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How to measure resistance? by rbell2915
Started on: 09-19-2013 11:56 AM
Replies: 15 (435 views)
Last post by: theogre on 09-20-2013 03:12 PM
rbell2915
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Report this Post09-19-2013 11:56 AM Click Here to See the Profile for rbell2915Click Here to Email rbell2915Send a Private Message to rbell2915Edit/Delete MessageReply w/QuoteDirect Link to This Post
Hi! I'm in autos and we're learning how to do some stuff with live work. I brought my car in. Got a lot of attention but anyway I have to measure resistance on the headlamps and the relay for the coolant fan. Everyone is confused as hell and I'm looking for some answers to these questions. Thanks!

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2.5
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Report this Post09-19-2013 12:38 PM Click Here to See the Profile for 2.5Send a Private Message to 2.5Edit/Delete MessageReply w/QuoteDirect Link to This Post
I believe you will first need a tool to measure ohms.

digital


old school

[This message has been edited by 2.5 (edited 09-19-2013).]

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Raydar
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Report this Post09-19-2013 12:46 PM Click Here to See the Profile for RaydarClick Here to Email RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
Measure on the "ohms" scale.
Ideally, the device being measured must be removed from the circuit (unplugged.)
Unless you're measuring a semiconductor (diode or transistor) the polarity is not important.
If your meter doesn't read, switch to the next highest range until you get a reading.
Analog meters must be zeroed before taking a measurement. Short the leads together, and adjust the knob for 0 on the scale.
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Robertzep2
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Report this Post09-19-2013 12:49 PM Click Here to See the Profile for Robertzep2Click Here to Email Robertzep2Send a Private Message to Robertzep2Edit/Delete MessageReply w/QuoteDirect Link to This Post
The resistance of a filiment-type bulb changes resistance with heat.
By Ohm's law, resistance is voltage divided by current, so the resistance of a headlight can be measured by observing the voltage across it simultaneously with observing the current through it. To measure actual resistance in a working circuit, wait until the lights have been on a minute, then measure amps with a good inline digital voltmeter like Forum member 2.5 showed you.Your calculated resistance would be the voltage (measured)divided by the measured amperage. Current (amps) must be measured in series with the bulb.
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rbell2915
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Report this Post09-19-2013 12:55 PM Click Here to See the Profile for rbell2915Click Here to Email rbell2915Send a Private Message to rbell2915Edit/Delete MessageReply w/QuoteDirect Link to This Post
The meters we have at school are digital. In order to measure resistance, I unplugged the connector to the lightbulb, exposing three metal pieces. I'm not sure which of the three gets the positive (black?) or negative (red?). And the meters only have one selection for ohms at school, just so you know. And where is the relay for the coolant fan?
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Robertzep2
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Report this Post09-19-2013 01:14 PM Click Here to See the Profile for Robertzep2Click Here to Email Robertzep2Send a Private Message to Robertzep2Edit/Delete MessageReply w/QuoteDirect Link to This Post
Headlight for a Fiero is capable of low and high beams. So, you have two elements both with different resistances. I going to bet your instructor will show you the method of using measured voltage and current to calculate resistance using ohms law! As for the coolant relay, you will need your service manual to show you the location. This is another tool as valuable as a meter in servicing your car. These are lessons you are being taught. Use approved data and proper tools.

[This message has been edited by Robertzep2 (edited 09-19-2013).]

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theogre
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Report this Post09-19-2013 01:29 PM Click Here to See the Profile for theogreClick Here to visit theogre's HomePageSend a Private Message to theogreEdit/Delete MessageReply w/QuoteDirect Link to This Post
Exactly what are you measure ohms?

Many car parts/circuits are too big or small for normal meters to work.
Relay coils can read ohm but any switch contact need 0 ohms.

If you have problem then first check grounds.
See my Cave, Wire Service

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Lambo nut
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Report this Post09-19-2013 02:57 PM Click Here to See the Profile for Lambo nutSend a Private Message to Lambo nutEdit/Delete MessageReply w/QuoteDirect Link to This Post
Pin out for back of headlight.



Kevin
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rbell2915
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Report this Post09-19-2013 09:49 PM Click Here to See the Profile for rbell2915Click Here to Email rbell2915Send a Private Message to rbell2915Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Lambo nut:

Pin out for back of headlight.



