EEVblog 1622 - Don't Make This Multimeter MISTAKE

[EEVblog]

This is the biggest mistake beginners often make when using a multimeter ohms function.
A customer returned an EEVblog BM235 multimeter because it was "Faulty"
Was it? Let's find out what happens when you try and measuse a CAN bus with a multimeter.

[derree]

Hi Dave. In one comment you wrote that the customer still insists that the bm 235 is bad and you don't know what you are talking about. I learned that measuring component values in situ is always prone to errors, powering the circuit up for sure doesn't help. Would be nice from the customer to explain here what you are "getting wrong" ...

[DimitriP]

For some, the CAN Bus is a can't bus.

Sometimes I say there are people that should never be in the same room with an oscilloscope - at least not alone!
I now realize some should not be touching multimeters either.
And certainly NOT MY CAR.


To summarize:

CAN Bus Termination

There should be a 120-ohm termination resistor located at each end of the bus to prevent signal reflections. When you measure the resistance between CAN HI to CAN LOW on a wiring harness you should measure 60 ohms. This measurement should be conducted with the device power off.

https://support.enovationcontrols.com/hc/en-us/articles/360038856494-CAN-BUS-Troubleshooting-Guide-with-Video#IntroductiontotheCANBus


In the next episode we might have to go into why when you have a 120ohm termination near EACH end of the bus, how you end up with 60 Ohms .....   

[vk6zgo]

For some, the CAN Bus is a can't bus.

Sometimes I say there are people that should never be in the same room with an oscilloscope - at least not alone!
I now realize some should not be touching multimeters either.
And certainly NOT MY CAR.


To summarize:

CAN Bus Termination

There should be a 120-ohm termination resistor located at each end of the bus to prevent signal reflections. When you measure the resistance between CAN HI to CAN LOW on a wiring harness you should measure 60 ohms. This measurement should be conducted with the device power off.

https://support.enovationcontrols.com/hc/en-us/articles/360038856494-CAN-BUS-Troubleshooting-Guide-with-Video#IntroductiontotheCANBus


In the next episode we might have to go into why when you have a 120ohm termination near EACH end of the bus, how you end up with 60 Ohms .....   

That's easy, the multimeter doesn't care----as far as it is concerned, they are in parallel, whereas to the signal things are somewhat different.

[quince]

Thanks Dave. Every time I say "don't measure the bus when it's powered" someone replies "but my other meter worked just fine!". Yeah, but not every meter is identical!

[vk6zgo]

This is the biggest mistake beginners often make when using a multimeter ohms function.
A customer returned an EEVblog BM235 multimeter because it was "Faulty"
Was it? Let's find out what happens when you try and measuse a CAN bus with a multimeter.

That was an easy lesson to learn with an analog multimeter.
There is no "magic" between the probes & the guts,so depending upon the circuit voltage, you either got a "funny reading", or twanged the meter, hitting either stop.

The latter was the most common result in tube times.

[EEVblog]

Hi Dave. In one comment you wrote that the customer still insists that the bm 235 is bad and you don't know what you are talking about.

Yep. I authorised return of the meter in any case.

[Shock]

"Your ecu is fried sir, yes I believe it was on it's last legs and gave up while I was testing it."

[Gyro]

"Your ecu is fried sir, yes I believe it was on it's last legs and gave up while I was testing it."

I once got exactly that from a main dealer!

[David Hess]

There are rare multimeters which support an AC resistance mode where the ohms converter is pulsed or provides an AC current and the change in voltage is measured to get a resistance measurement in the presence of applied voltage or current.  I think a keithley 2000 series model supported this because it is a necessary function for in-circuit current measurement.

[jhjove]

Geez, this is "electronics tech 101" stuff.
When I saw the DMM reading negative ohms, I'm thinking what, this reads siemens?? 

[Sensorcat]

There are rare multimeters which support an AC resistance mode where the ohms converter is pulsed or provides an AC current and the change in voltage is measured to get a resistance measurement in the presence of applied voltage or current.  I think a keithley 2000 series model supported this because it is a necessary function for in-circuit current measurement.

The Keithley 2001 has this In-Circuit Current mode. However, in ohms mode, my results are not different to what Dave has shown in the video: Very high absolute values, positive or negative, depending on polarity.

[David Hess]

Geez, this is "electronics tech 101" stuff.
When I saw the DMM reading negative ohms, I'm thinking what, this reads siemens?? 

My older multimeters would read negative under the same conditions.  Ohms mode uses an "ohms converter" to drive a current between the terminals, and then reads out a voltage.  It is literally DC voltage measurement mode with a current source across the terminals, so if the input is driven negative, it will read negative.

The meters which read zero have firmware to block negative voltage measurements in ohms mode.

[jhjove]

I'm sure my DMMs would do the same.

I was being a little sarcastic though.

I learned on old D'Arsonval meters (simpson, eico). I wonder if the current back-feed would cause the meter needle
indicator to bounce off the minimum reading (or even damage the movement); it's probable.

[David Hess]

I once saw an analog meter needle hit the stop so hard that it bent.

[Whales]

Could the cheaper meter be placing most of 9V on the bus, locking out the bus drivers?

I know that my cheaper meters can light blue LEDs on continuity mode but my expensive ones can't.  Typically a lower test voltage/current is considered "better" because it's less likely to activate other things in a circuit (ideally you want less than even 0.5V to avoid activating diodes, but I don't think I've seen that in standard DMMs).  Also the cheaper meters tend to use 9V batteries.

