How to Troubleshoot a Speed Circuit Malfunction
Speed circuits ensure that some mechanical part is rotating at the proper speed. In automobiles, speed circuits help transmissions shift at the correct times, and make sure that engine parts and wheels are turning at the right speeds in relation to each other. In large DC motors, the speed circuit ensures that the motor is turning at the correct speed as the load changes. Speed circuits can fail in two different ways: False positive failures are when the detectors report a nonexistent problem, and false negative failures are when the circuit does not report an existing problem.
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Instructions
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Find false positive speed circuit problems in automobiles with a code checker. These are handheld tools for diagnosing automobile problems. They plug into the solid-state electronics in newer cars and display codes for various abnormalities; for example, the code reader for Chryslers, Dodges, and Jeeps lists the ignition/distributor engine speed circuit malfunction as P0320.
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Fix a false positive such as P0320 when there is nothing wrong with the distributor or ignition system by checking the sensor and associated wiring. Cleaning off the sensor and reinstalling it often clears up the problem; otherwise, substitute a new sensor, and see if the problem persists. Check the wiring between the sensor and the central electrical system, as a short in this wire can also cause a false positive reading.
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Discover a false negative problem with the speed circuit when something goes wrong and it is not reported by the code checker. It is not enough to fix the problem; it is also advisable to check the sensor system as for a false positive – examine the sensor and the wiring.
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Troubleshoot speed circuit problems with DC motors in the same way as for automobile speed circuits – problems with no diagnostic warning are false negatives, meaning something is wrong with the speed circuit, and diagnostics with no real problems also mean something is wrong with the speed circuit. With DC motors, the best check is putting a multimeter in the speed circuit and making sure the voltage fluctuations match the specifications that came with the motor. The fix is often the same as in the automotive case -- clean or replace the sensor, and make sure that the wiring is intact.
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Tips & Warnings
Often, the problem is simply dirt. Cleaning the inside of a DC motor controller will make the electronic components run cooler, which often solves the problem. Cleaner automotive sensors will also correct many problems.
Sometimes, problems that do not actually show up as malfunctions, and are unrelated to the speed circuit, can cause what looks like speed circuit trouble. For example, bad brushes in DC motors and electrical arcing in ignition coils can both show up as speed circuit malfunctions. Fixing these problems often prevents major troubles later on.
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