The main difference between the megohmmeter and the milliohmmeter is that the milliohmmeter is designed to read a very small amount of resistance. In order to get a precise reading the milliohmmeter utilizes a 4-wire resistance measurement in which two of the wires pass the current through the circuit while the other two wires make the delta (difference between two points) voltage measurement. The milliohmmeter further depicts the deficiencies of a traditional DMM for resistance measurement.
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Temperature of the windings is also critical to the test procedure. You may recall that as temperature goes up, resistance goes up. When you are dealing in milliohms the temperature becomes incredibly important. Toyota for example uses the temperature of 20 degrees Celsius throughout their service information. If you were to take a measurement of a motor generator unit that was not at that ambient temperature the readings may mislead you and lead to a bad diagnostic outcome. A formula will need to be utilized to adjust for the actual tested ambient temperature. Toyota suggests that you store both the meter and MG unit in the same are before testing. Additionally, Hioki specifies an operating heat range of 32-104 degrees F with a relative humidity of less than 80 percent.
Toyota’s factory service information subscription is one of the least expensive in the industry. To access Toyota’s information visit www.techinfo.toyota.com Within the TIS system you will find a tab entitled QTG for Quick Training Guides. Toyota provides a quick training guide on the usage of megohmmeters as well as milliohmeters to use as a reference for testing. These are fantastic resources for first-time users. Well worth the cost of a 2-day subscription (about $15).