Hello, I am Professor John Kelly and this is
the WeberAuto YouTube channel. Today I'm going to show you an amazing tool that somehow I have
never used before and never heard about and it is the milli-ohm meter now I had never heard of
a milli-ohm meter until a few months ago when I was reading an article in motor Age magazine I'm a
contributing editor to motor age magazine now and again and there's an article written by another
contributing editor Mr. David Macholz I hope I'm pronouncing that correctly he's a Toyota T-TEN
instructor out of Suffolk County Community College in Selden New York and the article is about megohmmeters
and milli-ohm meters now if you do any work on hybrid or electric vehicles you probably
have heard of the megohmmeter or giga-ohm meter the Fluke 1587 meter back here, and I've had one
of these since 2007, but I've never heard of the milli-ohm meter so let's take a look at why this
thing is so cool and why you may want one if you do any type of electrical diagnostic work not just
on hybrids but any type of electrical diagnostic work I think once you see what this thing does
you may want one now this is a very expensive tool it's around $1200 - $1300
depending on where you buy it okay just some basics about an ohmmeter. An ohmmeter
has really never been a very useful tool in my opinion because all it would do is show you if a
circuit was open or not open. the resistance value that you would measure with an ohmmeter could be
inaccurate if the power was on, which of course the power in the circuit is always supposed to be off
when you use an ohmmeter, but I've never really trusted ohmmeter readings in all of the time I've
been a service technician Ana professor so we will compare just a regular ohmmeter reading to the
milli-ohm meter in this video the megohmmeter the Fluke 1587 meter we will come back to in a
different video okay if we use just a regular old meter at a high-quality Fluke 87 series 5
multimeter and we rotate the selector dial to the ohms position we can measure resistance and I
have a piece of wire right here just a small piece of wire it's 39.3 inches long which is 1 meter
in length and we connect some good quality meter leads to the meter I've got alligator clips on the
the ends of these meter leads whenever you measure low resistance values with any multimeter you
should zero out the meter to compensate for the resistance of the meter leads so on the Fluke 87
meter there is a relative button with this little triangle or the Delta symbol we just push that
and now with our meter leads connected together we are reading zero ohms all right now if I come
in with our 1-meter long piece of copper wire and I connect one meter lead right there and the
other one on the opposite end we are reading 0.1 ohms of resistance for this 1-meter length of wire.
This wire here that we'll measure next is 1/10 of that so it's 100 millimeters and if we measure
its resistance I get the same 0.1 toggling to zero so almost the exact same reading with the
tenth of the lengths all right then I have a 10 millimeter long piece of wire right here and we'll
just clip that in here and I'm reading zero ohms okay so this meter wasn't designed to read real
small resistances clear down to 0.1 ohms is its resolution, which for most automotive work is fine
but let's take a look at we're measuring smaller resistances than 0.1 ohms would be possibly
valuable now let's do the same thing but with the milli-ohm meter now this milli-ohm meter is
from a company a Japanese company I'm probably pronouncing it wrong HI OKI or HIOKI. Its model
number RM3548 and it's called a Resistance Meter and this will measure clear
down to 0.1 micro ohms. so milli-ohm is thousandths of an ohm. micro-ohm is millionths
of an ohm, this will measure 0.1 micro-ohm, that's 100 nano-ohms it can measure at
its finest resolution, so incredible accuracy. well, let's prove it. The milli-ohm meter
uses two meter leads with alligator clips of course, but these meter leads if you look at it
have two wires per meter lead, so when you plug in the meter leads to the meter there's two wires
per lead, okay so let's measure this 1-meter length of wire we will turn on the milli-ohm meter there's
an optional temperature sensor here for the meter and I would highly recommend that you purchase
that otherwise you have to do a calculation to compensate for whatever the temperature of the
conductor is that you're measuring the meter we'll assume that it's at a certain temperature and
then if that temperature is wrong then you have to do a compensation calculation to determine the
correct resistance with the temperature sensor as long as the meter the sensor and the conductor
and the leads have in the same area for around six hours is what they recommend to let everything
stabilize to the same temperature then the meter will measure the resistance of the air in the room
that will be the same temperature as the part that you're measuring and then the resistance value