Kevin


Thank you! Now which pins do I touch with what polarity contact? EX: Do I touch negative to ground and positive to low beams? Or vice versa? Or does it not matter?

I'm so illiterate when it comes to this, I'm sorry
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Lambo nut
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Report this Post09-19-2013 10:09 PM Click Here to See the Profile for Lambo nutSend a Private Message to Lambo nutEdit/Delete MessageReply w/QuoteDirect Link to This Post
When checking resistance, it won't matter here. Touch either lead of you checker to ground then the other to the top terminal to check low beam element, and ground and right terminal to check high beam element.

And don't worry, I nor anybody else here knew anything about this at one time in their life.

Kevin
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Robertzep2
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Report this Post09-19-2013 10:19 PM Click Here to See the Profile for Robertzep2Click Here to Email Robertzep2Send a Private Message to Robertzep2Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by rbell2915:I'm so illiterate when it comes to this, I'm sorry


From your first post, I got the impression you were in a class. If that is the case, where is your instructor/teacher? You need to learn basic electricity and how to use a multimeter! In a classroom situation this information should have been presented to you through lecture, video and books. This should have been done way before you ever were exposed to a "real world" live lab environment.
Learn to use a multimeter
There are thousands of good books on understanding automotive electronics.
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Report this Post09-19-2013 10:35 PM Click Here to See the Profile for phonedawgzClick Here to visit phonedawgz's HomePageClick Here to Email phonedawgzSend a Private Message to phonedawgzEdit/Delete MessageReply w/QuoteDirect Link to This Post
Incandescent bulbs change resistance when they get hot. If you measure the resistance when the bulb is cold you will find the resistance is quite low. The resistance changes as soon as the filament gets hot and starts putting out light.
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rbell2915
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Report this Post09-19-2013 11:06 PM Click Here to See the Profile for rbell2915Click Here to Email rbell2915Send a Private Message to rbell2915Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Robertzep2:


From your first post, I got the impression you were in a class. If that is the case, where is your instructor/teacher? You need to learn basic electricity and how to use a multimeter! In a classroom situation this information should have been presented to you through lecture, video and books. This should have been done way before you ever were exposed to a "real world" live lab environment.
Learn to use a multimeter
There are thousands of good books on understanding automotive electronics.


I think he's going to be helping us along as we need it, but yes, I agree. I now know how to test the resistance of the headlamp and coolant fan relay and how to prove power from a 12V source, thanks to everyone, plus my father too!

Right now I'm starting to get a grasp on this. Can't rely on school for everything. Sometimes the best education is done out of school.
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Report this Post09-20-2013 05:50 AM Click Here to See the Profile for RaydarClick Here to Email RaydarSend a Private Message to RaydarEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by rbell2915:
I think he's going to be helping us along as we need it.
...


That's why they're called instructors.
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Robertzep2
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Report this Post09-20-2013 09:10 AM Click Here to See the Profile for Robertzep2Click Here to Email Robertzep2Send a Private Message to Robertzep2Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by rbell2915:


I think he's going to be helping us along as we need it, but yes, I agree. I now know how to test the resistance of the headlamp and coolant fan relay and how to prove power from a 12V source, thanks to everyone, plus my father too!

Right now I'm starting to get a grasp on this. Can't rely on school for everything. Sometimes the best education is done out of school.


I full heartedly agree that sometimes the best education is found doing by researching on one's own! I am Electronics Engineer, and have been working in the field over 25 years, and still learn new things everyday. I made the statement to you about learning some basics on electricity and how to use some tools because I had an intern, who was in his third year of electronics, destroy a $20,000 prototype. He improperly hooked up the equipment and did not know how to make measurements. The moral of the story unfortunately, improper use of equipment can destroy electronic devices. Good luck in your education and learn the basics, because everything is built up from that.

[This message has been edited by Robertzep2 (edited 09-20-2013).]

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theogre
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Report this Post09-20-2013 03:12 PM Click Here to See the Profile for theogreClick Here to visit theogre's HomePageSend a Private Message to theogreEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Robertzep2:
The moral of the story unfortunately, improper use of equipment can destroy electronic devices. Good luck in your education and learn the basics, because everything is built up from that.

You can do damage from a meter...
wrong meter setup, especially Ohms and amps can cause problem even wreck the ECM etc.

Even some ECM "grounds" are input/reference for use by ECM.
1 example, See my Cave, Ground "Myth" notes
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