# Resistance measurement in the 0.5 - 1.0 Ohm range

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#### Phil Allison

Jan 1, 1970
0
"Chris"
You've got your 12V battery already. See if you can find a 100 ohm
resistor that's 3 watts or more. The better the precision and the
higher the wattage, the better. I like to scrounge power resistors
when I find them, so I've got a collection of 10 watt and up 1%
resistors to do this type of thing.

Now, hook up your battery in series with the 100 ohm resistor and the
coil, like this ....

** Then just measure the voltage across each resistor.

The readings are in *proportion* to the value of the resistors since the
current through them is the same.

Eg. say you get 12.7 volts and 55.0 mV.

Their ratio is 231 : 1

Tthe answer is then 100 / 231 = 0.433 ohms

......... Phil

C

#### Chris

Jan 1, 1970
0
Phil said:
"Chris"

** Then just measure the voltage across each resistor.

The readings are in *proportion* to the value of the resistors since the
current through them is the same.

Eg. say you get 12.7 volts and 55.0 mV.

Their ratio is 231 : 1

Tthe answer is then 100 / 231 = 0.433 ohms

........ Phil

Hi, Phil. You're right -- this is an easier way to solve.

Thanks for the spot.

Cheers
Chris

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#### redbelly

Jan 1, 1970
0
Chris said:
Even with a coil that is sensitive to too much power, you can make a
fairly accurate resistance measurement using a DMM with a 200mV range,
if you have a precision series resistor to give a known current.

Chris,

Looks like a good method. By the way, one needn't buy a precision
resistor, so long as you can measure the resistor precisely. Measure a
"5%" 100-ohm resistor to be 102.5 ohms (for example), and you're
precision is even better than blindly using a 1% 100-ohm value.

Mark

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#### Rich Grise

Jan 1, 1970
0
....

Oh, and by the way -- if your sparkplug wires are more than 3 years
old, and you're having intermittent misfiring, you should replace them
as a matter of principle. You can't really troubleshoot them. And get
the good ones -- the cheapies aren't worth it.

And when you replace the plug wires, don't forget to replace the coil
wire! I changed a set of plug wires once, and looked at the coil
wire, figured, "nah, looks OK", and about 3 months later had to have
it towed to the shop because it wouldn't even start. $100.00 later, it turned out to be the coil wire. If you found this advice of help, Rich Grise keeps the tip jar for s.e.b. ;-) I don't even know what this means. Cheers! Rich E #### Ed Jan 1, 1970 0 Chris said: Hi, Ed. Seconding Mr. Monsen here. They're moving cars through the service bays, and trying to fix things as quickly as possible. It looks like they made a mistake in thinking that the coil was the cause of the problem, even if it was out of spec. One thing you might try that I've found helpful. It's kind of like going to the doctor with a list of symptroms and things you want to ask. Write down in your own words what you're experiencing with the intermittent, and especially when the intermittent happens (particularly during condensing humidity, only when the engine is warm or cold, when it's below or above a certain temp out, when it's raining, &c). Make two copies. Make sure the service rep writing your job ticket attaches a copy of your observations to the job ticket. Never miss a chance to communicate directly with the tech actually troubleshooting your problem. If you have a problem with any technical service, take it back as soon as you discover the problem wasn't fixed, while the problem is still fresh in the mind of the person who worked on it. Nothing spurs a little extra effort in a conscientious tech faster than an immediate "Wrong -- try again". And there's no easier time to negotiate about the charges. Oh, and by the way -- if your sparkplug wires are more than 3 years old, and you're having intermittent misfiring, you should replace them as a matter of principle. You can't really troubleshoot them. And get the good ones -- the cheapies aren't worth it. If you found this advice of help, Rich Grise keeps the tip jar for s.e.b. ;-) Good luck Chris Thanks to Mr. Monson & Chris. However, you missed a subtle barb hurled towards GM, the dealer in question. When I said I was being pressured to go to the Toyota dealer I meant to imply putting the Buick Lucernes in the rearview mirror of a new Avalon But I do agree with the advice of your notes. I have been working this problem for months, and it has stumped my trusted independent shop as well as the Buick dealer. I have taken the car back to the independent several times without charge because they installed a$300 mass air flow sensor on a bad hunch. The Buick dealer replaced
the coils on
the second visit, having found nothing wrong with the car on the
first. And I did hand a
1 1/2 page description of the the symptoms and history to the service
rep. I have reason to
believe the tech didn't read it, so the next time I dealt with the
service manager, giving
him a copy and watching while he read it. Still, I noticed that he
summarized it in one line
on the service order and I'll bet that's all the tech paid any
attention to.

Regarding the plugs and wires, that I did myself. I bought the best
available at a local
parts store I've dealt with for years. The wires were Federal Mogul,
which at least used to be
a trusted name in auto parts, and the AC plugs the car came with.