that it comes up with will be accurate because resistance is affected by the material it's made
of the temperature of the material itself the length of the material and the cross-sectional
area of the material so let's measure this 1 meter long piece of copper wire the instructions
that come with the meter tell us that when you take a measurement that you should wait about 10
seconds to let the meter stabilize and come up with an accurate reading so this milli-ohm meter
is showing us that this 1-meter long piece of wire has a resistance of 53.06 milli-ohms now
that's 0.05306 ohms, now our other meter our Fluke 87 meter that
wasn't designed for this type of low resistance readings was measuring 0.1 ohms not 0.053
ohms and the rule of course is to round up so in round-up mathematically
point one was correct but this is 0.05302 ohms, 53 thousandths of one ohm
Alright now if 1 meter is 53 milli-ohms then one tenth of the length
of that wire so 100 millimeters should be 5 milli-ohms so let's try this let's take the 1
meter length out let's put in the 100 millimeter or 1/10 of a meter and look at that we get
5 milli-ohms how cool is that? so a tenth of the resistance which of course it should be if one
meter is 50 milli-ohms and 1/10 of that this little 10 millimeter long piece of conductor I have right
here copper wire should be 0.5 milli-ohm so let's measure it next 0.59 it's a little longer than
10 millimeters 0.5 milli-ohm that's incredible accuracy I mean you might be wondering well how
is that useful for On-Car Diagnostics well in a hybrid vehicle, which is why Toyota is recommending
this tool you, have a stator winding for MG1 and MG2, and for any vehicle with stator winding so any
hybrid or electric vehicle is going to have stator windings of some sort it could be this individual
wire a Wye-wound stator like this older design stator right here and we can come in there's
actual Toyota specifications to come in with this milli-ohm meter and measure the resistance
of these windings so let's do that next all right so I'm going to take this meter and go from the U
phase connection to the V phase connection and we are reading 104.3 let it stabilize for 10 seconds
and notice this is at room temperature here in the shop which is pretty hot, 28.3 degrees celsius
so the U to V resistance of the stator windings is 104.4 milli-ohms, now
that is a 12 strand connection so there's 12 wire strands there now let's go from U to W remember we
were at 104.4 now we're reading 104.2 and the toyota specification is there should be
no more difference than 2.0 milli-ohms difference between each measurement, we were 0.2 milli-ohms
different between those two, now let's go from V to W 103.9 that's still a 0.5 milli-ohm difference
between the highest reading and the lowest reading which means this stator is not shorted it does not
have wires that have shorted out overheated lost their insulation and shorted out and are touching
each other but that is something that can happen on cars, and I have a 2002 Prius right over here
that we will go measure this vehicle has a problem with its transmission there's something wrong that
caused the vehicle to lose power and now when you try to drive it it's really jerky goes to choo,
choo, choo, choo, choo, choo, as you try to move it and I believe that it has a shorted stator and so
we will use this tool to measure that but before we do that let's look at a different style of
stator a newer design stator is called a hair-pin style stator and it doesn't use the small
little wires like we saw in that earlier Toyota stator this is out of a 2nd generation Chevrolet
Volt the 5ET50 transaxle, but the latest transaxle from Toyota also uses this hairpin technology,
what we mean by hairpin technology is each one of these big heavy pieces of flat wire are
actually shaped like a hairpin, it's a u-shaped piece that has been bent laid in place and then
welded together on the back here with another hairpin stater element and now we have all of
these individual welds that can have potentially poor connections and so we can also use this same
milli-ohm meter to measure the resistance of this style of stator so let's do that let's
take a measurement from U to V notice now we get 34.8 milli-ohms much, lower
resistance than we had before on the wire-wound stator okay now let's go from U to W 34.68
milli-ohm and then W to V 34.82 so all of these are easily within the
2 milli-ohm spec that Toyota gives us I don't know what GM would give us for a specification
because I have not seen this tool used or called for in any GM diagnostic yet but I'm sure they
will so this is a GM stator the other one was a Toyota but both of them have used both the wire
wound and the hairpin style welded stators here what we're looking for is consistency, we want
each measurement across each phase to be very close to equal, a tiny bit of difference is okay
