A look at the newest diagnostic tool for hybrid repair: The milliohmmeter

Feb. 1, 2018
The megohmmeter and milliohmmeter are two of the tools that you are most likely to see when diagnosing faults within the windings of motor generator units and the high-resistance, high-voltage wiring that carry the current for the motor generators.

The digital multimeter (DMM) has been around for quite some time and is an invaluable tool for performing a variety of tests including available voltage, voltage drop, resistance, diode tests and more. With advancements in hybrid and electric vehicle technology, new testing equipment may now be required to perform advanced tests outside of the capabilities of the traditional DMM. The megohmmeter and milliohmmeter are two of the tools that you are most likely to see when diagnosing faults within the windings of motor generator units and the high-resistance, high-voltage wiring that carry the current for the motor generators. New tooling brings us to a familiar crossroad of the questioning of our diagnostic strategies and whether or not the hybrid repair business is one in which we want to venture. We will look at the ins and outs of this new tooling and help build a case for intelligent diagnostics. 

(Image courtesy of Toyota Media) Diagnosing faults in the windings of a motor generator will require the use of a megohmmeter and a milliohmmeter.

Before we dive into specific tooling it may be helpful to refresh our knowledge of the units of measurement for resistance. The base unit of measurement is the Ohm, named for Georg Ohm, a German scientist who discovered and named the principle of resistance. Units of measure smaller than an ohm include milliohms and micro-ohms. Units of measure larger than the ohm are the kilohm, the megohm and the gigohm. A megohm is equivalent to 1,000,000 ohms while the base unit ohm is equivalent to 1,000 milliohms.  

The megohmmeter  

The megohmmeter, often known as an insulation meter, functions in a very similar fashion to the resistance or diode check feature on your traditional DMM. These features utilize a current that is sent through the leads to determine a voltage drop which is then displayed as a unit of resistance measured in Ohms. We have been taught that using the resistance function on the DMM does not give us an accurate indication of a circuit’s integrity due to the fact that we are not testing the circuit dynamically. However, that notion is misleading. We are testing dynamically but the meter does not produce enough voltage or current to truly load the circuit. This is why voltage drop testing a live circuit is so effective because the current within the live circuit Is providing the “dynamic” portion of the test. The DMM is simply reading the results. The lack of voltage and current is the reason that there are two different settings for resistance measurements and diodes to begin with. The resistance function utilizes approximately 2-3 volts and relatively low current. This amount of current is not enough to properly forward bias a diode while yielding an accurate voltage drop measurement.

(Image courtest of Fluke) The Fluke 1587 is a popular megohmmeter for hybrid insulation testing.

In plain English, when using the Ohms function to check a diode your resistance measurement will most likely be inaccurate. You will be able to see that the diode passes current in one direction and block it in the reverse polarity but the actual voltage drop reading displayed on the meter would be wrong. The diode test function provides enough voltage and current to forward bias the diode and will yield an accurate voltage drop measurement. When it comes to testing the windings of an electric motor such as those found in hybrid electric, electric and fuel cell vehicles the DMM simply does not have the ability to reach the voltage or current required to “dynamically” test the windings.  Additionally, using a voltage drop test for this type of circuit is dangerous and should be avoided at all costs. The megohmmeter allows the user to test these high voltage components in an isolated environment while the high voltage system within the vehicle is disconnected. Think of these tests as you would a pressure test of a hydraulic circuit. If you were to pressure test a hydraulic circuit you would most likely pressurize the system in excess of the typical system pressure in order to determine if there are leaks in the system. The insulation of high voltage systems is very similar to that of a hydraulic system and insulation of the wire is very much like the hose or pipe that carries the hydraulic pressure. If the pressure compromises the insulation, a faulty reading will be displayed on the meter. For the sake of your safety, do not attempt to test these circuits while the high voltage system is operational. Follow manufacturer instructions and perform your testing accordingly.  

Diagnosing with a meg or milliohmmeter requires the component or harness to be isolated from the HV circuit.

Megohmmeters are readily available through the name brand electrical testing companies you are already familiar with and can be purchased for between $500-$700. Whether or not this investment may prove to be worthwhile might depend on how committed you are to hybrid repair as well as the frequency of these types of repairs that you might see in your shop.  

Reality check 

Last year I received a call from an acquaintance who does a fairly significant amount of diagnostic work. He called with the complaint of a P0A7F diagnostic trouble code with an information code of 555 on a late model Toyota Prius. This particular code had the technician concerned due to the fact that the diagnostic flow chart for this code within Toyota’s service information was over 150 steps long and included the use of a megohmmeter which he did not own. Additionally, following the flow chart indicated that there was a problem in one of three major areas: the hybrid transaxle, the inverter assembly or the high voltage ECU. None of these yielded themselves to a silver bullet fix or the ability to substitute a known good part. The call was one of those we have all made at some point looking for a work around for a diagnostic process that included tooling we just don’t have.  

Diagnosing windings within the MG units may require the use of a megohmmeter and a milliohmmeter.

One of the amazing things about Toyota as a company is their commitment to “Kaizen” processes or, “constant improvement” as it translates. This has led Toyota to build updates into their service information in the form of service hints which appear in TIS documents in green font and usually proceed service and diagnostic procedures. In the case of the P0A7F, Toyota indicated a hint that pointed the technician toward the hybrid inverter assembly. Surely it couldn’t be that simple could it? In this case it was. The choice then became whether to buy a new inverter or replace the Intelligent Power Module within the inverter. Both were prohibitively expensive. This led to a call to a local salvage yard and the acquisition of a complete inverter unit for $250. A few days later I received a call back that this car was fixed and returned to its happy owner. 

Diagnostics are never quite as easy as that example but this story points to a larger reality. The person who wrote that hint most likely used a megohmmeter to get to his diagnosis but the techs that followed were able to bypass those steps. Toyota had their fair share of issues with inverters and technical service bulletins and service updates were readily available to point technicians in the right direction. While every diagnostic scenario is unique, the frequency in which you will use the megohmmeter may help make up your mind as to whether or not it’s a part of your tool-set. For the hybrid specialty shops it may be a no-brainer but for the typical shop performing hybrid maintenance and repair work the answer may not be so cut and dry. 

Toyota provides quick training guides for megohmmeter and milliohmmeter usage with a TIS subscription.

The milliohmmeter  

While many are still adjusting to the concept of megohmmeter usage, certain DTCs relating to the motor generator units will now require testing with a milliohmmeter. The milliohmmeter specified by Toyota is the Hioki RM3548 which retails for about $1,200. Much in the way that the DMM was limited in its ability to test high resistance values, the DMM also cannot measure very small resistance values to verify good connections. These milliohmmeter offers the ability to measure resistance between phases of the motor generator units when dealing with trouble codes relating to the MG units.  

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.  

Toyota provides hints and TSBs for complex hybrid faults, which often bypass the need for advanced tooling.

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.  

Additional resources   

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).

About the Author

Dave Macholz | Contributing Editor

Dave Macholz is an instructor for the Toyota T-TEN, Honda PACT and general automotive programs at Suffolk County Community College in Selden, NY. He is an ASE CMAT and L1 technician and holds a NY State teaching certification in vehicle repair. 

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