EDIT: Yes I can imagine this particular CANbus driver doing nothing if the bus already has a voltage higher than what it can provide.  CANH > VCC and CANL < GND makes the circuit do nothing, the diodes keep it isolated and the meter will read 60R.  The TVS will probably not kick in until above 12V or 24V or similar.

[EEVblog]

Could the cheaper meter be placing most of 9V on the bus, locking out the bus drivers?

It would be doing something like that, yes, possible. If it is I'd imagine it also varies based on what car you are testing.
Still doesn't change the fact that there is nothing wrong with the BM235 or any other meter.

[vk6zgo]

"Your ecu is fried sir, yes I believe it was on it's last legs and gave up while I was testing it."

I once got exactly that from a main dealer!

In the early days of ECUs,before each was "personalised" to the individual vehicle, mechanics would buy a bunch of ECUs from the wreckers.
If some strange fault occurred, they would always say "it's probably the computer" & slot one of their spares in.

Our 1988 Australian Ford Falcon would gracefully come to a halt & as groping around in cars had lost its appeal, I called the "mobile mechanic" who chucked a spare ECU in & called it good.
The Ford strenuously disagreed, & finally gave up entirely, so I was stuck with borrowing an oscilloscope from work (a THS7xxx---can't remember the exact model) & checking the thing for myself.

Back probing the weird connector on the ECU, I found that all the signals were present, including most importantly, the ignition pulses, which were then fed to the old style distributor, but seemingly, didn't arrive there.

It turned out to be an old fashioned car type intermittent, where the wires in the plug which went into the distributor had fractured, making intermittent contact.

Digging round in the junk box, I found some bits that I could use for a bodge fix, & the old beast was a "go-er" again.

The Falcon had another, very intermittent fault, in that every now & then, it would hesitate when cruising at speed, then "come good".
This, over years, got a bit more annoying, so I revisited the ECU, this time using my old BWD which had a triggering fault.
All the pulses were there, sliding across the screen.

It turned out to be an incorrectly fitted injector in the throttle body, so again, nothing to do with the ECU.

OK, car electrics have moved on since then, but basic stuff like connectors still live in a hostile environment, so the old "trace the circuit" skills are still important.

[Kleinstein]

How much voltage is used in the ohm mode depends on the meter. At least the cheap old meters (based on ICL7106 or similar) often used the direct ADC input scale (some 200 mV or 400 mV) for this. With modern auto ranging chip sets this could be different. Especially mains powered bench meters may use more voltage, at least for the high resistance ranges.

The CAN bus signal can confuse the DMM. It is not just a DC background but has the extra pulses of some 5 V that may well activated clamps in the DMM. So it is hard to predict what the meter makes from this.
The zero reading of quite some of the meters looks like they don't like negative and show zero instead.  Ideally the meter would check for foreign voltage and show an error instead of the wrong resistance. This could still be confusing with RC parallel combinations - so not the normal way DMMs work.

[David Hess]

Compliance voltage of the ohms converter largely depends on the era of the multimeter.  The oldest ones have no limit and will happily drive 10+ volts.  This however will damage the base-emitter junction of a transistor, so later it was reduced to about 3 volts preventing breakdown of low voltage junctions.  More recently the compliance voltage has been raised slightly to 4 or 5 volts, so that GaN LEDs can be tested.  Of course exceptions abound, either deliberate or from thoughtless design.

[vk6zgo]

How much voltage is used in the ohm mode depends on the meter. At least the cheap old meters (based on ICL7106 or similar) often used the direct ADC input scale (some 200 mV or 400 mV) for this. With modern auto ranging chip sets this could be different. Especially mains powered bench meters may use more voltage, at least for the high resistance ranges.

The CAN bus signal can confuse the DMM. It is not just a DC background but has the extra pulses of some 5 V that may well activated clamps in the DMM. So it is hard to predict what the meter makes from this.
The zero reading of quite some of the meters looks like they don't like negative and show zero instead.  Ideally the meter would check for foreign voltage and show an error instead of the wrong resistance. This could still be confusing with RC parallel combinations - so not the normal way DMMs work.

DMMs can be confused on other ranges, too.

Working on my Mother in Law's colour TV set in the mid 1970s, I was chasing the cause of a vertical collapse.
Looking at a 150v rail obtained from rectifying the output of an overwind on the Horizontal Output Transformer, I found it read around 110v on my work Fluke 77 on DC range---certainly low, but not really enough to cause the symptoms.

I resolved to borrow an old BWD from work, as it looked like a more complex fault.
On the 'scope,the problem was immediately obvious, the rail voltage was a series of unipolar peaks of around 150v, appearing at line rate.
The fault was a dried out filter cap, but the Fluke 77 couldn't tell the difference between the result & a slightly low DC voltage.

[ballsystemlord]

@Dave , I wouldn't call this a beginner mistake. It's an "I didn't RTFM" mistake. I've never done this. Even as a child. I knew better, because I read the manual.

[ballsystemlord]

Not to drag everyone off topic, but my father still uses ohms mode to test non-polar capacitors. "If the range goes positive and then negative the cap is good" -- Dad. I just thought you'd all enjoy that little tidbit.

[Kleinstein]

In the old days of analog meters there was the additional problem that the protection was not that good. Measuring an active circuit in the ohms range can damage an older meter easily. I have damaged my old meter this way, measuring the resistance of a 150 V transformer winding. Most modern meters are better protected, but still not perfect.