but not a lot. Now before we go over to the car and take some measurements of the the stator
windings in the vehicle I want to show you a few more things that are amazing about this milli-ohm
meter so I I was wondering how accurate can this thing be and so I got just one of my alligator
clip style jumper wires here and I stripped off the insulation for about 50 millimetres of the
length of this jumper wire now I counted there are 24 strands a very fine wire in this flexible
jumper wire so let's measure the resistance from one end of this jumper wire to the other with all
24 strands and intact and then let's start cutting one strand at a time and seeing what happens
all right for the entire length of this jumper wire here with all 24 strands we have 36.44
milli-ohms of resistance, now if I just come in and pick out one wire strand I'm going to come
in and cut that wire strand and let's watch what happens to the resistance reading here so we're at
36.42 we cut the wire strand and we go up to 36.54 now
the reason we went up is because these wires are in parallel with each other so every time you
put wires in parallel with each other the total circuit resistance goes down so now the circuit
resistance has gone up so now we're at 36.50 let me pick out one more wire
strand 36.49 right now and we cut it and now we're at 36.65
so we went from 36.49 to 36.65 roughly 0.15 or 0.16
milli-ohm per strand. so I've got, I've cut two strands, so now I can predict what will
happen when I cut the next one if we go up by 0.16 milli-ohms per strand when I cut the next wire we
should go up to 36.80, 36.81 somewhere right around there so
I'm going to come and cut the wire here it comes it's cut there we go 36.81
exactly predictable is that incredible or what so we can get right down to determining wire strand
resistances and how many wire strands are broken in a length of wire so another incredible thing
here this meter can tell you how long the wire is so there's a length button right down here if
I go ahead and push that button turn it on we punch in the resistance per meter now we measure
roughly 50 milli-ohms of resistance per meter in this copper wire earlier so now if I hit enter the
word length shows up on the screen and I can come in and connect our meter leads to this 1 meter
length wire and now it tells us that it is 1.0662 meters in length so based
on the resistance of the wire, it can tell you how long the wire is that's really cool so now
let's go to this 100-millimeter length wire 100.11 millimeters it's now showing us and
now let's go down to this 10-millimeter length wire there 11.06 millimeters, now I
I still need to think about it, I'm still wrapping my head around how cool this meter is there's got
to be some really cool diagnostics things that we can do with this besides just check for shorted
stators on a hybrid or electric vehicle, alright one of my co-workers Will Spiegel was here in the
shop yesterday and he said hey I wonder if you could check the resistance of a fuse and measure
a voltage drop across a fuse based on that, so I thought wow that's a really good idea so I went
over to their 2017 Chevrolet Bolt EV and pulled out five of the spare fuses that are in the under
hood fuse block so now let me put the meter back to resistance measurements let's measure the
resistance of five common sized fuses so these are the little micro fuses the General Motors uses
this is a 10 amp fuse and we are reading 7.562 milli-ohm so roughly 7.5 milli-ohm
but we don't have to go roughly it's we can it's 7.55 milli-ohms at 28.3
degrees Celsius. Alright now that's 7.5 milli-ohms for a 10 amp fuse here's
a 15 so we were at 7.5 milli-ohms on a 10 amp fuse now we are at 4.783
milli-ohms for a 15, let's do the 20 3.486 milli-ohms on a 20 amp fuse
2.7195 milli-ohms on a 25 amp fuse and 2.3117, 2.3165
it's stabilizing milli-ohm 2.3 milli-ohms on a 30 amp fuse so each fuse has
its own specific resistance and I measured two other styles of fuses and their resistance values
are very close to what these measured here so for a certain size of fuse, it should have a
certain resistance now with that resistance if we go over to the vehicle and take a voltage drop
measurement in a live circuit right across the top of a fuse so here's a 15 amp fuse right here if we
take a voltmeter put it on millivolts and measure right across the top of that which I did as you
can see in this photograph here under the hood of the Chevrolet Bolt EV over there I measured 17.5
millivolts dropped across the left-front low beam headlight fuse with the headlights on so 17.5
milli-volts that's 0.0175 volts now if we divide that by the resistance
of this fuse that we just measured which is in milli-ohms of 4.783 milli-ohms
that's 0.004783 ohms so basically if we take 17.5 milli-volts
and we divide it by 4.783 milli-ohms the current going through
that fuse with the headlights on was 3.56 amps and it's just basic Ohm's law stuff.
