected colleague and friend of mine, Eric Ziegler, hit the nail on the head when he said, “My position is not whether or not you backprobe… that ship has sailed. The question is how you work around it with terminal breakout kits or by piercing. There are lots of options for piercing; pick the least intrusive method and leave the wire the same as you found it.”
There’s a variety of tools designed specifically for wire piercing. A plain old pin can do the job, but it’s more difficult to insert and more likely to do additional damage to the wire. A better option is the Pomona 6405, which holds the wire in place. Turning the end of the tool pushes a pin directly into the center of the wire. It also lets you access the wires running through some harder-to-reach areas.
Another option is the uSignalTap, available through www.aeswave.com, that’s similar to the Pomona 6405 but is longer for harder-to-reach wires. There are many other wire-piercing tools on the market and their prices usually range from $10 to $60, which often includes multiple piercing tools or sets (figure 1).
Breakout lead sets are also available that allow you to separate the connector and install a Y-adapter (figure 2). This technique becomes more difficult as the number of cavities in the connector increase. If the connector is easy to reach and doesn’t have too many pins, these tools could be a viable option.
Okay, suppose you pierce a wire and obtain a reading that indicates poor pin tension. How do you test pin tension? Again, this task boils down to having the correct tool. Often called drag testers, these tools allow you to insert the appropriate-sized pin into the socket and draw the tool back out slowly (figure 3). You should feel some drag, indicating proper pin tension.
Multiple cavity connectors are the easiest to test because you can compare your results to other cavities of the same size. Since there are so many different connectors, these tools usually come in sets that can cost from $60 to over $200. They can be pricy, but they’re worth the investment, especially when you need them.
If you pierce a wire, you’ll need to repair it properly. As quoted earlier, “leave the wire the same as you found it.” There are a lot of techniques for repairing wiring, but some are better than others. Crimp connectors, for example, aren’t a good choice. Crimp-type connectors with glue are better, but still not the best choice.
The best option would be to solder and heat-shrink the pierced wire, but this may not be practical due to space constraints.
My preferred option is liquid electrical tape (figure 4). It’s available in a variety of sizes and containers. The smaller tubes are the easiest to work with and allow you to apply a small amount of sealant to the pinhole in the wire.
Some technicians like to use clear nail polish to seal the damaged wire, which is another acceptable repair. Never use silicone sealant: It attracts moisture as it cures and could allow the copper wire to corrode.
A SIDE NOTE: CUTTING
Cutting a wire may also sound frightening, but the same rules apply. For example, a vehicle isn’t communicating with the PCM. You confirm power and ground to the PCM, but notice the 5-volt reference is at zero.
This 5-volt reference wire feeds multiple sensors, some of which aren’t very easy to access. If one of the sensors on the 5-volt line shorts out, it could affect the entire circuit.
To separate the PCM from all the sensors quickly, you could cut the 5-volt wire and see if communication returns. If communication returns, then you’ll need to chase the shorted sensor. If there’s still no communication, suspect the PCM.
Most importantly, you’ll need to repair the cut wire correctly before returning the vehicle to the customer.
Piercing and cutting wires can be part of today’s daily diagnostic process. You can still backprobe connectors as long as you’re aware of the potential pitfalls. Once you get over the fear, you can pierce, test, repair, and move on with your diagnosis.
Engine or electrical diagnostic issues you’d like to see addressed? Let Scott know. Send him an email at email@example.com and you just may have your question covered in a future issue of GEARS.