One big problem on top of the missing problem is the OBD II system
shows nothing. That's what the
techs kept telling me, and I finally bought a pretty good scanner,
Actron CP9145, on eBay for $160, and confirmed that there are no faults or pending faults. I'm thinking about driving around with it attached for a week or so and see if anything turns up, but the ODB codes are supposed to be stored for a while, so that shouldn't be necessary. I will attempt to do more precise measurements on the coils, and either way will be going back to the Buick dealer for a talk. Thanks for your interest and help. Now, about that tip jar.... where is the damn thing? I've gotten a LOT of good advice here in the last couple years, while building test circuits for my old Jaguar V12 which also has perplexing problems. So it better be a big jar when I find it! Ed E #### Ed Jan 1, 1970 0 Just to wind this up... I measured all three of the coil packs with the Kelvin method as suggested by Chris. All of them were in the range 0.347 - 0.349 Ohms. Then, I pull one of the new ones off the car and measured twice. The first time it measured 0.407 Ohms, but it was fairly hot. I let it cool down a couple hours and measured again and got 0.368. My guess is if had time to let it cool to the same temperature as the others it would have dropped further. So, there is no question in my mind that my coils are as good as the new ones, at least with respect to primary winding resistance. Thanks a bunch! Ed C #### Chris Jan 1, 1970 0 Ed said: Thanks to Mr. Monson & Chris. However, you missed a subtle barb hurled towards GM, the dealer in question. When I said I was being pressured to go to the Toyota dealer I meant to imply putting the Buick Lucernes in the rearview mirror of a new Avalon But I do agree with the advice of your notes. I have been working this problem for months, and it has stumped my trusted independent shop as well as the Buick dealer. I have taken the car back to the independent several times without charge because they installed a$300 mass air flow sensor on a bad hunch. The Buick dealer replaced
the coils on
the second visit, having found nothing wrong with the car on the
first. And I did hand a
1 1/2 page description of the the symptoms and history to the service
rep. I have reason to
believe the tech didn't read it, so the next time I dealt with the
service manager, giving
him a copy and watching while he read it. Still, I noticed that he
summarized it in one line
on the service order and I'll bet that's all the tech paid any
attention to.

Regarding the plugs and wires, that I did myself. I bought the best
available at a local
parts store I've dealt with for years. The wires were Federal Mogul,
which at least used to be
a trusted name in auto parts, and the AC plugs the car came with.

One big problem on top of the missing problem is the OBD II system
shows nothing. That's what the
techs kept telling me, and I finally bought a pretty good scanner,
Actron CP9145, on eBay for $160, and confirmed that there are no faults or pending faults. I'm thinking about driving around with it attached for a week or so and see if anything turns up, but the ODB codes are supposed to be stored for a while, so that shouldn't be necessary. I will attempt to do more precise measurements on the coils, and either way will be going back to the Buick dealer for a talk. Thanks for your interest and help. Now, about that tip jar.... where is the damn thing? I've gotten a LOT of good advice here in the last couple years, while building test circuits for my old Jaguar V12 which also has perplexing problems. So it better be a big jar when I find it! Ed Hi, Ed. Didn't miss the barb at all. But since the Toyota is probably as much domestic content as the Buick, and who knows how much of GM common and preferred stock is actually owned by our friends overseas (that's what happens when you have a$2 billion a day balance of trade
deficit -- they have to do something with all that money), I'm not sure
what to say, except to echo the words of Theoden King in The Two Towers
-- "How did it ever come to this?"

The first really reliable car I ever owned was a Toyota Corolla with
almost 100,000 miles on it when I bought it as a workmobile. I more
than doubled that, and shed a silent tear when it finally retired (due
to rust from Chicago winters and street salt). I kept the stereo on my
bench for years to remind me of what a fun little car it was.

Since you already got the plugs and wires, take a look at the
connectors on the oxygen sensors, particularly if you live in an area
where there's salt spray (like by the ocean, or where they salt the
except to try posting on alt.autos.gm and see if anyone there has
walked out this tough dog.

Good luck
Chris

R

#### Ralph Mowery

Jan 1, 1970
0
Ed said:
Just to wind this up...

I measured all three of the coil packs with the Kelvin method as
suggested by Chris. All of them were in
the range 0.347 - 0.349 Ohms. Then, I pull one of the new ones off the
car and measured twice.
The first time it measured 0.407 Ohms, but it was fairly hot. I let it
cool down a couple
hours and measured again and got 0.368. My guess is if had time to let
it cool to the same
temperature as the others it would have dropped further.

So, there is no question in my mind that my coils are as good as the
new ones, at least with respect to
primary winding resistance.

Thanks a bunch!
I have to admitt that I have not been in a car service department in a long
time.

What hapened to the big Sun machines and any thing like them ? They had
scopes on them where you could see the patern of the voltages on the plugs
and coils.

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Phil Allison
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