So you can measure the milli-volt drop across a fuse if you know the fuses resistance, which you
should probably measure, there's a lot of garbage fuses for sale out there, and there's high-quality
fuses for sale, I'm going to buy some less than reputable brand fuses and compare them to the
factory fuses that come in these vehicles and see if the resistance values are the same they're
milli-ohm resistances I hope they are but I know there's been some problems with some that weren't
and and the fuses weren't blowing when they were supposed to blow. Okay, so, with this meter we can
measure milli and micro and even nano ohms of resistance which is useful for measuring the
stator winding resistance to find a short circuit I've shown you that we can cut an individual
wire strand and it can detect the difference there I've shown you that we can not only measure
the resistance of a piece of wire but we can tell you how long that wire is that might be useful in
somehow determining where a short-circuit is I've still got to think about that but I this is not
the last video I'm going to do with this milli-ohm meter there's got to be other uses of
this meter I'm just barely playing with it now and I really like it. Now the last thing I want
to do is let's go over to this 2002 Prius with what appears to be a transaxle problem and
measure the resistance of the stator windings in the vehicle itself. okay here we are under the
hood of a 2002 Toyota Prius first generation Prius but this could just as easily be any other
Prius or electric or hybrid vehicle on the road it doesn't really matter what matters is that we
can access the three-phase cables that go down to the transaxle from the inverter converter
assembly because that's where we are going to use our milli-ohm meter so let's take some
measurements okay our first measurement will be from you the U phase to the V phase and we are
reading 32.88 milli-ohms of resistance at 28.6 degrees
Celsius here in the shop now the resistance will go up as it gets hotter it'll go down as it gets
colder so we're looking for consistency alright now let's go from U to W 35.12
milli-ohms of resistance so we were at 32.89 milli-ohms and now we are at
35.11, that is more than the 2 milli-ohm maximum allowed by Toyota so that
indicates a problem. Alright now let's measure from V to W now we're getting 26.98
milli-ohms so our very lowest value is right here between the V and the W phase at
26.982 milli-ohms our very highest value was between U and W at 35.11
milli-ohms and then we had one in the middle that has a shared branch of the
Y wound stator of 32.89 milli-ohm. so we have 26.9. 26.9
was the lowest, 35.1 is the highest that gives us roughly an 8.2
milli-ohm difference between the highest reading and the lowest which tells us that there
is definitely a problem with the stator windings for MG2 which is what this goes to on this 2002
Prius which is why it jerks really hard it has melted through the insulation around the
stator windings it may even have contact with the stator frame which in the next video on meg-
ohm and Giga-ohm meters we will go in and learn how to check for loss of isolation or check for
insulation it's an insulation tester so we will learn how to do that so by comparison I have
this exact same P111 transaxle in the backroom over here with a good stator winding for MG2 and
all three measurements that we've done here on that transaxle measure are identical they're
almost all 35 milli-ohms and so that's what we're looking for so the milli-ohm meter is a very
useful, but expensive, but very useful tool. we can tell when stator windings are starting to melt we
don't have to wait until it totally gets to where the vehicle is chugging and kicking and spitting
out trouble codes if it's acting strangely with a little bit of jerking or pulsating as you drive
even if it has a trigger to trouble code yet we can go in and take those measurements and see
because this seems to be a fairly common problem on high mileage Prius transaxles and the newer
ones with those hair pinned technology Staters with all those additional welds and connections
have even more potential of problems hopefully we won't see that this has been a review of the basic
use of a milli-ohm meter thank you for watching
great. now I want one of these.