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

Thursday, February 1, 2018 - 09:00
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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.  

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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.  

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