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Haas inexplicably created a troubleshooting document for their vector drives that uses RESISTANCE to check the power components. This is a flawed method. We have MANY people calling us to repair their vector drives after improper troubleshooting that incorrectly identifies the vector drive as the problem.
Due to the nature of IGBTS (transistors) in use with flyback diodes, using RESISTANCE on a meter may yield false positives in the troubleshooting process. This is due to the fact that a IGBT that is turned off has a very high resistance from Collector to Emitter. A typical meter should measure MegOhms or "OL" (out of limit). If the circuit was only measuring an IGBT, you could possibly get by with this check. However, these IGBTs have what is called a Flyback Diode in parallel with the IGBT. Measuring resistance across a diode can yield unpredictable results, and is dependent on polarity.
Basically, Haas is instructing you to check diodes with a Resistance meter instead of a Diode meter. The CORRECT way to do these measurements in the field is by using the Diode check on your meter. While polarity is still very important, it will give you predictable readings. Please visit our troubleshooting link for the correct procedure, and feel free to keep a copy of our downloadable PDF in your service bag. Troubleshooting | CNC Service Company