Mechanically Speaking: ‘Known good part’ not always best answer

"Many times automobile components cannot really be tested step by step. There is no simple method of testing them."

“Replace with known good part” is a statement that you used to come upon often when trying to diagnose what is wrong with a customer’s vehicle. The Japanese vehicle repair manuals seemed to rely on this step frequently. It was often used in the process of elimination deep within a diagnostic trouble tree.

Trouble tree? That is one of those flowchart things that asks your mechanic to perform certain tests where the outcome establishes the next step in solving a problem. If the voltage at connector B1 pin 36 is greater than 11 volts then go to step five otherwise go to step seven. Get my drift?  Many times automobile components cannot really be tested step by step. There is no simple method of testing them.

Some manufacturers believe that their mechanics have no need for any inside information on the way certain parts work. When your mechanic establishes by following the flowchart that all the signals going into a part are correct and all the wires leaving that part are in working order then it is time to replace the part with the aforementioned “known good part”.

We all (auto mechanics in general) have a dusty shelf or bottom deep tool box drawer harboring some known good parts. The known good parts of choice were ignition modules and alternator voltage regulators. You might have a wiper motor or an idle speed control motor as well.

In every group of technicians there are those that have a much larger selection of known good parts. These types are the ones that skip the testing part of the trouble tree and go straight to the known good part swap. It can be faster. And it sells parts. Does it make satisfied customers? I do not think so. A lot of good parts are replaced with good parts.

Modern day technology is making part swapping a very costly approach to diagnostics. In many cases it is now impossible. A modern automobile is a myriad of electronic parts. Many of these electronic parts are essentially small computers.

Most of these small computers are programmed to operate with the car they were installed in originally. The software within them may be locked to the car.

Most vehicles now have some type of immobilizer system (anti theft system)  built into them. When you put the key in the ignition and turn it or push the start/stop button a series of secret handshakes ensues. All the handshaking modules must be satisfied that they belong together.

Take the engine control computer from a running identical vehicle to your own and install it and it will not work. Without technical wizardry the other modules in your car will not shake hands with the intruder.

Fortunately some manufacturers make it possible for your mechanic to program their modules to work in different vehicles. This makes a known good used part a possible solution to certain automotive woes.

To succeed, programming equipment and access to the manufacturer’s software must also be in your mechanic’s drawer along with the known good parts.

Trail’s Ron Nutini is a licensed automotive technician and graduate from UBC. E-mail:


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